Hi,
I'd like to get some opinions on a 24' span.

The project is a garage/vehicle shed, final outside dimensions 24'x24' (12' height). I will be using old growth SYP (most logs with DBH of ~ 24"). My plan was to use 8x8 timbers for all the short runs (10-12') and rafters ~14-15'. The building would have three bents (two ends and a middle) - 12' connectors.

The question is if I can use a 24' beam across the bent (with no support in the middle) or should I frame the bent as two 12' sections (in other words have a supporting post in the middle under the 24' beam)?

I guess a secondary question is what dimension would the 24' beam need to be if no support was used? (8x10? 8x12?)

This is my first TF project, so I may not have given all the info needed to answer the questions - please advise.

Thanks.

http://www.tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=19471&page=7

Put the post on top of the tie and still have a clear span. The tie I used was 9"x12"ish, it also had a crown in it, the span is 24'. If you have access to a 24' tie you might as well have 24' top plates as well.

Tim

you meant put a post between the tie and the ridge where the rafters meet?.. and are you referring to this picture: [img]http://tinypic.com/r/whyxq0/3[/img]

A simple truss in the middle bent. The rafters are not part of the truss. Yes, that is the picture.

Tim

ok thanks

I tried it this way;
288" clear span
full 8x10 dimension..(8x10x288)/144=160BF
160/12=13.33 cu/ft
13.33x 36 lbs/cuft=480 lbs green
Using an E value 0f 1.2 million psi and with just the self weight of the beam I'm coming up with about 3/16" deflection. Hanging 1,000 additional lbs from it, it sagged 9/16"

Figuring deflection with just self weight of the beam, with an 8x8 I came up closer to 5/16", at 8x12 it was a bit over 1/8"

Not saying not to make a king rod truss out of it, but if the bottom chord is just a tie it is in no danger with the long bottom chord alone the way I see it.

Don -

I think he's trying to figure if he is going to build a truss or drop a post to the ground.

Either way he has 24' to take care of - either a continuous ridge, or a tie that is part of a truss...

As I mention in all theoretical engineering posts in the forum - it would help to understand loading - snow, wind, etc. Use. Roofing material, etc. Open frame? Enclosed? TG on ceiling? Metal roof? Shingle? Etc. Etc.

If you decide to build a truss - you will be carrying a tributary area of 12' wide (6' each side) of the total roof load. How this spreads to the truss depends on the type of truss. Do you plan on using a ridge with common rafters (point load onto a king post) or principle rafters with common purlins (distributed load on truss rafters)?

Self weight often causes very little deflection... the good stuff happens when we throw snow on low pitches with non-slippery surfaces, then unbalance it all with wind!

24' is very doable. Timbeal points to a king post truss design with dropped rafters (or struts) that would work, assuming the loading makes sense for how the truss ties into the posts, and how the load transfers down the struts and out through the tie (or lower chord). And the joinery you design needs to be able to transfer those loads as well...

BJC could ask more questions to which we could respond. Mike points out questions which would for sure need to be answered. My example is just that. I have seen many 24' spanned buildings where the roof only loaded the walls not the floor. The king would not have to be tied into the roof system, avoiding roof loads. You then have to look at what is going to keep the building from spreading.

My suspicion is the 8x10 would be bouncy. But is there even a floor to load?

In my truss above, the struts are using a 3" tenon and plenty of relish, no issues that I can see.

Tim

Tim, not sure I follow you here. What is the point of a king post if it is not tying into the roof system? What did you mean by this?

The King is supporting the floor, not holding the roof up. Without a post under the tie, what do you do? Span 24' without a post?

Tim

Guess I didn't follow where we added a floor! And didn't you post a pic of a king post truss?

Difference in language I guess. I never refer to a king or queen unless it is in a truss or bent that supports the roof.

You have to support 24' either way - in the ridge or in the tie... I'm a fan of supported ridges - even with a simple truss like you linked too.

I have not read where BJC added a floor either. There is the possibility of a floor in the future perhaps? Or at least some timber laid on top of the tie and endless stuff placed upon that. Shouldn't we look at the future?

Isn't a truss different than a bent?

Yes, I posted a picture of a king post truss with no ridge beam in the building.

Tim

Hey Guys,

I think I see what you all are talking about with two different arrangements.

One you can build a truss with a kingpost by either dropping the rafters (upper chords) which, allow for a continuous ridge (this would require building upon the chords to get back to the plane of the roof with purlins and commons)

Or you can have the principal rafters land on the kingpost at the roof plane requiring a interupted ridge.

But either way, in order to span without a center post under the tie, the compression member (chord or rafter depending) has to be joined to the two tension members (kingpost and tie). Therefore zeroing out the forces and defining a truss. On the other hand if you can find a piece for the tie that can handle a 24' span, you won't need a truss. But that would have to be one big honkin tie.


BJC, this reading might help http://www.tfguild.org/publications/kingposttrussTF72.pdf

p.s. these Guild publications are simply awesome. Give thanks.
notice the hand drawn joinery as well.

Thanks guys for all the feed back. I'm sorry that I did not give more info to start with let me see if this adds any flesh to the question:

We are in Alabama, so minimal to NO snow... occasional tornado but snow is not a problem.

Initially the structure will be left open as sort of a pavillion to park vehicles under. I may enclose the sides later to make a true garage out of it.

Again, initially I will put an aluminum "tin" roof on... my plan was to put 2x6 (or 8) rafters between the bents and then 2x4 purlins on them on which to attach the aluminum sheets. I say initially because I may want to later either upgrade to a true tin roof or possibly (if the structure ends up looking nice) do something more traditional (shakes, etc).

As a foundation I will either simply put the posts in the ground (3-4 ft) or will pour piers - again very little cold and thus a freeze line of about 6 inches and little to no heaving to deal with.

This will be my first TF project. I have the bandsaw to mill the SYP so I can custom the beams as necessary...

Not having a center post in the bent is appealing: nothing to hit with the van when backing in (except the truck), three less piers to pour, etc.

It sounds like the initial idea posted would actually create a situation where the 24' cross beam is "hung" from the ridge... is that accurate??

Thanks
Ben

Post in ground will rot, place on piers for sure.

If I were building this in my backyard using stick rafters and purlins it might look something like this:

I like it.

I do have a question. Do the opposing rafters contact each other at the peak?

If so what is the ridge beam doing?

What I am suggesting is to leave a gap between each rafter at the peak. If they do touch, in a few years of shrinkage and settling, the rafters could be free of the ridge beam and it would not be doing the job intended.

This type of structure will eliminate rafter thrust, allowing a simpler joint at the tie to post connection. If the rafters do touch each other they will add thrust to the side walls and more attention will be needed at the tie to post joint.

The tie is not suspended form the ridge. It is the truss that is suspending the tie. If this was not true, what would be supporting the ridge? The ridge is adding more load to the truss.

Last question, did I over analyze this?

Tim

Tim -

You could clip the rafters short or push the ridge up so the rafters hit the sides of the ridge.

The ridge is taking a distributed load from the rafters. This is 1/2 the span of the rafter x 2 x the area between bents - so you have in the center case 12' x 12' = 144 sq ft of area (not much). The ridge then transfers this to the KP. If the KP gets pushed down, the struts hold it up by diverting the load out to the ends of the tie. With proper joinery at the feet of the strut the KP is nearly useless as you could just push the struts up to hold the ridge - but the KP would be useful if you added a loft - and it makes for easier joinery. If the tie is strong enough to take a centered point load (in my neighborhood maybe 55 lbs sq ft roof load x 144) you don't really need the struts. And if the tie were designed with a tension joint at the post you wouldn't really need the ridge or the truss - assuming your top plate was stiff enough to take the distributed outward thrust and your tie to post connection was same.

But - the ridge gives us an easy place to set our rafters - and we can brace the building along its length - esp seeing as this will be open with no sheathing on the roof.

Not really anything too difficult in this example - its a small building. But it will make a nice learning frame and a fine structure.

Mike

Wow! Those are some neat graphics. Do you have a design program that generates those?

You know, even though I am now beginning to read some books on TF and learn the lingo, it would be really helpful I one of those diagrams had each piece labeled with their proper name - i.e. tie, KP, ridge, strut, etc. Some of these I know from modern stick framing but others (like KP?) I'm still learning.

Thanks

He has a FREE program that generates them. Evangelize him, Mike!

Its from Google.
Its free.
It is called SketchUp.

And Clark has written some cool utilities when you want to 3D model complete joinery and generate shop drawings... and label your parts, export lists to drop into Excel spreadsheets for estimating...

The only function it lacks is to help me lose 20 pounds and get faster on the bike.

KP = King Post, typically a solo post centered in a truss that runs to a ridge or the peak, usually ending at a tie or lower chord

QP = Queen Posts, usually paired posts dividing a truss span, both running to either principle purlins or to principle truss rafters.

This glossary first appeared in TIMBER FRAMING 68, June 2003.
Glossary of Terms.pdf

Thanks guys,
I have the glossary now and I'm downloading the program.
I liked the proposed design that was posted, however, it looks like it will take 6 24' beams to construct it... while I have standing trees where I can get the 24 footers, I'd rather put this little project together with logs that I already have down, which limits me to three 24 footers... I may have to put that center post in unless I can design a way to attach the three bents together with two 12 footers instead of one 24 footer...
Maybe after I get the SketchUp downloaded I'll play around with a couple of designs.

My plan was to cut 8x8 beams for the posts and rafters and 8x10 beams for horizontal members (braces probably 6x6) all out of green SYP.

Anyone have any comments about that?

I just noticed in B-mikes last sketch up that the tie seem very close to the top plate, perhaps it should be dropped 6" or so at a minimum. What does the joinery look like there, Mike?

6x6 braces? 4x6 is a more standard size, allowing a simplified mortice and tenon.

I only see where you need 5-24'ers, so you only need two more, it would be well worth the taking down of two more trees for the top plates. Skip the bloody ridge.

Tim

Braces were drawn as 4x6 but I'd make em 3x6 or 7 depending on what tools I have to cut mortises.

Scarf joints to get the longer stuff. Skip the 'bloody' ridge if you are comfy with your loading - but as I mentioned it will really stiffen the structure as you are building a lattice for sheathing and have no shear in the roof planes. And if you lose the ridge lose the kp's and braces and struts too.

You could also build a center bent and spline the top plates and ridge through the posts to use shorter material.

And sure - you can drop the tie on the posts 6-8-12 - but you'll have a shearing problem if you lose the ridge - the rafters will push out on the top plates which will want to tear the tops of the posts off - the more you drop - the bigger lever you have to do this. And make sure your joinery @ the post / plate is designed to resist the plate rolling as the rafters push out. I think a small drop is acceptable and if I spent more time on free design I probably would have corrected it when I started to design the joinery.

But - you live in the magical land of little to no snow - so figure out what works best for you.

Timber Frame Construction, has a nice solution, IMO, on how to address this issue on page 170. I would recommend this book to anyone.

You could loose the kings, braces and struts but then you wouldn't have much. The vast majority of historic barns I have in my area have no ridge beam or even ridge board, the rafters are butt cut and spiked, working just fine. They all have bracing.

I understand what the struts are doing, but dropping the king seem odd. I am struggling to understand this. Are there examples of trusses with out kings, only the struts? How does this work? Especially in the future when a floor load is applied.

Tim

Tim - why build a truss if it is not going to supprt the roof? Just to hold up the tie? You've suggested ditching the ridge - in which case if you are not using a principle rafter or purlin system and there is no loft you could just as easily control the tension with a very minimal tie.

I'm sure barns without a ridge or purlins or ridgeboard exist. Every barn that I've surveyed (PA, MD, NJ, and OH) has either.

Have you set rafter pairs that were butted and spiked for a 24' span? I'm curious how that goes. (Serious question, as I think the ridge (or purlins) add an ease of assembly function as well as rigidity and bracing.

So - ditch all the timberwork in the roof system. Stick frame it. Design the tie to post connection to take the outward thrust. Hope that the metal roof and latticework offers enough bracing in an open structure against the wind.

Could work just fine. The OP will need to do his homework. In my neighborhood - I wouldn't do it the way you propose. And the OP is seeking to timberframe after all...

Hi Tim,

The way I saw it there were three ties side to side(24'), two ties(?) front to back (24'), and then the ridge (24')... six total.

4x6 is no problem... since they are going into an 8x8 can I just cut a 4" mortise and put the whole piece in without cutting a tenon?

Ben

Ben - study up on how you will want to do the layout (differing systems) and how a brace works. You'll most likely do a housing and tenon. On 4x material this will usually be 2" and 2". If you have capacity to easily cut 2" mortises (the tenon is the easy part) then go for it. For me, I'd either be drilling and cleaning up or running a chain mortiser. Both tools that I have access to are more efficient @ 1.5" - so personally I'd design my joinery for the loads I anticipate and the tools I have at my disposal. YMMV (your mileage may vary)

TRUSS. A network of timbers forming a rigid support structure;
ideally, all members of the truss behave in either compression or
tension, none in bending. Trusses are used to span distances imprac-
tical for solid members, or to support unusual loads.

I think I tried this before. As per the above description a "truss" supports unusual loads. I don't see a roof as an unusual load. A 24' span with no support under it is unusual. So in my view the truss is only supporting the 24' span. I am only suggesting a truss within and under the roof to support the 24' span, not the roof.

So, yes, to hold up the tie.

If I was to build a building in the summer and not take into account the snow load of winter I would be making a mistake. Why not take in account the very possible load which will come in the near future, the floor load placed on a 24' clear span. Build for a "minimal" tie? Sure a cable would do the job and lessen the chance of a floor being built.

And again I would look at page 170 of the above listed book if I was the OP. I hope this all is part of his home work and he should not rush it.

I am trying to make the majority of my bracing as you say, Mike, 3x stock and 1-1/2" mortice and tenon.

I am adding onto my house this fall and will be butting and screwing the rafters, tapered spruce poles. The picture of the truss I posted did not have a ridge and the common rafters are tongue and fork, no ridge beam and they when up nice and easy.

Tim

Tim - but you can eliminate the spreading by simply allowing the ridge to sit on the kingpost, there is nothing unusual about it. I think it is unusual to be building a truss to hold up a potential future floor load and then not tying it to the roof. You are 95% of the way to holding up the roof and eliminating the need for tension joinery in the tie by having the KP hold up the ridge.

The roof is not unusual. 24' is not long or unusual either.

The tie only needs to hold up itself - if there is to be a second floor then we need to hold up the tie and the floor.



I think I'll stop and let someone else comment. We seem to be lost in translation.

Me too.

I have constructed just such a building as you have described for the purposes you have described, I am on the same page and understand fully, just wanting to point out there are numerous directions to approach this situation. It is up to Ben at this point. I surely hope this discussion does not deter him from further posting on his project.

Tim

Tim -

Doesn't the KP truss without attachment to the roof / ridge / principle rafters just look 'wrong' or incomplete? Do you just lop off the KP and let it end or just float somewhere up there? Or cut to pitch to snug under the sheathing? or do a pair of commons fit over the top?

And yes, you do not need the KP in a truss. You can make a perfectly working truss with minimum 3 pieces - 2 rafters / struts and a tie to hold them together. It might make for interesting joinery at the 'peak'... depending on loading.

Or flip it over and have a tie, post, and metal for a bowstring, probably the 2 simplest trusses (in terms of piece count) to build.

That three piece truss is limited, is it not?

There is a picture of a king post truss which does not go all the way to the tie, in Egypt some where, a 20' span.

I suppose one could end the king anyway you want, I ran it to the peak with the slopes cut on it and the rafters land on either side. I have a picture here somewhere. I don't have the time at the moment to look for it.

I thought we were going to drop this?

I am working on a queen rod truss, it is in the design stage now. I will give details later if anyone is interested.

Tim

No need to look for a pic. Just trying to figure out what you were building.

Hi Tim,

It was a picture on the front cover of an old Timber Framing magazine that featured the king post roof that you mention but you must take care in interpreting what you see because if that roof is as old as Lynn T Coutenay's article eludes than major alterations should be anticipated.

Regards

Ken Hume

This is it.

http://www.tfguild.org/publications/kingposttrussTF72.pdf

What alterations are you aware of, Ken?

Tim

Tim - are you talking about 2 different things? Not going to the tie or not going to the ridge or roof? I am really having a hard time following you in this thread.

I was following that we were discussing a truss that the KP did not tie into the roof system - as in the image you originally posted and the description you gave of struts to the KP and then commons to either side of the truss.

As to the PDF - Which page? Image?
I see KP trusses with principle rafters or struts, sometimes both. With either a ridge or common purlins.

If it is lower left on page 17... that idea should still work. If you do not load the tie (with a loft or by hanging old Buicks from it) the KP shouldn't go into tension. I'd be curious to know how that truss works should the rafters go into bending, as that will push down on the KP... but it will still be in tension from the top of the triangle and simply not push on the tie (but scroll a few posts or threads over for Ken's concerns with dynamic / wind loading and bracing along the building's length, as bracing may influence how the KP (or QPs) bring loads to the tie)

Note that from what I can see even that example is tied into the roof system - principle rafters supporting common purlins that I assume run from truss to truss. Which is what I was asking about in reference to your building - trying to understand why you did what you did.

Interesting article - thanks for the trip down memory lane.

Unless I'm mistaken, and correct me if I'm wrong, but I thought the purpose of a king post was to hold up the tie, as it is hung between two rafters.

If a king timber doesn't reach down to the tie as shown in the drawing on page 17 (of that pdf listed above) then it is not a king post, but a king pendant (not to add more mud to the water). But a king pendant only helps hold the roof truss strong, if I understand the king pendant's function correctly.

I have seen king posts that don't reach the ridge:





This was at the Hancock Shaker village in Western, MA in a building being currently used as a woodworking shop. However, it was my understanding that the original use of the building was a leather shop. And that the second floor, where this post is, was where a lot of leather was stored.

In the background you can see a wheel that had a drum as an axle used to lift heavy loads through a large hatch in the floor. These heavy loads (of leather?) were stored on this second floor of this building.

I believe this post as added after the fact to create a truss to hold up the floor system, but again if I'm wrong about this, please correct me.

My point is that there are several different ways to use (king) posts to create trusses for supporting either the roof or the floor systems.

Jim Rogers

I agree. I was in the mud of language and confused about which direction Tim was describing.

That piece looks like a retrofit for sure. Cool though. Appears to be half lapped over the collar tie? And I assume the struts push down on a tie beam at floor level?


I guess I think it looks 'incomplete' when lopped off without interfacing with the balance of the structure. For a retrofit it doesn't make much difference - but if one is already running struts or principle rafters... why not let the KP span from tie to peak, assuming there aren't other compelling reasons to chop it short (either on top, or below)?

The king pendant is an interesting idea. It does offer the option of stiffening the rafters...

I'll be at the shaker village on the 25th and I'll see if I can get some more detailed pictures of this setup in the loft of this building.....

There are some different things going on in that building for sure:



Jim Rogers

STRUT. An axially loaded minor member in a truss or frame.

CHORD. In a truss, the major uppermost member (top chord) or
lowermost member (bottom chord). In a roof truss, the principal
rafters serve as top chords, the tie beam as bottom chord.

A couple more definitions for more clarity.

Mike you are calling the top cords struts. The struts are minor members, most likely supporting the rafters.

I think the confusion is in the definition, I am speaking of a truss not a roof truss, they are two different things, but could be one in the same.

I happen to believe that a 24' span of any member is too much, uncommon. In the beginning and through this whole process I have been sticking to that. The truss I proposed was only to support the center bent, allowing a clear span which the OP stated in the beginning as a concern, the first and third bents will most likely have walls under them, and no need of a truss. I further, do not see the need to build a roof truss for a 24' wide building. It is the economics of construction which lead me to that conclusion.

I was not the one who brought up leaving the king out. So I thought I would find an example of a truss where the king was compromised which is found in the portable document format which Mo and now I have posted in the same thread. It is there in a monastery, St Catherine's at Mt Sinai in Egypt (which Ken has pointed out has some concerns).

Furthermore, I would like to comment that a quick perusal of the pdf shows no truss with a span between members of more than 20' and unsupported from above, leaving 24' uncommon.

Tim

Hi Tim,

I tend to agree with your concerns about span and my experience is that when building widths go over 20 feet or so then additional support timbers are added generally in the form of aisles. I have seen buildings with roof trusses that clear span up to 68 feet but these are quite a rarity. Since this forum is providing help mainly to beginners then advice given on large spans is probably beyond the scope of this forum.

The St. Catherines monastry roof looks very odd to me. I have not seen any other roof that features these dangling king pendants / posts and given the great age it would be prudent to examine the whole building to establish what, if any, major modifications have been made to the timberwork in this building over the centuries (or is that millenia). Generally not many timber buildings survive beyond 6 - 700 years though recently I saw a TV programme that showed a Norse building in the Faroe islands that was over 1000 years old and this had been brought to these islands in "ready to assemble" kit form from Norway by the Vikings in their longships.

Regards

Ken Hume

I find it odd that we are concerned about designing a truss to hold up a 24' timber.

Yes, you can build a truss to hold up a 24' tie. But your argument of efficient building fails me here - build a truss to help hold up a single timber and then fail to allow it to do any other work in the structure? Without a loft or second floor in the design, the tie really needs to contain the spreading from the outward thrust generated by the rafters. Lower the pitch and this becomes a larger problem. Using that 'simple' truss that you designed solely to hold up the self weight of the tie... to do some real work up top... that to me makes more sense.

My point was that you are almost to the roof, and with little effort can solve the tension joinery required by allowing 24' of roof to push out on top plates and posts.

Strut, chord, correct.
A principle rafter truss like many in the PDF - the rafters are the top chords and internals are called upper chords. Anything internal to the rafters I would call a strut - mainly as that is what folks I've worked with / learned from would call them - and if they are not being used as principle rafters I would still call them struts - but your by the book definition works too. I have a hard time calling compression members 'chords'. Apologies for the confusion.

And note, from historical examples - aside from Jim's pictures of a retrofit - I think the common use of a truss is to hold floors and roofs. I'd love to see more examples of trusses that just hold up themselves. Those PDF examples are all integrated with the roof - mainly being principle rafter / chord systems with some arrangement of common purlins / rafters - much like mill and industrial trusses that I've seen in large masonry buildings from the turn of the last century and before that employ trusses on reasonable spacing holding up solid decking or common purlins and roof decking.

The only time I see trusses that are only holding themselves up (no structural ridge, no purlins, and no floor) is on new work, never old. I always assume that the designer either didn't understand what trusses are for or (more likely) he/she just wanted some eye candy. (which isn't necessarily a bad thing)

I see this most with architect-designed timber frames. (I'm not trying to start anything with that statement, just making an observation)

Hi Gabel,

I quite understand your experienced based observations however it is worth noting that the practice of sitting rafter couples (with and without collars), scissor braced trusses, principal trusses, tie beams or similar on wall plates without the aid of purlins, ridges, windbraces or the like is actually a very old practice nowadays referred to as "sans purlin". This practice can be found in very early buildings (11-1200's) through to about 1450. This type of roof is stabilised mainly by the use of masonary gables or hips and roof lath. It amazes me that this type of roof still survives but this simple approach to roof construction appears to be seriously long lived !

Regards

Ken Hume

Ken -

What holds up the sheathing / lathe / roof surface? Commons in between that sit on plates and butt or half lap at the peak?

Or are the 'trusses' packed together tightly to let the sheathing span?

Would love to see some pics or drawings.

Ken,

I, too, would love to see such a roof. I assume the trusses/rafters/etc are closely spaced, much like our modern conventional truss roofs?

Hi Mike & Gabel,

You can download a 3D panoramic photo rendition of such a roof from :-

http://cid-a449afe501ee8113.skydrive.live.com/self.aspx/Movies/Bishops%20Camera%20Roof.mov

This does not play like a regular movie but instead use your mouse to navigate inside the roof space.

The scissor couple spacing is 18" on centres but the gap between same is much less at about 10 - 12".

Some of the lower timberwork here is dated early 1300's but the upper scissor braced couples are a bit later at about 1375 possibly indicating a rebuild of the roof. Originally there was no ceiling at tie beam level and the roof was hence open to the camera below with only 2 tie beams crossing between the masonary walls.

Regards

Ken Hume

Ken -

I had to click the 'filmstrip' image and then save. I also had to manually change the name of the file after I downloaded - there were underscores before the file name and after the '.mov' which had to be removed.

Very very cool Quicktime VR photo.


That is a ton of work to create that roof!
And it looks like there are some spreading problems - I see steel rod and some extra bracing in that photo.

Hi Mike,

You can be forgiven for thinking that there is a forest of wood in that roof but as I recall when I calculated the volume of timber there was only about 750 cu ft used to cut the whole roof which has an internal span of 20 feet, external span of 26 ft and length of about 56 feet. This roof has double wall plates with a corniced flying plate and ashlars. The timbers are so dense that it's almost impossible to see the rafter peaks.

The main reason for the movement in the roof is probably down to the insertion of a 3 flue chimney which cut through the wall plates on one side and hence also through some of the scissor couples. In due course the chimney was removed and the roof yielded. This roof is in a castle and it has a number of other roofs intersecting and leaning upon it which also add to the deformation and yet at the same time provide a little more longitudinal stability. The roof was originally plastered over the 7 internal cants and presumeably this must have started to fall off centuries ago due to the various deformations. The original barrel valuted ceiling was then torn down and a new suspended ceiling inserted at tie beam height. This removed more than half of the longitudinal stiffening for the roof since the external tile battens are set at 4" spacing whereas internal lath is probably set at about 2" spacing. The metal tie rods are interesting in that since each of the scissor couples are independant i.e. no purlins then the metal ties will tend to provide only localised point load type pulling on the rafters and then rely on the external tile lath to transfer this restraint to the rest of the roof.

Inspite of all this abuse the roof still survives as a very early example of the kind of roof under discussion.

Regards

Ken Hume

Hi Mike,

Re file download :-

I should have referenced the following link :-

http://cid-a449afe501ee8113.skydrive.live.com/self.aspx/Movies/Bishops%20Camera%20Roof.mov

Regards

Ken Hume

Hi Guys,
Been on vacation the last week, but just wanted to pop in and let you know I'm still here. I'm enjoying the conversation and learning as I go.
I'm going to have to put this garage on the back burner for a while (which will give me more time to consider how to frame it anyway) since the fuel delivery guy somehow set fire to my diesel and gas tanks today and burned down their shed. Thank the Lord the tanks did not explode and no one was hurt. Now I have to rebuild that also.
I am also trying to learn a bit more about the Google Sketchup.
However, I will go ahead and cut my beams and get them ready. I think the consensus was that 8x8's would probably work, with maybe the 24footers being 8x10's.
Later
Ben

you could easily replace the 24' plates with sliced 12 and 14' members if you are trying to use timbers you already have available. however, i have never seen a spliced ridge so that would still leave you with 4 24'. im jumping in late in the discussion but ive read most of the thread and i would have to agree that the kp is necessary to carry a 24' span. also some were talking about removing the ridge beam but the only cases that i deem ridge beams unnecessary have principle rafters at the roof level. removing the ridge beam would also leave the tops of the trusses suspended in mid air and put rotational stress on the main ties.