Rafter design for Eaves

HI,

I'm new to the forum and am planning my first structure to be built this summer.

It will be a small (16x20) barn with no interior finish or insulation, clapboard siding, metal roof. Likely three bents if I can get a crane in there or find enough muscle.

I've been studying Steve Chappell's "A Timber Framer's Workshop". There's a lot of information in there but he doesn't get much beyond the frame, all his designs/examples have the rafters end at the walls. I would like 12-14" eaves. I'm GUESSING that extending the roof sheathing that far with out support won't work.

Suggestions? Also, suggestions for other good basic reference materials would be appreciated.

Thank,

Carl

I'd suggest a more traditional design. Jack Sobon's books feature a lot of traditional timber frame details. I especially like "Historic American Timber Joinery", which shows many examples of how to frame an overhang. It is available here from the TFG as a free download (http://www.tfguild.org/joinery/joinery.html) or in print form here as a booklet (http://tfguild.stores.yahoo.net/tifrbo.html) along with his other books.

Regards,

Creating a step lap rafter tail will give you overhang you want. As mentioned it is in the rafter joinery section of the Historic American Joinery series.

Or you could just add a sprocket, shown in the same section.

Thanks for the references, there're a big help.

Thought I should mention why I hope to use bent construction. For the most part I will be alone on this project. There will be a couple weekends with a lot of help but otherwise I'll be chiseling and sawing in the Adirondack boonies by myself. My thinking is that I can get the three bents jointed and assembled on my own and then when help arrives use those "resources" to get the frame erected. Once it's up I hope to be able to do the rest on my own. Make sense??

Looks like the rafter to post joints with be either, housed birdsmouth rafter with through tail (#7 illustration), or step-lap (#10 illustration) from the Graphic Guide. The tie beam in the bents will be joined to the posts with a wedged dovetail below the rafter and intermediate plate joints.

I have no idea yet for the size or number for rafters of the 8/12 pitch roof. Besides the three rafters in the bents I'm thinking two more between each bent for a total of seven (7). I'm thinking collar ties and queen posts in the bents and just collar ties on the extra rafters. What do you think, is this overbuild/under-built?

Metal roof--I had planned on dovetailed purlins every 4 feet. I've been told metal roofs attach every 2 feet. Should I instead use a larger # of small purlins, continuous purlins in notched rafters, plywood, or planking under the metal?

That's enough questions for now, I'd appreciate any help,

Carl

I think you should stop what your doing and read several more books on Timber Framing before you cut anything do any joining. I don,t mean to sound harsh but that's what I think based on your questions. Start small with you first frame like a wood shed or a garden shed. You will then look though different eyes on your second frame.
Just my $0.02

Thane

I would use continuous top plates, skip the CG's which is how I read your post. By using the continuous top plate your rafters are independent of the post, simplifying the work and raising.

Also note/read that #7 has a tricky aspect which can be overcome by using the rafters in #9 and alluded to by Jim Rogers, I believe.

Rafter spacing on a 20' building and only 16' wide you could use 2' but I would fall on the 4' side, nice, neat and half the work. Full 1" boards and roof top guard with the metal on next. No purlins needed. That is what I am doing and commencing cutting in the morning.

If you use collar beams will you have enough head clearance on a narrow building, have you drawn it yet? Might as well use the space. How far below the eve line are you planning on placing the tie beam?

I don't see this as all that big of a frame, a nice starter to learn on. Now you have $.04 worth in debt notes, not silver or gold, btw.

Tim

Carl,

Your use of purlins on 4' centers implies that you're using principal rafters. And principal rafters and step lap rafter seats don't often work out well, because you have to take too much meat out of the plate.

Like Tim said I would stick with continuous top plates, common rafters, and nail 2x4 purlins over the rafters, metal roofing over that. I like the purlins at 20" OC because it's a little easier to crawl around the roof.

If you want to use clapboard siding, you may want to consider a Dutch frame, with smaller bents spaced closer together. That way, you don't need any studs, just claps right on the posts. Or use girts if you want vertical siding.

I have plans for a little 3 bent 16x20 if you're interested. I could send you a picture as well.

Brad

Question:

How much stiffness does a metal roof alone add to a surface? In the case of 2x4 purlins atop common rafters, will the metal roofing adequately prevent racking? Or, should some additional wind bracing be used?

I ask the question because I'm putting on a roof similar to what Brad describes. The utilitarian buildings that I've looked at with metal roofing on 2x purlins don't seem to have wind bracing, so I've assumed that the attached metal skin will stiffen it up on its own. Is this a correct assumption?

Kevin, the ordinary metal roofing is not a means to a strong diaphram. The roofing is weak in structural connection and is easily crumpled and torn. Wind bracing is required for better assurance of stability. Compare the fastening schedules for roofing and plywood decking.

Roger,

Thanks. While I understand that it is by no means a substitute for plywood, diagonal planking, etc., I was curious to what incremental amount metal roofing might bring to the structure's rigidity.

I know this isn't the answer you are looking for, how much force does it take to buckle sheet steel? Not much, it would depend on gauge, fastening schedule, scheme and the specfic product. Call a supplier's tech rep. Ask youself what does failure look like, how rapid and how well can you predict and avert failure during a storm? Lumber truss roof pole barns require bracing across the webbing and so should other forms.

The metal skin on a modern post frame building is a quantifiable diaphragm and does contribute greatly to the building's lateral load resistance. The truss bracing is to prevent buckling of the deep, thin trusses. In testing a braced but unskinned 40' post framed building was laterally loaded to design load. Midspan of the building length the plate bowed out 6". Going from memory when they then applied the metal skin the deflection was under an inch. This is all part of design that Roger mentions above.

The upcoming frame builders expo, the post frame industry conference, will be in Louisville Feb 17-19. Dr Harvey Manbeck will be presenting his senior class lectures in post frame construction including the engineering behind the diaphragm models they use. Dr Bonhoff who modelled the ground/post interactions will also be presenting as will others. These guys wrote the book and they're offering a crash course.
http://www.nfba.org/i4a/pages/index.cfm?pageid=1
http://www.nfba.org/files/conf/2010brochure.pdf

Dr Manbeck wrote a program that has been available on the NFBA website, DAFI, Diaphragm and Frame Interaction. You'll need to be an old school computer geek and engineer to use it. He has been working on a more user friendly version. I attended one of his presentations about a year ago and had plenty to think about afterward.

Do I sound bummed that its been a slow year, maybe next time.

Don, the bracing in the web addresses other critical concerns, gable wall and post wind loads are transfered to the side walls and the whole roof frame with purlins is structured into a skeletal unit. The use of steel cladding for structure is a brittle affair due to lack of stiffness, so take care...

This is a link to the referenced documents the post frame industry uses as the basis for diaphragm design;
http://bse.wisc.edu/bohnhoff/Publications/Copyrighted/EP484.2%20PDF%20File1.pdf
http://bse.wisc.edu/bohnhoff/Publications/Copyrighted/558.pdf

I found the graph of the test I described above

Don,

Thanks. That's exactly the kind of info I was looking for.

So, we can build a timber framed building without the bracing and fasten 2x4 horizontally to the post, screw on sheet metal and have a stabilized building?

Tim

... essentially, diaphragm construction is another way of bracing a building, you can use architectural steel panels to form a diaphragm.
Is that different than a sheet of ply or a sip?

Kevin,
I would like to take this opportunity to apologize for my previous post. Although I meant only to be constructive, it has come clear to me know that I was hasty. After reading some very positive responses from other Guild members I began to feel very ashamed of myself. Kevin, I'm sure you are very capable in what you are planing to do.
To all other Guild members, I hope that I did not cause disrespect to this forum.

Regards
Thane

Hi Thane,

I came into this thread somewhere in the middle with a question about the diaphragm effect of a metal roof. Is it possible that I'm being confused with someone else?

Best,

Yes, (face red) The appology was meant for Carlmill.

you should definitely design your structure, or any structure for that matter, with at least a 2 foot overhand at all sides. Unfortunately, people frame things with a small (16 inch or less) overhang. Sure it costs more, but the price far our weighs the protection your building will get from the elements as well as provide a great area to get artistic with exposed rafter tails and nice stained pine soffits. If the design you are trying to use as a reference does not have good overhangs, either redesign the frame, or use 2x4 sleepers across the entire roof system to get additional framing for it. Cheers!

Tim,

Continuous Top Plate- This would seem to make the rafter design and joinery easier. I'd be concerned about the availability of 20' timbers in the Adirondacks and also the difficulty of lifting them into place.

Top Guard- Is Top Guard the stuff designed to replace roofing felt?

Metal- I've been advised that metal will have to be mounted to furring strips rather than directly on a flat surface to allow "breathing" and minimize heat transfer. Is that a concern?

Collar Ties- They certainly would be head knockers though I wasn't originally planning on using the space within the rafters. I might change the pitch from 8/12 which matches the pitch of the nearby main camp building to 10/12.

Tie Beam Placement- Haven't worked that out, understand the some? space from the plate joinery is necessary. Posts will be along enough to give 8' clearance from the floor to tie beams

Thanks,

Carl

Brad,

Would appreciate any plans and pictures. Thanks also for the "vote" for continuous top plates.

Principal rafters and too much meat- Just read in a Bensen book that the for the top of a post cross section was three joints with no more than 50% involved.

20' timber should not be a problem, 40' could be but still possible.

Roof Top Guard is a brand, I believe, and yes it replaces tarred paper/felt. I am about to put some down in stead of tp, and a metal roof directly on top of that. I am after the additional mass of the roof surface to keep the metal from changing temp too quickly and easing the sweating problem. I don't see any problems with doing this, maybe someone will point out a reason.

Collar beams, even so, someone may one day, and what a bugger it would be.

Tim

Thane,

No apology is necessary. The rookie nature of my questions was evident. The good news is that I am not new to chisel and mallet having built a number of dressers, tables, etc. using traditional joinery. Though the scale is different, many of the principals are similar. I also have built two Rushton style small boats from the late 1800's which might not be relevant to timber frame construction except to illustrate my desire to use traditional techniques.

"read several more books" is a great suggestion, you might notice in my original post a request for reference materials. SInce then I have acquired and read the Benson and Gruber book besides the Chappell book. Any other suggestions? I'm not too worried about the joints which are, I undertand, central to this form of construction; right now I'm more concerned about sizing and spacing of the components, deciding which tools I HAVE to have, deciding whether to use local Amish milled Hemlock or second growth Douglas Fir sold by a lumber suppler, etc.

"Start small" - The idea of a "training structure" makes sense, though age and approaching infirmity probably mean this barn will be my first and last timber frame structure (unless I can get some help).

In summary, I really do appreciate your "$0.02",

Carl

Thanks Carl,
Not sure how far along you are but have you considered a ridge beam and common rafter design? The ridge beam being simply a third plate with which to rest the top end of the rafters on. Just birds mouth both top and bottom ends of the rafters and lag them down. No notches are required in the plates as the outward thrust has been eliminated by the ridge beam. As for bracing, just brace as if it were a wall from the plate to the king post which sits atop of the tie beams.
Also, there is a Simple Beam Calculator which has been seen a few time on this forum and is very handy for these simple structures. I can send it to you if you don't already have it.

Thane

Carl,

20' plates shouldn't be a problem. I would be careful in selecting a mill though, as most only saw boards, and their mills often don't cut square. I would bet a good percentage of them don't even have a square in their mill. As far as raising them, do a search on the forum for parbuckling.

I can imagine which Benson joint you're referring to, and I'm not a big fan. Nor am I fan of some of the joints in the Chappell book. Buy a copy of the Historic American Timber Joinery book from the Guild, and also check out Sobon's books. They're both essential reading.

There's really not a lot of good reasons to truck doug fir across the country, when you've got at least a dozen different species you could build with locally. You can even use a dozen different species in your little 16x20.

Unless you have very tall kneewalls, I think you can skip the ridge. You'd also be point loading the tie beams with 1/2 the roof load, so you'd have to beef up the tie beams a lot.

Thane,

A search for "Beam Calculator" shows only you recent post, please send it on to me when you can.

Carl

Carl,

Try this link:

http://www.forestryforum.com/members/donp/beamclcNDS2.htm

Ok, that's the one I was talking about.
Thanks Kevin

The link above will be using another forum's bandwidth. I was given that space to use and at least in my mind consider it for the benefit of their members. I'm sure it's no big deal but I don't want to abuse them, hope you all understand.

I have been putting the calcs on a site I set up here;
http://windyhilllogworks.com/Calcs/beamindex.html
The same calc is at the top of the list.
You can right click and "save as" and save them on your computer to run offline or modify if you want.

Don, thanks for pointing that out and providing the calculation information.

Tim

Thanks Don. That one is even more useful.

Here's an update:

I’ve been doing a lot of reading since I submitted the original question in this thread. Besides this esteemed group I’ve read Chappell, Bensen/Gruber, Sobon/Schroeder cover to cover. Compared t a lot of the work I’ve seen (and many of you have done) this is a relatively basic project; hopefully, well matched to my rookie status.

I’d like to review my plans from the ground up and encourage your critique. The basic concept is unchanged but the dimensions have been changed to 16x24 in four bents. I realize that this size could be done in three bents. Four bents will cost more in materials, take more labor but, as mentioned in a previous post, I’m trying to keep the pieces as manageable as possible. This barn will be used for firewood, tools , a small pickup and some heavy stuff like an old iron stove and a couple 5 hp hit/miss engines that I hope to restore.

The foundation will be 4” reinforced concrete floating on stone. There will be one course of block with a total of 20 “J” bolts and an 8’ wide opening in the block wall (for the door) at the gable end.

I’m planning a 1x8 pressure treated sill and a 6x8 timber sill all held in place by the “J” bolts. For the corner joints I will use either a tongue/fork joint locked by a 4” square tenon from the post or, a pegged mortise and tenon and a stub tenon on the bottom of the post. If I have to scarf the long timber sill I’ll use a pegged half lap at either a “J” bolt location or under a post with a 4’ tenon.

The posts will be 8x8 and be long enough to allow 7 ½’+ headroom under the tie beams and allow 14” distance between the tie beam joint and any post top joinery. There will be 4x4 horizontal girts dovetailed to the posts, one between each bent halfway between the sill and plate. On the end with the door I plan two 4x8 posts between the sill and the tie beam defining the door, at the opposite gable end there will be one centered post between sill and tie beam.

The tie beams are 8x8 and joined to the posts with a pegged/wedged half dovetail.

Still haven’t decided on continuous or intermediate plates. It seems no one had anything good to say about the latter. I understand the concerns about tensile strength and the need to not take too much “meat” from the top of the posts. If I can a find a trustworthy source of straight 24’ 8x8 beans and can figure a way to get them in place with minimal manpower (thanks for the info on Gin Posts and Parbuckling) I’ll go with continuous plates.

For the rafter design I’m considering seven (7) 4x6 rafters with pegged tongue/fork joints at the peak. At the plate I will use a step lap joint with 2x4 tails. The tails will be elevated 1’ for strength and the resultant gap will be screened allowing for ventilation. I’m considering some dovetailed purlins especially if I end up with intermediate plates. I’m also considering collar ties 1/3 the way from the peak and queen posts on the gable end rafters to take some of the strain off the tie beams. Is this overkill?

Knee braces will be 30” 3x5.

The sides will have vertical 1” planking (T&G if I can afford/find it) and painted clapboard matching the main camp building. The roof will be metal also matching the main camp. I’m still not sure what to put under it but I suspect horizontal 1” T&G would work. I assume one can get a peak vent for metal roofing to work with the ventilation under the eaves.

I suspect that with small 8’ bents this design is probably overbuilt. Do you think the posts, tie beams or plates could be downsized? I’d appreciate an aggressive critique of any portion you consider faulty as I hope to have the pad in place and supplies/tools ordered by mid-April or as soon as the ice is out of the ground.

Thanks for the help; I’ll send pictures,

Carl

Carl,

Sounds like a good plan in general.

It depends on the species, grade, and load, but I don't think your tie beams are big enough. Realistically, they're probably not going to catastrophically fail (plenty of barns have been built with smaller timbers), but you might as well do it right.

The rafters also seem to be too small and too far apart (intuitively). With metal roofing, either use closer common rafters with 2x4 purlins over, or principal rafters with closer common purlins. It's a lot lot more work fully sheathing a roof, and it doesn't really gain you much with metal roofing (maybe with condensation, but it's easier to just open the doors up).

Lastly, you can use either regular boards, or T&G, but I often use shiplap.

Timberwrestler,

Thanks for taking the time to review my proposal.

Tie Beams- I realize that 8x8 is a bit light but "assumed" having 4 ties with wedged half dovetail joints and collar ties on the principal rafters to reduce spreading force would be adequate. I'll go to 8x10 for a little extra margin.

Rafters- You're clearly right on this. Looks like either heavier and/or more rafters.

Metal roof- 2x4 purlins on 2' centers makes a lot of sense, guess I'm having trouble with the look from the inside.

Carl