I have a new project involving a Hip roof. I must admit that this is an area which I have never ventured.
The structure is 24 x 24 with one corner cut off where the entrance will be. The roof pitch will be 6:12 and equal on all sides. The client prefers that there are no posts in the floor area.
If anyone has a concept they may want to share or a picture of something they came across it would be appreciated. I have not seen any good examples from which a concept could be adapted.

Thanks in advance for any help.
Thane

What do you mean by:

Corner cut off? Does the roof still cover this area, but it is 'exterior' space?

No posts in the floor area? Does that mean 24' clear span from wall to wall?


Initially - without knowing snow, wind, live loading - I picture 2 king post trusses along the hip lines. 1 truss can be 'complete' and flown in as such. The other fits into the sides of the first truss, tying through the king post. Or you can do 4 triangles that meet in the middle...

Or one could do 'drag on' beams, or maybe a boss pin and tension ring top plates...

from a building in montreal that i visited:



random images linked from the web
small:




large:

Um, there is some good food for thought.

"Or one could do 'drag on' beams, or maybe a boss pin"

I like that idea. With a square wall plan and an equal slope around you are building a "pyramid" on top of the walls.

I think Mike's second picture(with the addition of more jack rafters of course) would be the way to go.

Mike, good looking roofs.

Those are awesome pics Mike.
I will propose a plan with a full roof but missing a corner post.

Pic #3 looks very suspicious, I like the next one though, nice direct joints in tension, heavy splines to take the remarkably high horizontal forces resulting from a low pitch roof.

I've never been a big fan of curved timbers in bending or tension, not natural. They are pretty but I lean toward simplicity, form follows function. However the second to the last is very sweet! The reverse shoulder cut on the "spring loaded" curved ties...that's nice... the only reason not to like that one is I didn't think of it!

I always have to ask myself "Why, Why do they have to take the corner post out?" Or in many other examples I find myself asking why over and over for other reasons. Yeah, there is likely a solution to the corner post being removed, but don't we have enough complication in our normal life.

I don't see any examples of tie's at the plate level running from the corners, this puts a timber at 34', not all that bad. The king pin comes all the way to the tie, forming the truss. Most of the pictures look like smaller structures, with the exception of #3, suspicious to say the least. Also the roof slopes appear to be steeper than the proposed 6/12.

Tim

Folks, FYI, I with the exception of 1 and 2 I snagged those from the web.

#3 is from Trillium Dell, a respected group of framers. It is a barn restoration.

Here is more information:
http://www.trilliumdell.com/barn-restoration-barns-for-sale

i'm sure i'd break something if we analyzed it for snow / wind... but its a start.

Handsome!

However, my intuition tells me there will be a LOT of tension in those crossing collars.

You could reduce this somewhat by adding a post or strut between the dragon beam and the hip, as far inboard as you can get it.

What have you got for analysis tools? This discussion could turn into a very interesting 'process' article for Timber Framing.

Mike, that frame looks a lot like the frame in Rick Collins & Co.s barn in Pic #3, the one I described as suspicious. It's not entirely clear (to me) how the horizontal forces are resolved... apparently echoed by our fearless leader. Of course much of the worry can be relieved if you locate the barn somewhere that it never snows... of course a little steel can go a long way...

Far far from fearless.

I've seen a bunch of detailed pix from the TDTW barn, but confess I did not pay enough attention to the tension.

Rick and Matt at TDTW are very generous with what they know.

Let's inquire.

Thane, Whats the reason for the missing corner? In my opinion, strong looking corners are stronger.

Mike, you are quick with the program!

I was just messing around with axis last night. However, I realized just before sharing that it is 12/12 not 6/12. Again in my opinion, 6/12 looks too squatty. Is that a word?



How bout buttressing with a shed roof around the entire plan if need be?

looks like steel bands around the exterior corners. and might be a tension connection @ the horizontal beam across the corners. and they have a metal roof. looks like the barn is located in aledo, illinois. doubt they have the snow load we have in NH or VT out there. 42' clear span... much larger than we are talking for Thane's project.


joel - yes, i'd need to work out the tension / etc.
just tossed it out there as a 'what if'. i can run a 2d FEA on it to see what happens. typically i only rough these things out and let the pros handle the final number crunching. i'm dangerous enough...

i'd tensile tie the corners. on smaller projects i've done essentially a king post type of joint, all threaded through from corner to corner. the post up from the 'drag on' is a nice addition. we did that on the 2nd picture i posted - but that was a tiny tiny room.

paul - metal roof, some thoughtful steel detailing... could get you most of the way there.

Mike, the steel banding in the image will help but I think is sitll inadequate, the fasteners look thick and in line and the band doesn't extend far enough... but this is all seat of the pants conjecture. My confidence in Trillium far exceeds my capacity to analyze a frame visually from a couple of photographs. I am curious however what it is you feel the metal roof does to alleviate matters? I have tried that argument with engineers claiming snow loads are minimal because the snow slides off. But a little voice in my head knows its all BS, what if in the future the roof is shingled, what about those very heave snows that pile up before the roof warms the underside enough to allow the snow to slide off. After all we frequently claim a metal roof is the preferred way to go because it is a cold roof. Hmmm, actually this is a barn so perhaps it is cold below anyway, low pitch, no heat, snow could pile up...

Thane, Is this what you are thinking about for your missing corner. 5 hips, this would be fun.



some of those them there dang metal straps on the corners for the "octagon" hips meeting the plates.

I think he is thinking of a square frame, the clipped corner might actually be an exterior wall that aligns itself with one of the diagonal corner ties that our humble, poker faced leader was concerned about. This I think is where the dragon might be employed (simultaneously alleviating our "jobless recovery").

Mike, I like this concept a lot. It even has the corners cut off.
Of course I would change it a bit to make it my own... sort of.
I will calculate beam sizes to withstand 50 psf combined load.
I'm also changing the roof pitch to 8:12. They will learn to like it.

Thane

paul, metal roof allows you to claim 'slippery surface' as a deduct when running the numbers. is it BS, possibly.

yes, it could be shingled.
i agree, the straps look minimal.
but the images are tough to read.
not trying to debate it, as i have lots of respect for TD and simply stumbled on the barn when searching for images for thane.

here's what my software tells me.
12 pitch, cold roof, rough surface, unheated structure.
generic span of 28 ft with 10 ft between trusses.
15 pounds dead load, 50 total. (and a bunch of other stuff 'standard' that i only change when needed)

15960 point load @ peak.
total roof load climbs to 57 psf

same load using 'slippery' for roof surface:
9545 point load @ peak
total roof load drops to 34 psf.

so, it can change things quite a bit.
obviously, higher pitch, the better!

dropping the pitch to 8 we get:
rough: 57 psf
slippery: 43 psf
this doesn't take into account unbalanced loads

thane, have at it!
8 is better than 6.
i'm always weary of how flat hipped roofs can look...

Well, here is where I'm at so far.
The mat. is white Pine except for braces, splines and pegs which will be White Oak.
The lower Ties(8x10) will be half lapped into a large brace extending from plate to plate(8x10). Have not figured out how to connect the corners of the plates yet. Currently trying to come up with a simpler way to connect the common rafters(6x8) to the principle rafters(8x8). The compound angle M&T joinery will be time consuming for sure and I'm not sure if the client will appreciate all the unseen labor. Any suggestions to how I could fasten at the upper ends. I will Birds-mouth the bottom onto the plate.
I'm also looking for some opinions on the Truss design. Things are easy to change at this point.

Hi Thane,

A good way to connect the jack to hip is with a housing and GRK screw. The housing must have a bearing surface below though. With the "jack rafter" being 8" in depth and the "hip rafter" being the same, the plumb cut of the jack rafter is going to fall below the bottom of the hip.....

I would also move the abutment between Hip and Lower chord inward instead of at the end. like this:

Good idea Mo. That will give better Relish. Will M&T be required here or will large screws work just as well.
Why the GRK screw. Never heard of them before.
I will reduce the Jacks at the ends to 6 in.

Thane

Thane -

M&T at that location may remove too much material. I usually do the girt shoulder joint with lags. You need to figure out how much relish you need, and then you run in lags from the top.

As to jack rafters... do not M&T them. Housing of appropriate depth with panel screws or lags from the top. You can do an adzed or band saw reduction curve to bring the jacks down to size so they fit in the hips.

And - you may not need to push the hips all the way up to a fine point. A 1" or slightly larger flat on the top works just as well. If the jacks are properly sized, you can always up the size of the hips to make life a bit easier.


Here are the screws I like:
http://www.ozarkloghomes.com/loghogscrew.html

Some food for thought: I do not like to reduce the depth to fit the joining member. Reason being, is that I am under the philosphy that your timber is only as strong as its weakest section. I wouldn't want to create a possible shear point there. I would just upsize accordingly.

I recently had to wrap my head around something like you are doing now. It was a 24' X 24' 4 hip structure with a 3/4 pitch instead of your 1/3 pitch. It was a doozy. One thing that I learned though from it and recommend to you is that you find your longest rafter (excluding principals) and determine what cross section that needs to be and work back to the principles from there. A 24' triangular roof plane is a lot to fill in.

I'd be happy to show you an easy way to find the angles for the top of your jack rafters, as well as the backing cut of your hip, when you get to that if you like

In a simply supported, uniformly loaded beam, shear requires a significantly smaller section than bending. Therefore a truly "form following function" timber in this condition would look like a whale or for those familiar with Monty Python: the "theory of the Brontosaurus".

Paul, You lost me! Does that mean it is ok to reduce at the joint between hip and jack? Can you explain further please? Thanks.

"My theory by A. Elk, brackets, Miss, brackets. This theory goes as follows and begins now. All brontosauruses are thin at one end, much much thicker in the middle, and the thin again at the far end. That is my theory, it is mine, and it belongs to me, and I own it, and what it is, too."

EUREKA! This makes things a little easier. Learn something new everyday. Thank you.

P.S You know my father told me something once, he said, "You are not stupid if you don't know something, its just that you have not yet learned otherwise."

It is my understanding that a timber needs more size in the middle section. It can be reduced at the ends where, as Paul points out shear could require a smaller section. So, an adzed reduction would be pretty sexy. Brontosaurus went extent a while back.

More screws
http://www.grkfasteners.com/en/RSS_1_2_information.htm

Tim

Does anyone supply Adze handles? I have a good head for one which I have never used. Another option would be a drawing of one showing length and shape. I will have some time to practice once the timbers are ordered.
Also, it would be nice to make some braces with curves.

Thane

Finished design concept for client approval.
posts- 8x10
plates- 8x10
hip rafter- 8x8
jack rafter- 6x8
collar tie- 8x8 & 8x10
corner brace- 8x10
king pin- 12x12
queen pin- 8x8
Oak braces & Splines
8:12 pitch roof

Roof decking- 1.5x5 t&g pine
stone wall around perimeter installed after frame.

Wow! That is a thing of beauty, I like it.

Engineering fee will be high:

Check for uplift at the outside corners
Check the size of the corner crossing ties
Heavy load at the ends of the crossing ties
Check shear in the through tenons

phew...that gets complicated, its sort of like a hammerbeam truss except there's no buttress or big ass post to anchor the outside.... on the other hand you could argue a well fastened roof diaphragm could do much for this frame...

but its very creative...looks great!

Paul, thanks for response.
The corner crossing ties are 8x10 and resting on the posts.

Thane - I think the posts should move closer to the corners, and the plates will need to go up in size. Those braces need to get out near the corners. Lots of shear going on in those plates that are cantilevered. That corner joint will be interesting!


I would definitely get an engineer involved when the client says 'Go'.

Shear plane in the roof will help hold things together - but if there are no walls, uplift will still be a concern.

Another option would be to add a center post on each wall and make your trusses span from center to center (getting them into the plane of the roof and out of the corners). Then your truss bearing locations will be over posts, and your hips will drop into corners. With this method the corners only need to hold up the end of the hips (and their distributed load).

Thane: we've got handles for sale, drop me an email for details and photos.
jrsawmill (at) verizon (dot) net

Thanks Jim but I made an Adze handle yesterday.
Now I need to learn how to use it.

Thane

Wow, people sure can come up with some great things if they put their heads together and help each other out!

Looks like you already designed the jacks with a clip. Here are some pictures of the same situation. A little bit different with two different slopes intersecting at the hip (offset backing cut (image 2)) and no overhang. Other than that they could look a lot like these at the top. Drop em in and screw em.

yea, that's me always a day late.....

All is good! Client approved, Engineer approved(beefed up the corner crossing Ties and added steel strapping around the corners of the plates.)
Now just have to round up a few timbers... and a bunch of whittling.
Thane

Picked up some White Oak this week for braces.
Thought I would show my progress of making curved braces for my first time.
Stock size of brace is 4x8x60 for a 36" brace.

Step one: Planning.


Step 2 & 3: Layout & score. Yup... a chainsaw.




Step 4: Adze work


Step 5: Tenons and finishing touches.

they look great Thane.. great looking timber cart btw.

where in ontario are you? Do you have good supply on white oak? I'm looking for some 4x6's, no white oak around my neck of the woods..

Thanks. My friends tell me I should make wooden wheels for it... and then laugh. Nice guys.
.
I'm near London. Lots of White Oak around here. Try Townsend Lumber in Tillsonburg.
http://www.townsendlumber.com/
Or call 519-842-6015 Mac Troyer. Great Guy.

thanks for the info, I've chatted with Mac before looking for a monster 26'er..

wooden wheels would take it to that next level no doubt...lol
I'm sure that thing is handy as can be, especially if you're working solo...:)