Shed roof attached to small cabin frame design

sheneman · February 01, 2007, 06:10:03 PM

Hello -

I am designing a 12x20 timber frame cabin with a shed roofed porch extension off of one side. The essential cabin design is from Scott Stevens at Grand Oaks Timber Framing, although I have modified it a bit.

My current working design uses principal rafters and purlins on the main cabin, while the shed roof extension uses common rafters at a shallower pitch than the 9:12 pitch on the main cabin.

In the initial CAD sketch-up of the cabin design, I attached the shed rafters directly to the 7x9 continuous top plate. However, I am not sure how to do the joinery there for the rafters. The best I could come up with was a kind of shouldered dovetail cut at an angle. This would avoid the need for pegging the rafters to the top plate, since I can't see how you would do that.

In these pictures, the principal rafters are 7x7, the purlins are 4x7, the shed roof common rafters are 4x6, and the top plate is 7x9.

Any ideas? It would be appreciated!

Thanks,
-Luke Sheneman

Jim_Rogers · February 01, 2007, 06:33:30 PM

The rafters of the main shed to the plate look ok, but you might move them a little higher so that the bird mouth cut is in the bottom 1/3 to 1/4 of the rafter. This will help prevent the rafters from splitting.

If you wanted to cheat you could attach a ledger beam to the side of the posts and rest the lean-to rafters on top of that. But again that would be cheating.

You could make a tenon on the end of the rafter and a horizontal mortise in the side of the plate. It would be different but could work.

Jim Rogers

sheneman · February 02, 2007, 02:41:47 AM

Thanks for the response, Jim. I really appreciate it.

Here is the kind of joint I think might work to attach the lean-to rafters to the continuous top plate. A kind of angled, shouldered dovetail.

Any thoughts?

Much appreciated!!

-Luke Sheneman

Jim_Rogers · February 02, 2007, 07:09:01 AM

This might work, but you have to understand what's going to happen when that dovetail tenon shrinks.
It will pull out of the plate....
And by cutting the plate upper surface your weakening the plate.
A tusk tenon and matching mortise would be better and maybe easier to cut
Where are you and what is the snow load in your area?

Jim Rogers

sheneman · February 02, 2007, 05:32:30 PM

Jim,

Do you mean something like this? The first picture shows three distinct joints that might work. Is the third one from the end the tusk tenon/mortise that you had in mind (shown more clearly in the second picture)?

Thanks,
-Luke

Quebecnewf · February 02, 2007, 06:18:09 PM

Great pics What type of software are you using

Quebecnewf

Jim_Rogers · February 02, 2007, 07:50:41 PM

The third one for sure.
You can't do the first one as the standard joint rule is that the mortise has to go with the grain not across it.
And you are just about right but you may consider lowering it to the center of either the plate or the center of the rafter. These centers are known as the central axis and there is very little stress going on there. No compression, no tension, this is the best place to remove wood or create tenons.
Great drawings.....

Jim Rogers

sheneman · February 02, 2007, 08:07:52 PM

Quebecnewf,

I am using Google SketchUp. You can download it from Google's website. The non-commercial version is 100% free, and I've found it both easy to use and very powerful.

-Luke

sheneman · February 02, 2007, 08:25:27 PM

Jim, okay thanks. You've helped me a ton with this. One last question I have is about the two ends of the plate. As you can see, there is a lot going on there, with the post mortise, the principal rafter joining, and the shed-roof common rafter mortise. Fortunately, the whole thing is above a post, but there is still a lot of wood removed. (see picture below)

First question: Is there too much meat cut out here? The picture below is a 7x9 plate. I could widen it if necessary...

Second Question: On the ends of the plates, the tusk mortise/tenon should generally look like this picture, right? Should one worry about latteral stresses with this kind of joint, since the joint is open on the end?

In general, anything I should consider doing differently on the ends?

Thanks again for all your help.

-Luke

Don P · February 02, 2007, 09:43:12 PM

This is a real rough sketch for an idea. The steeper the rafter the worse the slope of grain in the tennon. The sloped area above the tennon helps reinforce it. The bending forces within the beam are greatest at the extreme fiber, the uppermost and bottommost faces. The forces of tension on the bottom of centerline and compression above the centerline each diminish proportionally as you approach the center, or neutral, axis. The slope of the mortise roughly corresponds to the diminishing nead for wood towards the center. I left some flange at the top edge, it doesn't take much. Think I beam. Horizontal shear within the beam trying to split it under bending force does run greatest in the centerline plane, and accumulates at the ends... right where there's a whole lot gone. I think you should check vertical shear in the remaining section of the tennon bearing also. Not meaning to butt into a good subject, just some out of scale thoughts... carry on

Jim_Rogers · February 02, 2007, 10:03:42 PM

Luke:
The mortise on the end of the plate timber for the lean to rafter, shouldn't go all the way out to the end of the plate. The standard rule is to offset this mortise 2" in from the end. And make a smaller tenon so that there is some "relish" on the end of the plate.
This goes for the mortise that is for the post top tenon. This mortise doesn't go to the end of the plate either.
Also, the regular rafters coming down to the plate, don't need the standard "Fox Maple" center tenon, known as a cog. It can be omitted without any harm. You can peg the rafter to the plate down through the top if you feel the rafter may shift. Or cut a small housing into the inside surface of the plate for the heel of the rafter to set into. This will prevent any side slip in the rafter.
Remember the rule stated above, all mortises must go with the grain of the piece. Your mortise is vertical and this is wrong, in my opinion.

Yes, Don-P has corrected me. It's called the neutral axis..... I must have had a mental hick-up....

Jim Rogers

sheneman · February 03, 2007, 04:11:31 AM

Thanks for all of the help Jim and Don. I've simplified the end of the plate, switched to using a tusk tenon/mortise in the neutral plane of the plate, and I've extended the plate a bit to add relish on the end. I've simplified the joinery where the principal rafter meets the plate (now its a housed birds-mouth with no cog). There is a lot less taken out of the plate now. I think I'm zeroing in on a final design for this particular joinery.

The one piece I don't understand in Don's comments and illustration is regarding the slope of the mortise. How important is that slope? Is it to distribute stress and avoid splitting on the sharp angle of the tusk tenon? If so, should I have a slope on both the top and bottom portions of the tenon? Can I likely live without that slope? Finally, should the tenon be pegged, as shown in my picture below?

I really appreciate the comments and suggestions. I've learned a ton just from this thread.

Thanks,
-Luke

Jim_Rogers · February 03, 2007, 07:21:51 AM

Well, you never did tell me where you are, or what the snow load for your area is, if any.
So, it's hard to say whether or not you need the peg. It may need a peg but it really depends on the load, and the thrust as well as what the joint is on the other end and whether or not it can withstand the total thrust by itself.
I like the old story about the ladder leaning against a house on top of a puddle of frozen water, yes ice. Due to the slippery ice, just about any angle that ladder is at it will slip out as you try to climb it. When it is nearly straight up would be about the safest, but imagine it at the angle your lean to rafter is at. You wouldn't get pass the first rung and it would slide out and you'd fall down.
Now imagine that this ladder is hooked to the house. It really doesn't matter how slippery the ice is the ladder can't slip out unless the weight of the person climbing it exceeds the strength of the fasteners holding it hooked to the house......

You have to look at all aspects of a frame design to see if the entire frame is strong. And that there isn't any one weak point....

Jim Rogers

Jayson · February 03, 2007, 09:51:47 AM

Couldn't you just do a birds mouth on the upper end of those shed rafters and rest them on the plate? That would illiminate a lot of tedious joinery. And you would also expose some nice end grain inside if you made the rafters long enough to be seen above the plate from the interior. Planing that end grain with a skew plane can really glass it up and is fun to do. Oh yeah you could tie those rafter(shed) with a log ollie or staying traditional a square peg. If you did this you would have to come up with an alternative attachment at the principal rafter location. I don't do drawings but if this idea interests you I could emails some pics depicting this method. Or maybe one of these illustration gurus could help out. Good luck

sheneman · February 03, 2007, 11:04:14 AM

Jim,

Sorry. I'm in Northern Idaho at about 3,500 feet. We get quite a bit of snow from time to time. The roof needs to withstand significant snow load. 75 psf, is the official number, I think.

Thanks,
-Luke

Quebecnewf · February 03, 2007, 01:06:55 PM

Sheneman

I to have been using Sketchup but i have not been able to do drawings like you do. I guess I will have to work at it some more. The frame structure in your first pic is something that I would like to be able to do for a shop I want to build. Now that I see it can be done in sketchup I will keep trying. Did you take very long to get to the stage you are at now?

Quebecnewf

Raphael · February 03, 2007, 01:24:35 PM

75 psf is pretty significant.

You may need to go with a principle and common rafter approach with corbels or jowls supporting the principles.

sheneman · February 03, 2007, 06:03:11 PM

Quebecnewf,

It took me a couple days to really figure out all the tricks with SketchUp, but I'm entirely self-taught. I recommend going through the tutorials and just playing with it. I'm convinced that it can handle most anything a timber framer might need.

Thanks,
-Luke

Quebecnewf · February 04, 2007, 06:25:38 AM

Sheneman

The shed I want to buid is 26x32 with 10 ft walls and an open ceiling. It will be a type of timbefame. I plan to stand up regular 2x6 stick built walls then stand 6x6 posts at 8 ft spaces along the walls and 6x6 toplates. Raftes will be 3x6x16' and a 7/12 pitch. With this pitch snow load will not be a problem we have high winds in this area. As I type this this morning we have a temp of -12 winds out of the west at 67 kph and a wind chill of -28C. Supposed to go to 90 kph later today.

I will keep working with sketchup. If possible could you e mail me a couple of sketchup files of your shed and I could play with them and get some idea how you did yours. Just an idea no pressure.

Thanks
Quebecnewf
prowsell@hotmail.com

happyj · February 04, 2007, 07:53:08 PM

i do not mean to turn this in to a sketchup thread and maybe it should be somewhere else.

I was thinking while reading this that it might make some sense to create some templates for common connections. I will try to attach an example.

woohoo only took an hour but, i get it now
I believe there are enough common joints e.g. mortise, tenon, dovetails, housed and non-housed... and standard sizes that could be templatized? Is that true?

with a joint template you could just build a timber say 8"x8"x*' then put the template on it
Let me know if this makes sense please?

sheneman · February 05, 2007, 05:15:43 PM

happyj,

You could certainly create a library of sketchup components (templates), but I wonder how much it would gain you. It doesn't take long to create joinery in sketchup, and most likely you would need to customize any component anyway.

-Luke

happyj · February 05, 2007, 05:45:24 PM

point taken. I had started last year with the sketchup beta and i think the tutorials were not as informative. i spent several days last year but had completly missed the push pull piece to create those. I went back through all the tutorials and you are right it is pretty simple to just create them on the fly. If i were to create a library i would have to find what i was looking for every time (easier said than done) and that could be a pain.

Once again thanks for the assistance.

happyj