Best way to keep a shed roof from racking
Experts, I’m installing a shed roof over my existing deck. I have the ledger board lag bolted to the house, from there I have 3×10 green fir rafters extending to the header at the end of my deck. The rafters are spaced 36″ centers to leave me enough room for some nice skylights. The walls are merely 6X6 for the corner posts and 3×6 spaced so I can pop in standard width 32 inch screen doors. The roof will have tongue and groove pine, not sure how thick yet, running perpendicular to the rafters with 5/8 plywood on top of them. I’m hoping tying all this together with stabilize the entire frame. I was looking for some ideas or best practices when framing in a situation like this. I’m also planning to have railing running between the posts but not sure how much they will actually add to the stability of the frame.
Thanks in advance for comments
Joe
Replies
Once you have
the T&G and the plywood nailed off, you will have plenty of lateral strength. I would tag a temporary diagonal to the bottom of the rafters while you're working on top, to keep it where you want it.
I'm assuming you're using shingles on that roof.
When I've wanted to keep the look of board sheeting from the underside, I've used 3/4 furring run on top of the T & G / rafters b/4 sheeting with ply. This to keep the roofing nails from punching through. If you think you've got enough thickness of material, then don't bother.
36" centers on the rafters is pretty huge-hope the sheeting combo doesn't get squirrly. Will your plywood be T&G?
The plywood will be T&G
I'm using T&G 5/8 plywood over 1x6 T&G pine boards. The furring strips might be a good idea, I didn't think I would need it but I'll give it a look for sure. I've seen others use 36" centers with glue lam without problems but not sure what they're using for the roofing
A diagonal brace in an X can actually look decorative in this situation.
Generally, the concept is that the plywood will transfer the lateral/racking forces to the house wall behind it. I did a wall w/ all windows in it and it couldn't meet prescriptive shear wall requirements ... so I engineered it to transfer the forces to the wall behind. My situation was much more complex than yours and the result I'm not going to reveal here. You are talking an open porch ... but you are also talking about future enclosure. You also mention skylights ... which will hamper the structure. I hope these skylights are north facing.
I kind of fail to see the point of skylights in an open wall situation like you have ... but I admitedly know zip about a lot of your specifics. Skylights IMO are a potential source of discomfort (open sides or not) and they are expensive to buy and they are roof penetrations. Use them very carefully, I say.
In most places your project requires a permit. The BO will let you know what he will allow and what might be required.
An open porch might benefit from some structural diagonal bracing at upper joints between end posts and the beam.
The reason for the skylights with an open porch is to keep the room, in the case the kitchen, from gettingto dark if I were to completely close the roof, similar to a sun room. Surprisingly, in the case I'm using fixed skylights, which are much more affordable that a skylight that opens. The fixed skylight isn't much more than a piece of glass with a frame. I'll have to build the curbs out of 2x4 or 2x6, flash them and pop the skylight in.
The porch will be open to a point so I can either pop the screens in or leave them out for the winter.The skylights actually are north facing and you make a good point on this. I have no desire to cook like an ant under a magnifying glass on a sunny day.
Thanks for the comments
Ever see a sheet of plywood warp? Of course you have .... that's what sheets do. It's pretty basic geometry- a rectangle is but a pair of triangles, triangles determine a 'plane,' and there's absolutely no reason for two planes to align with each other.
Ever see a doorstop warp? I haven't. So ... turn your plane into a doorstop.
You do this with trusses. Even a shallow truss, making the roof into a wedge-shaped box, will add an enormous amount of stiffness.
Couple of Questions What's
Couple of Questions
What's the span of these 36" O.C. rafters?
What's the roof pitch ?
I would not use the 3/4 strapping method suggested,because of the skylights, and other reasons I won't get into now... I think you would be best served using 2x6 T&G pine for the roof sheathing....plywood on top of that would not be necessary unless the T&G has a v-groove on the upper side, then you might want to add the 3/4'" or 5/8" T&G ply to maintain a smooth roof surface.
Since your framing is 6x6 you might want to use 4x6's as corner braces (diagonally) for structure and style
You mention running the sheathing (pine) perpendicular to the rafters, which is standard practice, but you should also run your ply sheathing perpendicular as well, just don't let the top edge fall directly on a V-groove seam.
One more thing....your T&G should come end matched (T&G) like hardwood flooring....this will allow you to have butts at mid-spans instead of having to land all your butts on a rafter...which will save on material as well ....less waste.
I would also suggest you go to 36" screen doors instead of 32" less framing material to look at and more fresh air flow and visibility looking out.
Geoff
The roof pitch and rafter span
Roof pitch is 2/12 and the rafter span is 18 feet. I didn't know you could buy 2x6 T&G with the ends matched. That's a good idea and I like the 36" span for the screen doors. I would rather not have corner braces because it just won't fit with the style of my house, that's one reason I was thinking 5/8 plywood, maybe over kill but I'll sleep better at night.