New Concrete porch with Shed Roof

clerihew82 · 03-04-2012, 05:57 PM

First post here. I had a concrete pad poured for a back porch. It is 10' wide away from the house and about 37 foot long. I want to build a roof over the top. I did a (really bad) drawing on kinda what i want to do. I need to know how far up the roof i need to go to get 3/12 pitch.. I need to know so i can get the correct length 2x6 runners. Also will I need to build a knee wall over the wall header on the edge of the roof to support the length of the 2x6 runners?? Thanks!!

jklingel · 03-04-2012, 10:06 PM

If you want an exact answer, you'll need to provide more information, like the pitch of the present roof. Solve this for x and you'll see how high the new roof will be at the house wall. You can then measure the height of the house there and fiddle a little bit, it looks, and get approx 3/12. 3/12 = x/10. (Multiply both sides by 10.) More importantly, do you really want the new roof to hit the old roof? Tying them together will take some doing, assuming you want the joint water proof. Why not take off your soffit and facia and install the patio roof joists next to the old roof's trusses? I just did exactly that. I would find that easier, and there would be no danger of creating a leak on the old roof. Why do you want a 3/12 pitch, anyway? Who has calc'd that you only need 2x6 roof joists? Any snow load where you are? You may be able to rest the new roof joists on the old wall, or you may need a beam and posts just for them; it depends on your load on the new roof and your house wall. Does you new slab have any edge beam to support the new roof and its load? Is and edge beam advised? I think you need to consult an experienced builder or engineer in your area.

joecaption · 03-04-2012, 10:19 PM

Before you even concider following this posters advice take some time to look for another poster that had someone install a roof as he suggested. Slopes way to shallow, whole roof leaks even with metal roofing.
Every DIY roof I've seen done just attaching to the soffit leaks.
It's the weakest possable attachment point.
Going up onto the old roof is the best way to go by far.

I doubt 2 X 6's will be wide enough for that long a run.
Why the steel post?
Did you prepore footings while you were poring the slab for the post to sit on?
Why did you pick 8' 2" for the height not 8'.
At some point someone's going to want to enclose this, make it 8' and make it easyer later on.

jklingel · 03-04-2012, 11:19 PM

Joe: I guess you've never seen my roof then; it does not leak. It is also not attached to the soffit; I have no idea where you got that idea. I removed the soffit and facia so that I had access in the eaves, and set the new roof's 2x12's on a beam next to the house. The 2x12's are next to, but not attached to, the old roof's trusses. The beam is on its own set of posts, so the whole new roof is completely independent of the house. There is no reason you can not DIY a simple roof; you just need to pay attention to the pitch, whether you are a pro or an amateur. Sure, you won't have 8' of height at the outside, but it's just a porch, not a living room. Going up to the old roof MAY be the "best", depending on how you define "best", but not necessarily for a DIY guy; that was my point. He'd have to do some homework on that, and it requires a lot more work.