[ponch37300]

We have a 8' by 24' cement patio behind our garage that I would like to cover with some corrugated metal roofing. I need to tie this into my garage roof about 4 feet up. Plan is to put up some 4x4 posts anchored to concrete to support the outer edge. Then a double 2x6 across the 4x4s for the beam. Then 12' long 2x6s roof joists, not sure if spacing will be 2' or a little less. Then some 1x4s perpendicular to them to support the metal panels. Have a few questions.

1. Can I use regular wood since it's all going to be covered or do I have to use treated anywhere? I will use treated 4x4 posts but what about the 2x6 roof joists?

2. What is the best way to tie into the existing asphalt shingle roof? Just carefully peel the shingles back and stick some metal flashing up under the shingles? How to attach it to corrugated metal roof panels since there are humps in it?

3. Are 1x4s what I need to use for purlins? Or something else?

 

[Old Thomas]

How far apart are you planning to put your posts? Based on that, how many rafters are supported by the beam between posts? The reason I ask is that both are 2x6s. Posts at 8 foot centers and rafters at 2 foot centers means 3-2x6 rafters are supported by a 2-2x6 beam. It is not good design to have fewer supporting more. The beam needs to be upgraded unless your posts are close together. You should check span tables for the rafter size based on the species and grade of wood used. Usually 2x4s are used for purlins. They are cheap, strong, and easy to screw or nail to. All that needs to be treated is the posts. In fact, treated wood and metal roofing are not good together. I would flash from under the shingles over the metal.

 

[Nealtw]

You may need a footings under the concrete deck.
I would use 6x6 treated posts set in a saddle with 2 ply 2x10 beams untreated wood is fine above the posts.

I would open the roof sheeting on the garage over the wall and extend the wall up under the rafter to take that weight away from the roof rafters of the garage.