Help building flat roof for screened porch

[Bud_Man]

I need help building a flat roof (1.2/12 pitch) for a screened porch I am building. I have framed 2 2x6 side walls (20' long x 8' high) and a 2x6 end wall (approx. 18' long x 8' high) on a 19'x20' concrete pad. The side walls tie into the side of my house under the overhang (soffitt) which is approx. 10' high. My plan was to take 2 20' long 2x6's as supports in the middle from side wall to side wall. Then somehow take 11 12' long 2x6's that are attached to the 2x6 fascia with joist hangers to the 20' 2x6 supports and another 11 12' 2x6's from the support to the end wall cap plate. I am not exactly sure how to make sure the sheathing will match up with the sheathing on my house roof and how to tie the 12' 2x6's together at the support beam. I hope you understand what the heck I'm trying to say. My father-in-law says it can be done but I am just not seeing it. Please guide me through this whole process.

[Cole]

Bud Man, anyway you can sketch out your situation and what you want to do?

[Bud_Man]

I'll try.

[Joe Carola]

Bud-Man,

Is this drawing something like your talking about?

Joe Carola

[Bud_Man]

YES YES! Your drawing is much better than mine. How do I attach the 12' 2x6's to the supports? I mean, do I set the 20' supports on the top of the side wall first and then put the 12' rafters in the joist hangers and then toe nail them to the supports or do I make a frame and hoist it up? How do I make sure the pitch is right? I see how it looks but I'm not sure how to actually do it.

[Joe Carola]

YES YES! Your drawing is much better than mine. How do I attach the 12' 2x6's to the supports? I mean, do I set the 20' supports on the top of the side wall first and then put the 12' rafters in the joist hangers and then toe nail them to the supports or do I make a frame and hoist it up? How do I make sure the pitch is right? I see how it looks but I'm not sure how to actually do it.

Bud,

Sorry to say Bud but you can't do it that way at all. You need at least 2x12's @ 12" o.c. in one piece and I would run them to the house top plate not the end of the overhangs.. You can't run double 2x6's 18' long at midspan and then nail joist on each side with hangers. That double 2x6 wouldn't hold anything up. You would have to have a big structural wood beam or steel at midspan if you wanted to do that or you have to go with 2x12's or I-joist for a 20' span with almost a flat roof.

Your in over your head on this one and should get an Architect for this. What I said above is just a couple ways I've done it over the years but it's all done on plans from an Architect.

Joe Carola

[Sweep]

Joe is right.

19 x 20 is not a small structure. You should be very careful how you frame it. The roof framing sizes will be determined by the snow load and quality of wood you intend to use. The kind of stuff they sell at Home Depot would require what Joe has suggested. Higher graded lumber and no snow load might be 2x10's @ 16" o.c. but I haven't checked it.
The layout you show would require an LVL beam 3.5" x 14" if it were built where I live. I doesn't seem like a good idea.

I would use a gable roof with the ridge perpendicular to the main house. Then the rafters would only span 9 ft. or so. This is the main advantage of a gable roof and why they are still so prevalent. You could probably get away with 2x6's @ 16" o.c. (depending on snow load and lumber quality) and use shingles instead of a synthetic rubber roof. The framing would be easier and the final result much better looking. You would need to tie the bottoms of the rafters together with 2x4's etc. to avoid installing a very big ridge beam.

How do you propose to support the rafters at the exterior walls. I assume you will want the walls open so beams will be requred there. How frequent are the posts?

[Dragon]

If you build it they will come.

They being leaks in this particular case.

[Bud_Man]

Thanks for the professional advice. But can't I put the 12' 2x6's on top of the 2 18' support 2x6's (basically 1 18' 3x6). So for example, 1 12' 2x6 goes from the house and over the support by a foot. Another 12' 2x6 (overlapped and connected to the other 2x6) goes from the support to the end wall and hangs over by a foot. And so on, and so on. The joist hangers would only be at the house. You guys are probably right and I'm way over my head. You guys probably think I'm insane.

[Sweep]

The answer is still NO

Your question has already been answered twice before. You're not insane; you're just not paying attention. I'll put it another way.

Why didn't you propose spanning the full 20 ft with 2x6 rafters? Why use a beam? If a double 2x6 beam 18 ft long can hold up 180 s.f. of roof, why can't a single 2x6 20 ft long hold up 27 s.f. of roof? The single span rafter layout would be stronger but it is a trick question because they would both be overloaded and would collapse.

For the double 2x6 beam that you propose spanning 18 ft.:
• The dead load of the rafters, sheathing, and roofing would exceed the capacity of the beam by about 30%. This is a trampoline.
• If you add the live load of one carpenter to build it, you would have exceeded the load capacity of the "beam" by a factor of 2 which is about the limit of the legendary "safety factor". This is a trampoline close to collapse.
• With two carpenters on the roof, the capacity of the beam would be exceeded by a factor of 3. It would collapse without warning.
• If you could find someone light enough to build it, it would sag until the roofing leaked or people were afraid to stand under it.
• It would collapse in a moderate snow storm.

What part of "collapse" don't you understand?

Wake up! It is ridiculous to put a beam in the location that you propose. It is the most expensive possible layout. It requires a big double LVL beam on 4x4 posts which must be carefully placed to avoid a bump or dip in the roof. The beam must be braced at the ends and middle. It would weigh between 220 and 260 lbs.

Frame the shed roof with good quality 2x10 rafters (or 2x12s for snow) @16" o.c. or use a 6 in 12 sloped gable roof with 2x6 rafters). Either method will meet the building code without an engineer. The gable would be less likely to leak and would look better.

DO NOT put ANY load on the ends of the existing rafters.

If you are looking for UNprofessional advice you have come to the wrong forum.

[Joe Carola]

Thanks for the professional advice. But can't I put the 12' 2x6's on top of the 2 18' support 2x6's (basically 1 18' 3x6). So for example, 1 12' 2x6 goes from the house and over the support by a foot. Another 12' 2x6 (overlapped and connected to the other 2x6) goes from the support to the end wall and hangs over by a foot. And so on, and so on. The joist hangers would only be at the house. You guys are probably right and I'm way over my head. You guys probably think I'm insane.

No, Bud, you can't put them on top because they will never hold. You can't span double 2x6's 18' flush holding up two rows of 2x6's with hangers or even dropped with the 2x6's on top overlapping like you sdaid or even just using one long continuos 2x6. That double 2x6 @ 18' wont hold squat and will fall down on your head and also your families head and even your Father-in- Laws head. So tell your Father-in-law he's 1000% wrong and if he disagrees, tell him his way will fall on his daughters head.............

Get an Architect and do the job right or don't do it at all or you will KILL someone. I also assume you aren't getting permits so anything I say or anyone else says means nothing but if you have an Architect design it and your not getting permits at least you know you will be doing it the RIGHT way and SAFE way.

What I am suggesting for your roof is from past experience but everything I've framed over the course of 20 years has all been built from Architectual plans and is the right way and safe way to do it.

I like Sweeps idea about a gable. It would be a lot easier to do.

Joe Carola

[Dragon]

And flat roofs are bad.

I've made more money fixing flat roofs.

By reframing a pitched roof over them when people finally got sick of the leaks.

[myron]

Sweep has it

Myron