Expert: Mark Sundberg - 6/28/2009

Question
QUESTION: I'm in a double wide mobile home and i want to add on a screened in porch. i would like to install a roof system with a gable that's perpendicular to existing. the porch will be 12ft wide by 10ft. i want to use a ridge pole and rafters. my question is, can i use the existing exterior wall ( 2x6 const. ) to support / build from for the roof system. if i can't , and i need to install a support post for the roof.can these posts be set on top of an exsisting concrete pad. i believe the pad may only be 2 or 3 inches thick.the porch itself will be anchored with a ledger.or can the roof be completly supported by the porch via the ledger. i do have the room on the pad to use those "bases" like the ones used for a floating deck. i looked at these on line but couldn't find anything for them saying that they are capable of supporting a roof

ANSWER: Hi Ron,

Assuming your ridge is running the 10' dimension with the 12' running parallel with the wall of the home, the load in the post bearing the ridge beam will be about 5x6x30 = 900#, at most. While that's not a lot, it is more than a little and I'd actually be surprised the walls of your home are 2x6. Might be if it's very new and well insulated, just they tend to be tight on area and one place they can save a bit is in the walls. People don't buy walls, they buy the space between them.

In any case, I'd not rely on that existing wall for the post supporting the ridge. I also wouldn't rely on a very thin concrete to support it as well. I'd suggest you dig out a space under the existing concrete that's 12" deep and 12" square and fill it with concrete. Then get one of those foundation blocks you describe, one for each end. An alternative that might look better would be to carefully chip out the concrete so the block fills the hole you dig below and just barely protrudes above the existing concrete slab, but make sure you lay about 2" of sand under the block and get it good and wet and wiggle that block into place and then compact dirt back around the sides well.

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QUESTION: MARK
thank you, your correct with the ridge length being 1st. the home is of newer construction with 2x6 studs. for the support of the ridge, i understand what i need to do. but as far as the lower side of the rafters...
will i need to install the same support system for the top plate or will the existing wall be sufficient ?
FYI.. I'm in PA. so I'll be digging down 36in.
the less of that, all the better
or am i going overboard with it ?

Answer
Hi Ron,

With the ridge running perpendicular to the existing structure, your rafters shouldn't be falling on the existing wall, except for the gable closure which shouldn't be a problem as it's not really supporting any roof.

However, yes, any new walls you build should have a double top plate or a beam, size dependent on it's span, for the rafters to bear on. Those walls could easily be of 2x4 studs or if screened or glazed, supported with a couple-three 4x4 posts topped with a 4x6 beam. That would save you some digging. And yes, if you are in a frost zone, you really want to dig down below the frost line for the foundations, otherwise you'll be getting heaving.