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MaximumAsh 06-04-2012 09:50 AM

Addition Foundation Question
Hello All - working on plans for a small addition to the back of our home, and had some foundation-related questions. The details:

Addition: 3-season sunroom
Size: 14' x 14'
Location: Southern NH

We currently have a 12" thick slab butted up against the house for a patio, not merged with the existing foundation. It was there when we bought the home a few years ago. I'm wondering if we could build a 3-season sunroom on that slab, or if we need to modify/replace it first.

Given that it's not merged with the existing foundation, and doesn't hit the 42" standard depth, I'm guessing we would have to replace it if we planned on attaching the sunroom to the house, is that correct? Is there a way we could simply add a footer or something along those lines to the existing slab, or is a complete remove/replace required?

Is there a suggested design for the foundation for a project like this that someone could suggest? Thanks in advance for everyone's help!

GBrackins 06-04-2012 10:24 AM

You need to check with the local building official and find out what is the minimum depth of foundation for your area. I'm in Massachusetts and the bottom of our foundations must be a minimum of 48" below the finish grade (ground elevation). I'm sure NH is at least the same if not more.

You may be able to excavate under the edges of the slab, but it would take more time, plus the energy code section of the code requires installation of rigid insulation behind your frost wall (foundation) if you are heating/cooling the space, so you might as well bite the bullet and just remove the slab. IMHO

MaximumAsh 06-04-2012 11:08 AM

I called the local building folks and I thought they had said 42", but you could be right with the 48". I don't have all my notes with me from the conversation.

I've done a ton of home improvement over the years, but mainly interior or non-foundation exterior work like sheds, etc. I've done plenty of slabs and such, but never a full foundation. Is this something I could look at tackling on my own to save on cost? Given how important the foundation is overall, I want to make sure it's done right, but high cost will kill the project.

When I asked some local contractors for some ball parks on the foundation work, the numbers were widely spread. They also seem to have different foundation design ideas, so I'm not sure which is best.

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