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Old 04-21-2014, 03:59 PM   #1
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Build house on existing concrete slab


Hi!

I am planning on building a house on an existing foundation from a previous building that had burnt down. I am new to DIY house construction, so am looking for some general advice and guidance, as well as some good starting points for where I can begin my research.

Here are the details of my existing foundation:
- It consists of 3 concrete slabs, and a gravel portion which is surrounded by concrete support blocks
- 2 of the concrete slabs are laid on either side of the third, and are about a foot vertically higher than the third
- All the concrete slabs are flat with no wall on the perimeters
- The gravel portion is adjacent to the lower concrete slab in the middle
- The center, lower slab contains rebar
- The side, higher slabs were poorer quality additions with cement poured directly on river rocks and no rebar, to my knowledge
- The support blocks extend into the ground only by about 6 inches
- The support blocks have a piece of connecting rebar coming out the top
- None of the poured cement foundation has connecting rebar extending out the top
- The total foundation area is about 572 square feet
- The foundation does not have any visible cracks

House plan:
- The house will not be very large (as per the square footage)
- The first floor will be raised above the cement foundation such that there is a crawl space underneath
- There will be "one and a half" floors, with one section of the house being a high-ceilinged open room, and the other side having a second level loft.

Initial questions:
- Is using an existing foundation that had been through a fire safe to begin with?
- What are the advantages, disadvantages, and alternatives to building a cement block wall on the perimeter of the cement slabs.
- Is supporting the joists on wood posts using cement support blocks that sit on top of the cement foundation a reasonable option?
- Will the lack of rebar "connectors" (not sure what to call them) extending out of the top of the concrete slabs be a problem for structural integrity? If so, how can I ensure a strong structure while reusing what is there?
- What measures can be taken at this stage of construction for good heat insulation?
- What measures can be taken at this stage of construction for good air ventilation to prevent mold, etc.?
- Where can I find information on building code for my area (in BC, Canada) that a newbie can understand?

Thanks!

Troy

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Old 04-21-2014, 04:18 PM   #2
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Build house on existing concrete slab


Way to may factors going on there for anyone on the net to even guess on this one.
To prevent moisture from getting in there would have to have been a vapor barrier under the slab before it was even pored.
To keep it warmer there would have to have been foam insulation under it.
Only place rebar would have been used is the outside footing that should have been below the frost line and anyplace there was to be supporting walls.
How far above grade is the slab? In the US it needed to be at least 6" to keep the siding from rotting and keep water from getting in under the walls.
What happened to the plumbing that was coming up out of the slab when it burned?
Got some pictures?

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Old 04-21-2014, 05:03 PM   #3
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Build house on existing concrete slab


if I was planning on building a new home ($$$) on top of an existing concrete slab exposed to a residential fire I would first hire a structural engineer to evaluate the existing foundation.

They would be familiar with your code requirements and besides determining if the slab is safe would also let you know of any upgrades you'd need to perform based upon what you want to build.

I think it is money well spent.
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Old 04-21-2014, 05:12 PM   #4
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Build house on existing concrete slab


Quote:
Originally Posted by GBrackins View Post
if I was planning on building a new home ($$$) on top of an existing concrete slab exposed to a residential fire I would first hire a structural engineer to evaluate the existing foundation.

They would be familiar with your code requirements and besides determining if the slab is safe would also let you know of any upgrades you'd need to perform based upon what you want to build.

I think it is money well spent.

What Gary said, one of concrete's worst enemies is fire.
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Old 04-21-2014, 05:20 PM   #5
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Build house on existing concrete slab


We'll probably never know who started the rumor that concrete needs a vapor barrier or your house will drown with water and moisture problems. It seems it only takes one person to start a disease plague that spreads like wild fire. There are tens of millions of buildings without plastic and doing very well and will continue to do so.

But I would determine if the remaining footings would support your plan.
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Old 04-21-2014, 05:55 PM   #6
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Build house on existing concrete slab


I agree with the engineer IF you are bound and determined to use the existing slabs (foundations).

But I can see huge problems with the unknowns. Problems and unknowns usually cost extra engineering time (and possibly tests, like core drilling) and can result in higher costs during construction.

I think that I would probably just demo the foundations. It isn't that expensive with good equipment on site.

That way you are not constrained by the size and shape of the existing stuff. You will know how deep the foundation goes. It can be designed for the second story (loft) etc.
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Old 04-21-2014, 06:05 PM   #7
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Build house on existing concrete slab


Hire someone with a front end loader and a couple of trucks. That is the simplest and qucilest way to get going and avoid all of the other construction costs, limitations and problems in the future. - The insurance company probably would not pay to have the concrete removed and make the land worth more.

Dick

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