I have a garage built on a slab and it needs rebuilding. It was origainally built before pressure treated lumber, so the sills are rotting as well as some of the studs. But, I've noticed that the concrete slab has cracked and small pieces are missing at or below grade in areas. Is there any way to fix the slab without having to demo the whole building? If I demo it I have to get a variance to rebuild it in the same place becuase it doesn't meet set-back codes. Can I get away without a true foundation and footings?

There are three different problems here.

We have a foundation which may be essentially sound, with a few pieces missing. I would simply patch it.

We have a ground slab which is broken. I presume from your question that the slab extends under the walls and rests on the foundation. If so, no wonder it is broken. You could saw-cut the slab around the perimeter, leaving the part under the studs, and rebild the rest, using mesh, and keeping it free of the foundation walls.

Then we come to the rotted sills and studs. You can replace the sills in say 4-foot pieces, using treated lumber. The studs will just hang there and you can drive the new sills in under them. Cut the new sill pieces to fit snugly between the existing anchor bolts, and use big washers to re-bolt them. Then you can sister up pieces of stud where the existing studs are rotted, again using treated lumber.

A lot easier and cheaper than ripping it all down, fighting with the town for a variance, and then building it all back up.

Unless of course you REALLY want a new garage...

Richard, thanks for the advice, but I realize now that I wasn't very clear about the problem. The slab as a whole is not broken. It is just cracked around the perimeter in a few places only. Probably from the lack of gutters and plantings too close to the garage. Can I patch that AND add a course of block so the siding/sills aren't in close contact with the final grade? If so, do I use concrete or cinder block? Also, does the slab need to be dampened when laying the block? Any thoughts would help. Thanks,
Rob

I'm an architect, not a mason, so the questions of actual techniques of building are best left to those who are expert in those areas.

Yes, you cann add a course of block, and it should be concrete block, preferably 75% solid load-bearing block.

I"ll bet the cracking around the perimeter is because the builder erroneously supported the slab on the block walls. That method of building almost always guarantees cracking, and a few inches in from the perimeter is where it will happen.

Okay, if I cut the slab from the perimeter and keep it free of the foundation how do I keep the moisture out? And how far from the foundastion? 1/8", 1/4"?
An architect, eh? I will be looking for one in the near future for an addition to my home. I'm in New Jersey, where are you? I'm considering using SIPs. Have you had any experience with them? Any thoughts or concerns?

f you cut the slab and replace it with a new free-floating floor slab, you can use a premolded joint filler at the perimeter. If you're just planning to cut your existing slab, it most likely doesn't have a vapor retarder under the slab anyway, so moisture can migrate through the slab or through cracks, so I wouldn't worry about the perimeter.

I practice in Rockland County, New York as an architect, and in northeastern Pennsylvania as a home designer. I have no experience with SIPs, and such things are not on top of my list of things to experiment with. I'd use them if a homeowner specifically requested them, but otherwise, conventional construction offere many fewer potential problems.

Okay. About the moisture barrier. I would like to remove the old slab and pour a new one. I am also thinking of heating the garage as it will actually become a woodworking shop instead of a true garage. What would be the order of materials starting at grade? Crushed stone, vapor barrier, insulation panels and then floor?

And about SIPs — I've seen them used on "This Old House" quite a bit and I like the speed in which a structure can be raised. As far as fewer problems with conventional lumber I'd have to disagree. There is much more waste which goes into depleting landfills; the lumber is hardly ever dry and walls will bow over time; less of a thermal berrier with studs, etc. I could go on, but I won't. I just think that using them is a greener way to build. I was hoping to find someone who actually has used them to see what I'm up against.
thanks

If you're going to heat the garage, you will have to insulate the entire structure, and if it's a typical garage, it won't have been built for the purpose. For example, no attic ventilation is probably provided.

For the floor, usually the preferred and code-cpmpliant method is to install rigid insulation such as Styrofoam extending twoo feet down along the foundation wall, and two feet in under the floor slab. In your case, the vertical insulation will not be possible without considerable excavation, so insulation under the slab should be sufficient if you use 2 inches of Styrofoam, R-10.

As always, the vapor retarder should be on the winter warm side of the assembly, so in your case it goes right under the slab, and over the insulation, which then lies on top of the gravel.

I probably won't live long enough to encounter the problem, but I dread having to do additions or alterations to a house built with structural insulated panels (SIPs). As for your other comments, lumber is only not dry if it isn't specified properly, and the impact of construction waste on the ecology is far less than the impact of foam-producing plants and panel-producing plants, so I hardly view the use of any plastic foams or wood panels as "green" in any way. That doesn't even take into account the outgassing from the foams or the wood panels once in place. I don't ever remember reports of walls bowing in over 40 years of architectural work. The effect of studs as a thermal bridge is vastly over-rated by the panel manufacturers.

I can think of many conditions that would be very difficult to accomplish with SIPs, that are quite simple in conventional construction. If you are building a simple box and don't care what it looks like, then SIPs may be an answer, but for most houses, conventional construction is a superior way to build.

Thanks for the info on insulating the floor and I will be putting a new roof on so I will be adding a ridge vent along with soffit vents.

As far as the SIPs go, we could probably go back and forth on the whole green building thing. But, as far as not being able to build something interesting, well, I'd have to disagree. Take a look at this home designed by a firm in Brooklyn: http://www.facedesign.com/portfolio.php?project=tracyresidence
While the whole house wasn't completely built with SIPs, they did use quite a bit of them, and they are even used in the curved part of the roof.
I'm not saying that I'm right or your right, I just think there are better, more efficient and greener ways to build. you just have to do a lot of research to it.
Thanks again for the info on my garage. I've got to get to work!
Robert

With reference to the picture you linked to, I rest my case.

Just what does that mean?

I got to thinking, and after seeing your reply I was wondering what you've designed. Can you send me examples? It might help me understand where your coming from…

Hmm. i didn't think so…

I don't know how to post pictures here, and in any event, it's not about what I do, it's about the house pictured in the link that was posted, and it sure didn't make me feel all warm and fuzzy and want to call it home. For a bus treminal, maybe, but for a home...no thanks.

The home I showed you wasn't presented for you to decide weather you like it or not. It was in response to your ealier post in which you stated that you can only build a simple box if you're building with SIPs.

Hahaha...touche'