I have a customer who would like to add on to their existing house with a garage addition. The only thing is that they want the garage on the main level of the house and want to have additional room underneath. They want to make the lower level for storage and safety during a storm. I have seen in articles where they have poured structural slabs. I know it can all be done, but my question is, can it be done with wood instead of concrete?
The garage will only be approximately 15′ x 24′. They want to be able to park one car inside and have steps to the basement. I know i’ll have to get an engineer and really do some research, just curious if anyone has any other ideas or have done anything similar.
Replies
I think it will either be theoretically un-doable, or it will require so much lumber that you won't want to do it. Don't get me wrong, I'm not saying that it CAN"T be done with wood, I've seen it done with wood. One of my best buds from childhood's dad had a barn with a space underneath, and it was, I'm guessing 6x12 beams 16" o.c. with two layers of full 2x planking on top. Probly all old growth fir, as the barn is about a hundred years old now, and still standing. When we were kids, he'd think nothing of driving his pickup in there to work on it, and I don't recall the floor moving a bit....
But I think nowadays it would be tough or impossible to find an engineer willing to put his stamp on it. I'm no engineer, but I'm thinking that the weight of a car on 4 spots is a heck of a lot of live load per sq ft. Why not conc poured in place on a steel deck with reinforcement or better still, pre-cast slabs? They can be core drilled for plumbing ( with some restrictions ) and it's got to be cheaper than about any other method.
my .02
Bing
Certainly it has been done with wood. Might be better to use some steel support beams, though.
Also, in snow country (where are you?) you need to consider ice falling off the car and melting -- where will it go? (Concern even with concrete, as the ice may have a high salt content and attack the rebar.)
Structurally there is no problem at all with making a wood garage fllor that supports an automobile. We do that with raised garages quite often here.
The problem I foresee is that of having habitable space under the garage. gas leaks and other toxic fumes can sink to that lower level, made even easier with the stairwell approach. That is why most codes call for an attached garage to be at a lower floor level than the living unit.
so I had anticipated that your Q was going to be how to design around that problem
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I've only seen it done using precast, hollow core concrete beams with a 4" slab poured over it.
As Piffin has noted, it will be critical to seal the surface and slope all run off out to the exterior. There are products avaialable in membrane form. There are concrete surfaces that can do it too.
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