I need to build my garage floor with wood planks, cover with waterproof membrane then top with snap together PVC floor tiles. My site is so steep the garage is on top of the house and the house is on top of wooden posts i.e. house on stilts therefore I cannot afford to use heavy concrete slab for fear of heavy top structure in an earthquake. You experts think this is feasible? Any advice is much appreciated.
Almost anywhere it would be illegal and with good cause. Even with your “waterproof membrane” some oil and fuel may make it’s way past or though (few waterproof membranes are oilproff) and soak that wood that would become a torch.
You don’t need a automotive expert you need a construction design professional who knows local codes and can design you a safe home. This would be even more important in a earthquake area.
You have to check local building code. My guess is that you will have to have an approved coating over the wooden floor for fire prevention. This coating could be something like a 1" concrete with proper admixture(aka grout likely fiberglass as reinforcement). If you are permitting the project remember you need to check codes.
hmmmmm. you are afraid of an earthquake, have a 4 or 5 thousand pound auto suspended above living space, and have the house on stilts??? so this is a three story structure with an auto on top, and you are looking here for construction solutions before you consult a structural engineer???
you may want to re think your operation. consult an engineer first. find out if this is feasible from a technical/ engineering/ load bearing standpoint, then; if it is possible, find out what local building codes require for a garage floor surface.
If I understand your situation correctly, you have a house built into the side of a hill and the roof is about at the top of the hill. The hill is so steep that you need pilings to hold the house above grade. I’d level off the top of the hill and put the garage on solid ground. It would be a lot cheaper to build a corridor with steps into the house than build a garage on top of the house. You will need to make sure you meet set-back restrictions for the garage if it is closer to the street than the house.
Doesn’t matter what anyone here things. It’s what your building inspector thinks. We can make suggestions, but if it doesn’t meet code then you’re out of luck. In many states using any kind of wood for flooring won’t pass inspection. If you’re in California (i.e earthquake zone) then I suspect their laws are pretty strict and you won’t be able to use wood either.
You need an architectural engineer. Proceeding without one may easily result in a disaster that will alter the rest of your life. And your families lives.
I cannot strongly enough urge you to hire an architect. Please.
This is the most insane thing I’ve ever heard.
For one, if you can’t build it stout enough to hold a concrete floor, what makes you think it can hold a 4000 lb vehicle?
If you are worried about an earthquake dropping the floor on you, why not worry about that car on top of you?
The answer is No. Find another location to build.
Skipper
Any structure is feasible but, for a structure like yours, normally a civil or structural engineer would perform the design work and apply his/her “stamp”. If the structure you described is somewhat common in your area, then an experienced local builder may already have an approved design - though a building permit will be required as will inspections.
You need a structural engineer and I suspect would recommend ditching those wood posts. Concrete adds structural integrity not just adds weight. Planks add nothing. You need a substantial pier structure properly anchored and I suspect should be concrete piers with poured concrete beams or steel beams. Then a concrete pad poured for the floor along with a membrane. My folks had a house with a room under the garage and a poured garage floor. Only one center concrete post was required and no problem.
Thank you very much everyone who has replied to my question. I’m sorry as I was not clear enough. My Seattle lot drops immediately off the white line by the side of the road and that explains the house on stilts. I actually have building permit in hands and the structure is designed by proffesional architect and engineer and is designed to carry 3" concrete slab plus the cars. Well it is my family and I who live under the garage and I’m not comfortable with the idea of a heavy top structure (the 1000 s.f. slab weighs in at 37,500 lbs at 150 lbs per c.f.) in an earth quake zone, no matter the structure engineer and city said it should be OK. I just wonder if anyone else has done this and it works out OK? Light weight concrete still weighs in at 30,000 lbs. Again, I appreciate all who responded.
A concrete slab that size would be a no-no in my opinion. The lumber could be feasible. I vaguely remember visiting a new house under construction here in OK many years ago on a steep bank overlooking the Arkansas River. Standing on the road one looked almost straight down on top of the house and unless memory has failed me due to the years it seems like this house had a parking spot over part of the house. I know concrete was not used but I do not remember exactly how they did this.
It would seem to me that considering the slab size (roughly 30 X 30?) one would need to run at least 4 or 5 steel I-beams and cross that with 4" thick wooden beams. I hope we’re not talking 2 bys here! 
Depending on the garage width that could put the steel beams approx. under the wheels of the cars.
Just an opinion here and I’m by no means a structural engineer. I would just prefer a little steel underneath that wood and whenever I do something it’s usually overkill rather than underkill.
In retrospect, what kind of walls are you going to have? Even steel and wooden beams are going to require a pretty healthy support.
as an alternative, why not have the ‘garage’ part of this be outside, not under roof. so it can be sealed from the inside.
the rubber roof membrane would suffice then, but the fire code is going to have to be settled between you and the local building dept.
kienbien: Yes I do have four steel I-beam underneath the house that rest on a retaining wall and the four 12x12 pressure treated Douglas Fir, all are cross connected horizontally and vertically with tensioned steel tie rods. The garage floor is supported by 4x12 parallam. The structure can carry the weight no problem but I shiver thinking about an earthquake. Has anybody tried corrugated steel for garage floor? or paerhaps special fiber glass concrete that only needs to be 1-2" thick? Many Thanks,
Think about what you’re saying. You hired a STRUCTURAL ENGINEER to devise a solution and now you’re not comfortable with the design and are considering alternatives. ??? Are you QUALIFIED to design such a structure? I doubt it because you hired someone who has the credentials and skills to do it. It’s even more insane to log onto a public automotive forum to seek advice from people who are not structural engineers. Even if someone here has a similar arrangement, it is not YOUR situation so how can that apply? Regardless, no building inspector is going to accept design deviations without proper credentials signing off on it. And if something bad does happen, your insurance isn’t going to cover it either.
If you’re uncomfortable with the PROFESSIONAL design and assessment, then seek a second opinion from another PROFESSIONAL that can see the situation first hand and DESIGN a solution.
Gagage on stilts…can you say “Ferris Beuler’s Day Off”
There’s a house near me with a corrugated steel floor in the garage. But it’s under the concrete floor and above the living area below it. It seems to me that the steel underlayment is there to provide compression to the bottom of the concrete pad and prevent chunks of concrete from falling onto or through the ceiling of the living area.
I doubt that you could afford the stout beams required for this floor. Imagine how much just one 16"x16" beam costs then shock yourself when you count the number you need. The can be manufactured (thick really thick plywood) but that is still probably too expensive - and heavy. Even if the volume of the wood floor and the concrete floor are the same, the wood floor would weigh about 8000 lbs. And I suspect that the wood floor would need to be thicker than the concrete floor.
Stick with a local structural professional. Don’t try to outguess them. I know what you are talking about with that drop off, but so does the architect. They also know earthquakes. Talk to the architect about your concerns. You don’t need to reject his design, but rather you need to express your concerns as you would to any design. I suspect they have had the question before and should be able to give you some good answers.
In any case, I would not worry, when Mt Rainier goes it won’t make and difference anyway.
BTW I expect to be moving out that way if a few years.
Tell your structural engineer your concerns. Let him either make you comfortable with his design or suggest options that will provide more of a safety margin. Work with him, don’t try to second guess his designs.