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Garage floor made of wood...

Sco's picture

A friend asked me to help her build a garage with a wood floor in central Maine.  The garage will be 20' x 32' with a 4' frost wall, and a 14' wide garage door on the gable end.  I'm struggling with how to design the wood floor and what materials to use for the floor framing.

Currently we are planning to run a beam down the center of the long dimension and use either 12' - 2x12 PTL, 12" o.c.. along  each side of the beam, or 20' I-joists run the full width, again 12" o.c..  My first question is whether or not we need to use PTL?  The building site is fairly well drained and we plan to install a perimiter drain to daylight along the foundation footing as well as a 6 mil vapor barrier on the bare ground beneath the deck.  We plan to use full dimensional 2x6 hemlock for the decking.  Currently we have no plans to allow for ventilation in the crawl space.  Under these circumstances, do you think it's necessary to use PTL???

My other question is how to design the joists under the garage door opening.  Currently my friend plans to store snowmobiles, a tractor, atv's and a trailer or two in the garage, but no doubt the day will come when she decides to put a car or truck in the garage.  I can't help thinking that the joists below the garage door will need to be reinforced somehow in order to withstand the force exerted when a car or truck is driven into the garage. 

We intend to build a ramp for entry through the 14' garage door.  My concern is the rotational forces exerted on the first few joists when someone drives a vehicle into the garage.  Would it be sufficient to build a beam of 3 or 4 2x12's for position directly below the garage door opening?  Would that be sufficient to overcome any tendency for the first joist to want to topple inwards when a large vehicle is driven up the ramp???   Or is this not an issue at all???

Thank you in advance for any assistance!


Wow ... haven't seen that in (post #187311, reply #1 of 12)

Wow ... haven't seen that in a while. I'm NOT an expert, but building science is my general profession. We had wood floor garages all over where I grew up ... lots of steep hillsides and garages w/out a lot of concrete.

PTL ... pressure treated lumber, right? What about the vehicles covered in snow/rain when you pull them into the garage? That would be my biggest concern. As such, I think I would avoid TJI joists as they potentially will get wet regularly ... unless you are designing a waterproof deck.

A few years ago, I saw this builder do a garage over a living space. He did joists like 12" oc then poured a concrete deck on it. Don't know if it was entirely successful ... he seemed like not the best contractor, but wasn't the worst, either.

It seems by all rights all the framing/decking might need to be PTL. I know what it is like to drive through a foot of snow and arrive home and pull the car into the garage ... LOTS OF WATER! Your ground moisture issues seem like largely a non issue in comparison.

There ain't NO free lunch. Not no how, not no where!

Actually, these folks already (post #187311, reply #5 of 12)

Actually, these folks already have a 3 car garage attached to their house where they park their everyday vehicles.  This garage will be for their toys - 4-wheeler, motor cycles, lawn tractor, etc.  So, theoretically this second garage should stay much drier than the other.  On the other hand, your point is well taken - maybe i am worrying about the wrong source of moisture...

If the job will require (post #187311, reply #2 of 12)

If the job will require engineered plans, will go through a plans-check process in order to acquire permits, you will want to examine what the code says in your building jurisdiction.

I know that many rural areas have nothing like this going on, and you can pretty much build what you like.  If that is your situation, then you need not be concerned about code, but you still may want to get a local engineer to help with your structure.

We think of cars and pickup trucks as really heavy things, when we think of a wood-joist floorframe they might sit atop, but they are really not that much, if we start comparing them to things like bathtubs full of water, and upright pianos.

A 6,000 pound pickup truck occupying a rectangular space of 6.5 x 18 feet, is equivalent to about 50 pounds per square foot of loading, while mainfloor living spaces in houses are designed to 40 psf today. 

Here is a handy link to see what you might need, as far as floor joists go:


"A stripe is just as real as a dadgummed flower."

Gene Davis        1920-1985

but wouldn't the bigger issue (post #187311, reply #3 of 12)

but wouldn't the bigger issue be that the 6,000 lbs is point loaded on four six inch by six inch spots?

A very good piece of advice I once got is not to always ask "Can You (do x or y or z)?" but rather you should be asking "Should you do x, y or z?"  The answer to the first question is invariably YES while the answer to the second is most often NO.

Can you build a wood floored garage? Yes

Should you build a wood floored garage?..................................

Here it is back to you. (post #187311, reply #4 of 12)

How was this addressed for you in engineering school?

Is the floor frame structure a loose collection of joists, sheathing, planking, etc.?  Or do all those parts, fixed together with their glue, screws, nails, whatever, produce a diaphragm of some type?


"A stripe is just as real as a dadgummed flower."

Gene Davis        1920-1985

It is a diaphram ... but (post #187311, reply #8 of 12)

It is a diaphram ... but what's your point? A beam is a beam. Point loading vs. evenly distributed loads command much different considerations in structural design. Four 'point' loads of 1,500 lbs each on a diaphram is much different than 50 lb/sqft. 1,500 lbs on a 6x6 area is ... that's right 6,000 lbs/sqft. While the diaphram/structure can hold say 6,000 lbs evenly distributed, it may not hold the point loads imposed by a vehicle. That may simply be the difference between e.g. 1/2" sheathing and 1 1/2" sheathing/flooring.

A piece of paper will hold a pair of heavy scissors laying on it and held in place along two edges. But try to hold the same scissors on the point ... no chance. Diaphrams are very similar.

I remember my first house ... I was young and ... semi-smart :) ... to backfill our basement, we drove a bobcat onto our floor structure - floor trusses and 3/4" plywood ... plus some more plywood to avoid puncturing the subfloor. Boy was that a scary thing to do ... but we got away w/ it.

There ain't NO free lunch. Not no how, not no where!

Are you a structural engineer? (post #187311, reply #9 of 12)

Because if you are, please proceed to tell the O.P. what he or she needs for those point loads.  Size the beams, the deck, and size the joists.

And be sure to allow for the vehicle to be parked within some reasonable range of available space.  Furthermore, allow for those point loads as they move along when the vehicle is driven in an out of storage.

You might as well, while you have your sliderule out, figure the dynamic loadings from starting and stopping, and add that into your beamstress analysis.


"A stripe is just as real as a dadgummed flower."

Gene Davis        1920-1985

No need to get upset, Gene. (post #187311, reply #11 of 12)

No need to get upset, Gene. I'm just making comments. I have had enough structural training to be a structural engineer ... I don't use that education regularly to enable me to engage in a detailed conversation of structural theory or calcs.

I do know that you design structural elements very differently if they have a point load(s) or a uniform load. A floor diaphram is not much more than a series of beams laying along side each other. You may likely design the point loading for the worst case positions on that beam given your anticipated potential load (e.g. your 3/4 ton truck with toys in the back).

And yes, you may very well have to accomodate the dynamic load of the vehicle in motion ... which doesn't affect the floor much I suspect, but may affect the shear resistance of the supporting structure ... although I suspect that in many or most situations, that dynamic load is satisfied by wind or seismic requirements that may easily be much larger than the dynamics of a vehicle coming to a sudden stop from say 1 mph ... although I don't really know.

There ain't NO free lunch. Not no how, not no where!

I hear what you're saying.  (post #187311, reply #7 of 12)

I hear what you're saying.  My friend is adamant about a wooden floor.  As i thought about this it occurred to me that i've been in barns over the years that have wooden floors and these barns often times were used to store tractors, back hoes, and other large equipment.  So it occurred to me that maybe i was calling this by the wrong name - maybe i should refer to it as a barn with a wood flooor...

Thank you for that link!  (post #187311, reply #6 of 12)

Thank you for that link!  That will be very useful.

Do some reading on another forum (post #187311, reply #10 of 12)

Go here and read the whole thread.  All of it.


"A stripe is just as real as a dadgummed flower."

Gene Davis        1920-1985

Wood Floor (post #187311, reply #12 of 12)

I made the mistake of not using pressure treated lumber when I decked in my Garage Floor when converting the space to a kitchen.  Even though I had a vapor barrier under the slab and used PT everywhere it contacted cement, the joist were not 18" off the ground and the building inspector in my county would not approve it.  I eventually had to cut in ventilation, install a vapor barrier above the slab,  and after three months of hagling they reclassify the joists as sleepers.  Finally the job was done, but the lost time and aggravation was enough to make me wish I used all PT lumber to begin with.

Might want to check if you are having inspections.  Otherwise, the floor should look sweet!

Scott Jarvis