Wood framed floor strong enough for a car?

IHDiesel73L · Mar 23, 2014

I have some issues with the site for my pole barn such that I don't think I'll be able to build one. Basically I've got a lot of old tree roots and stumps that were ground down and covered with about 6" of topsoil right around the perimeter that will make boring the holes for the posts impossible. If I were to dig them out I'd likely have holes the size of a Volkswagen to backfill, so that's not really an option either. Now I'm looking at shifting the footprint slightly and putting up a building on skids. At least this way if I get a little settling I can just jack it shim it easily. I've poked around online and seen pre-fab garages that use 4x4 runners spaced about 4' OC with the platform on top built from 2x4s 12" OC and 3/4" CDX subfloor. I don't know what kind of loads (LBS/PSF) to plug into a span calculator for a car, plus all of the other junk I'll be putting in there. Can anyone give any guidance on this? The building will be a 16' x 24' gambrel barn with storage loft. I'll be keeping all of my OPE in it (lawn tractor, power washer, generator, log splitter, push mower, chainsaws, etc...) all of the time, and pulling cars in (5000-6000lb SUV/truck) for maintenance once in a while.

JimRB · Mar 23, 2014

6000 pound truck weighs 4000 up front and 2000 in back. Divide front by 2 and you have 2000 pounds. Pretend the foot print of tire is 1 square foot. Load is 2000 pound per square foot. 2x4 and 3/4 plywood will not handle that load. If you look at old bank barns you will see people parked tractors and trucks inside. Bank barn is a barn built in the side of a hill. They use real 2" or wider lumber for joists and often 10-12" deep. Those joists are covered by full 2 or 3" thick 2x8 or 2x10s.

When I ran spruce/pine/fir structural 2x4 40 pounds a square foot load it showed a span 6' 4". Load only goes to 100 psf.

What you see on line are probably prefab sheds. Not really what I would think of as a garage. I think of garage and I think of the ability to park heavy equipment like trucks in and the ability to use a floor jack to change the brakes. I would not trust a single layer 3/4 plywood on 2x4.

Have you poked around with a probe to see where every trash hole was dug and marked the edges of every trash hole? Maybe there some sort of array that would let you pour rebar reinforced footers that spans trash holes and build a traditional 2x6 framed garage with a dirt floor. One of my front porch supports is right where my geothermal line was dug. I made a form that spanned that trench and landed on undisturbed dirt on both sides. I stuck some rebar in the middle of the concrete. No sag in 16 years but the dirt in the trench has settled a few inches.

dave1949 · Mar 23, 2014

Think about, or look underneath, how an equipment trailer deck is built. They will handle the weight. You could consider an I-beam "skid" frame with C-channel stringers. Then screw PT wood 2x6 floorboards to the stringers. A step down from that for strength, but no welding, would be PT skids and stringers.

If you built it so that the floorboards could be replaced without disturbing the wall support, (don't set the wall on top of the floorboards) that would allow you to replace/repair individual boards easily if needed.

KennyG · Mar 23, 2014

Jim is being a little conservative. When you put plywood on a beam structure, you create a diaphragm structure and transfer the load over a greater area. However, 2x4s are a little light. Your 6000 lb truck is probably going to spread the load over at least a 100 square foot area. Look at the span tables for 60 to 100 psf and see what it will take.

IHDiesel73L · Mar 23, 2014

KennyG said:
Jim is being a little conservative. When you put plywood on a beam structure, you create a diaphragm structure and transfer the load over a greater area. However, 2x4s are a little light. Your 6000 lb truck is probably going to spread the load over at least a 100 square foot area. Look at the span tables for 60 to 100 psf and see what it will take.

I ran #2 Doug Fir 2x4s through a span calculator with the following specs and was surprised:

2 x 4
#2 Doug Fir
12" OC
100lbs/psf Live load
20lbs/psf Dead load

Max span - 5' - 0" :applause:

So it appears that 2x4s are adequate, but I think I'd go 2x6s for the extra margin.

newbury · Mar 23, 2014

Have you thoroughly checked out your previously determined post hole sites? How were you going to dig the holes?

With a strong powered auger you might be able to dig thru them.

Just yesterday I was powering thru roots with a PHD on my Kubota M4700 (42HP PTO) .

vtsnowedin · Mar 23, 2014

Don't screw around with it. Dig out your 16X24 foot hole 24X32 x three feet deep and backfill with clean sand or gravel capped with a foot of crushed stone and a concrete slab set high enough so the water drains away from it. Then you can build something on it that will last longer then your grandchildren. Most stumps are only 18 to 24 inches deep and the main roots usually are small enough to break with a back hoe bucket four feet from the trunk. Once you get the one in the center out the rest will come easy.

ace10 · Mar 23, 2014

How about driving the posts?

HillStreet · Mar 23, 2014

My family home growing up had a barn with partial basement underneath. My father parked small trucks and tractors on the main deck over the basement.

The deck was plank boards over 4" beam joists. That building was about 70 years old when we took it down.

Talk to a sawmill about your timber, rough cut wood is a good thing to use. Plywood won't give you the strength you need.

IHDiesel73L · Mar 23, 2014

newbury said:
Have you thoroughly checked out your previously determined post hole sites? How were you going to dig the holes? Just yesterday I was powering thru roots with a PHD on my Kubota M4700 (42HP PTO) .

Holy crap! That's one **** of a machine! I was planning on renting one of these: