Reinforcing wooden garage floor for vehicle

Submitted by nalc t3_11mxkt4 in DIY

Hi, I recently bought a house with a 12x24 ft garage. It's got a wooden floor and the previous owner would park his car in it, but I'm a little nervous about the floor holding up.

The floor consists of five longitudinal 4x4 PT skids on a compacted stone pad, 34" on center. Above that are 2x4 PT joists laterally, on 16 inch centers. Then it's 5/8" plywood for the floor proper.

I identified a couple courses of action to reinforce it and was curious if folks had any suggestions.

  1. Remove the existing plywood flooring then add 14.5" pieces of PT 4x4 longitudinally above the center three skids (which are about the track width of a car) so that there is additional compressive strength. In this setup the floor is prevented by sagging between the 16" OC joists, and the entire weight of the car should mostly go through the stacked pair of 4x4s

  2. Add a second layer of 5/8" plywood to the entire shed floor

  3. Add two longitudinal 2x12 planks for the length of the shed to support the wheels, above the floor.

  4. Using a circular saw set to 5/8" depth, cut out two 12" longitudinal strips out of the floor the length of the shed, then add the 14.5" 4x4 pieces above the skids from Option 1, then install the 2x12 planks above that. Add 7" vertical braces from 2x4s or 4x4s to the sides of each joist to prevent sag in the middle

  5. Add the PT pieces from Option 1 and the second layer of plywood from Option 2.

Options are diagrammed here: https://imgur.com/oPnCNGq

Big picture wise I'm not sure if the approach should be to spread out the load of the car as much as possible by reinforcing from the top, or whether to reinforce from below by adding more structure in compression directly below the wheels, or both. PT in compression should be good to at least 1ksi which is a 20x static safety factor for a 50 psi tire load, but it's not directly below the tires. I've looked at a bunch of span charts but they don't seem to have what I need, which is a ~1,100 point load that is not centered on a very short span.

There's also some more aggressive options not shown, such as adding a second set of 2x4 joists between the existing ones (so it's on 8" centers, between the three center skids) probably in conjunction with the Option 1 pieces. Or doing Option 1 but then filling in the entire center cavity with gravel, which would give it a ton of compressive strength but I'd have to be worried about drainage/rot.

And I guess to head off the inevitable, yes I know that a wood floor is not ideal for parking and if I was starting from scratch I'd get a concrete pad poured and build the garage above that, but at this point is is not feasible to go that route. So my options are reinforce the floor (which has had a smaller car parked on it for several years without any noticeable damage) or park outside.

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Comments

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athermalwill t1_jbk7k05 wrote

Does the floor show any signs of buckling or sag where the previous owner parked?

If not, I would probably just double down on the sheathing with a layer of 3/4” advantech flooring.

If it does, more sleepers and blocking between the 2x4s is probably the practical way to go.

It sounds as if nothing serious would happen in the event of a failure.

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foxhelp t1_jbkduuk wrote

Replying to athermalwill (#2,194,289)

Agreed, if there is no sagging I would go this route.

if there is sagging then I would just add another 4x4 skid mid point between the skids and then add the 3/4 advantech.

But I am not a professional builder!

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nalc OP t1_jbkhl9y wrote

Replying to athermalwill (#2,194,289)

There's no buckling or sag, but I think my car is a fair bit heavier (and higher tire pressure) than what the previous owner was parking.

The Advantech seems pretty reasonable for 9 sheets, and certainly easier to put down a second floor than to try to cut up the old one.

I might go at it with a pry bar and see how easy the current floor panels come up. I like the idea of prying them up for the 4x4 sleepers on top of the skids but if they're a pain in the ass to get up then IDK, maybe it's better to cut out slots with a circular saw set to 5/8" depth. Or perhaps only add sleepers in the specific joist bays where the car would be parked, not all of them?

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ValidGarry t1_jbld188 wrote

Loadspreading would give you peace of mind without structurally altering the floor or ripping it up. Rather than 2 x 12, you might look at ripping some sheets of ply to 24" strips and putting them down. Since you want to spread the load wider, going up from 12" would give better results. 12" would probably work, but spreading is spreading and wider is better.

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nalc OP t1_jblpxmo wrote

Replying to davidmoffitt (#2,195,372)

>WTAF would tire pressure have to do with anything?

The contact patch of a tire is the weight of the vehicle divided four divided by the tire pressure. So if you're calculating a maximum loading it's kind of important. I don't have any doubts of the floor's ability to support the weight of the car if it was evenly distributed over 280 sq ft. But it's not, it's distributed over four 0.2 sq ft contact patches each totaling about 1,000 lbs. So obviously the worst case scenario is the weight of the tire right in the middle ~2" of a 14.5" unsupported span of 5/8" plywood causing the plywood to crack. The compressive strength of wood is on the order of 1 ksi so I'm not worried about the 4x4 skids or even about the joist compression (since each 2x4 joist has about 5 sq in of contact to the skid). I have struggled to find good resources on plywood strength over short distances with very concentrated loads so that's why I'm asking for advice.

Would you rather someone drop 100 lbs bag of feathers or 100 lb of water on your head? I know which I'd prefer.

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nalc OP t1_jblqdpe wrote

Replying to ValidGarry (#2,196,093)

Hm, that's a decent idea. 3/4 T&G plywood cut into 2 ft x 4 ft strips and nailed down with the joints over the joists would be quite easy to install.

Any thoughts on OSB vs real plywood?

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bigloser42 t1_jbm2z5m wrote

Replying to nalc (#2,196,650)

For the purposes of what you are doing, the tire pressure & contact patch size is almost irrelevant. You’re talking about a difference of a few square inches at best. Weight is far more important.

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ToolemeraPress t1_jbock1j wrote

Ok. You’re attempting to analyze carpentry when you have no experience in carpentry. Forget all the online “resources”.

Forget messing with the substructure which is part of what holds the shed upright.

Layer 5/8” exterior grade ply at right angles to the existing ply. Use construction adhesive applied in lines approx every 12”, drive deck screws onto the substrate and 2x4s. Paint with garage floor paint.

Stop watching DIY youtube by people who know nothing.

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