Plywood decking thickness

[Ken]

How thick should plywood decking be assuming you are using joists 16" on center?

In my earlier years, perhaps 20 years ago, I worked for a builder who typically used 5/8 floor decking and 1/2" on the roof and exterior walls. Now I'm faced with an architect that insists 3/4" on the floors is a must and 5/8" on the roof and walls is needed.

Does the floor decking depend on the finish flooring. The builder I worked with would always install wide plank 3/4" hardwood flooring. But when I mentioned that to the architect, he insisted that I should still use 3/4" decking under it. His concern was regarding the holding capacity of the nails for the hardwood flooring and that it would avoid squeaky floors later.

Any comments?

[hallerb]

I agree with him and it might be code in your area, besides floor decking today is probably chip board, and that should be heavier

[tbasc]

3/4" T&G deck sheathin is pretty standard in Charleston SC for quality work. 1/2" walls are usual. Roof sheathing might be 1/2" in cheap work or 5/8" in engineered work. Are you in a seismic or high wind area? TB

[BiloxiBoy]

But, is it "really" 3/4??? or 23/32??? Whatever happened to the good old days when 3/4 was really 3/4?

Anyway, I'd agree with your architect - there will be less give, it will nail better to the joists, and it will hold the nails better for your finished floor.

[SQLit]

Cant answer, the loading of the floor is unknown (pounds per square foot) , and the span is unknown. 3/4 T&G plywood is springy under my 280 pounds of weight with 16 inch centers.

[pontiac]

1/8" is minimum. I always use 3/16" hardboard paneling across 48" spaced joists. Just be sure to run a strip of duct tape across the joists every couple feet and be sure the carpeting is tight to support any extra weight. I have found it satisfactory except the time I moved a refrigerator and both I and the fridge fell thru into the basement. I know I should have put more duct tape across the joists where the fridge goes.

[chris jung]

Our house (a raised ranch) was built in 1967. The floor joists are 16" apart, the subfloor is 1/2" plywood with 3/4 tongue & groove oak over that. The floor is squeaky everywhere and you can actually see it flex. In March, we are having the main hallway and a smaller hallway/foyer floor ripped up and new 3/4" subfloor and new 3/4 oak put in. It's a pity since the original oak flooring is very handsome. We can't afford to redo the subfloor & flooring on the entire house at this time.

If the builder had done it right the first time (put in 3/4" subfloor) we wouldn't have to through all this expense and hassle now. So my advice is to go with the 3/4" - you want that subfloor as stiff as possible.

Chris

[Michael Keefe]

Anything less than 3/4" of the plywood you get today will be unsatisfactory.. I used 1/2" on one of my floors and even though I'm only

150, I can still feel it give (1/2" underlayment, vinyl).. The rest of my house I used 3/4" and it's rock solid.. Screw it down, too..

[Chub]

why not just put new flooring on top and not go through ripping out old stuff?

[Goedjn]

The architect is right, if you want a good floor. the builder is right if all you want is one that's code-compliant.

Of course, if you wanted a good solid L/480 or better non-squeaking floor, I hope you're bringing that up before the framing starts, cause you'll also want depper joists, more blocking/bridging, glue, and spiral-shank nails instead of whatever crap comes out of the pnuematic nailer. All of which is going to cost you extra.

[chris jung]

For a few reasons. First some background. We have a apartment in our lower level. One bedroom is under the hallway and there is a metal furnace duct running the length of the hallway between the joists and ceiling. Sound (both airborne and through vibration) travels and is even amplified by that duct. To stop this it's recommended to wrap the duct in Mass Loaded Vinyl ( go to

for more info). To get to the duct we would have to either go from below (through 2 layers of drywall) or above. We decided to get to it from above.

If we were having all the wood floor redone (1250 sq foot) then we would consider having the old subfloor & oak floor screwed together to make a new subfloor and then put a new floor on top. To put a new floor on the entire

1250 sf would cost a big chunk of change. And as well, the kitchen and 2 bathrooms are tile (at least they were done right) and we would end up with them being at 3/4" lower than the wood floor. With our plan, the difference in floor height between the tile and wood floor will be 1/4 inch.

And lastly our house is a split entry with two half flight of stairs leading up to the wood floor. From what I understand, it's important that stair risers are the same height otherwise people will catch their toes on the highest step. I'm not sure if a 3/4" taller top step is a true safety issue but it was one last factor that influenced our decision.

Chris

[Calvin Henry-Cotnam]

For floors, the subfloor makes a difference on how much it flexes. We used an engineered system (I-joists) and the span capabilities depended on the thickness of the subfloor (5/8 versus 3/4) and whether it was just nailed or glued and nailed. For a firmer floor, we went with the 3/4" and glued and screwed it in place. Only tongue & groove for the floor.

I used 5/8 T&G for the roof (24" O/C trusses), but the first framer I hired balked at using this -- they often just use 3/8" plywood with H-clips. I hired someone else to finish the roof.

My parents' home was build in 1963 and has 1/2" plywood on a stick-framed structure that is 16" O/C -- there is no way I would use that on 24"!

As for wall sheathing, our code allows 1/4" OSB or plywood -- even 1" styrofoam sheets can provide the anti-racking support that is needed. I went with 7/16" OSB, but it starts at the sill plate. This means that instead of two horizontal runs of sheathing on each 8' wall and short strips at the end of the floor structure, the first course starts at the sill and extends about 3' up the first floor walls with another course above it. The third course of sheathing starts a foot below the top of the first floor wall and continues to about 2' up the second floor wall. This IS harder to install, but it provides extra anti-racking strength, plus it provides a huricane tie between the walls and the floor structure (not that we are in a huricane zone).

[Calvin Henry-Cotnam]

It likely isn't even 23/32!

Here in the land of half-assed-conversion-to-metric (Canada), our plywood and OSB are manufactured in millimetre thicknesses, despite being 4' x 8' in the other two dimensions.

3/4" is about 19.05 mm, so it is rounded down NOT to 19 mm but to 18 mm!

18 mm is 0.70866", while 23/32" is 0.71875".

23/32" is the closest fraction (without going below 1/32" increments) to 18 mm.

[JillAdams]

depends on how heavy the people are who will be walking and standing on the decking...