Subfloor and Rain

[rothbard]

Howdy,

Was wondering if anyone used plywood subfloor and left it exposed to rain for several months.  I'm getting ready to lay it down and probably won't have a roof finished for 6 months.  Predicted amount of total accumulation is 5-7 inches in that time.

I had planned on using 3/4" plywood since I need a little bit of give due to slightly unlevel joists from one to the next.  Otherwise I would use the stiffer advantech OSB stuff that seems to have a better moisture resistance.

[jsahara24]

I only have experience with advantech, which was a positive experience but I only left it exposed for a month or so.  We did get quite a bit of rain...

If you aren't able to create a temporary tarp roof (maybe billboard tarp?), then make sure that there is a way for the water to get out....We drilled holes to let the water drain through, and make sure you cutout your door floor plates...

[MountainDon]

Personally I would not want to leave any subfloor for any appreciable time unless it was Advantech. That can be left for a couple months w/o problems.

[rothbard]

Thanks Don and Jsahara.  I'll have to go buy a sheet of advantech and see how flexible it is.  Guessing I'll need it to flex up to 1" / 8 feet.

I did see one interesting idea online, possibly I could double plate the bottom of the walls and then build the walls without the subfloor in.  I'll have to keep that one in consideration.

[JRR]

I wonder how it would work to cut up a sheet of plywood into strips, perhaps a foot wide and use these strips around the edges. .. between the floor joists and walls? This would leave the joists open with no large area plywood membrane to catch the wet elements.  Or. with careful measuring and planning, full sheets could be used in the same way as strips with the idea for full sheets to be added later to finish the floor.  It might make for a better outcome .. .. as long as someone doesn't fall on or through the open joists.  Ouch!

[rothbard]

I wonder how it would work to cut up a sheet of plywood into strips, perhaps a foot wide and use these strips around the edges. .. between the floor joists and walls? This would leave the joists open with no large area plywood membrane to catch the wet elements.  Or. with careful measuring and planning, full sheets could be used in the same way as strips with the idea for full sheets to be added later to finish the floor.  It might make for a better outcome .. .. as long as someone doesn't fall on or through the open joists.  Ouch!

Yep that's another good idea.  Only consideration is adding backing in the areas at either end where it no longer falls under a joist and the T&G would need to fit in after being boxed from both directions.

I ended up buying advantech and got it all laid down.  They claim it's good for 500 days without needing sanding to fix it.  We will see.  Despite being stiffer than OSB and Plywood it conformed to my joists just fine.  Guess they were more level than I thought.

Hopefully there is no pooled water next time I come back but if there is I will drill hole in any low spots.  That will force me to delay putting in the vapor barrier in the crawlspace until the roof is up but I imagine they'll be OK waiting until then.

[MountainDon]

I wonder how it would work to cut up a sheet of plywood into strips, perhaps a foot wide and use these strips around the edges. .. between the floor joists and walls? This would leave the joists open with no large area plywood membrane to catch the wet elements....

One thing that full sheets do, when nailed to the floor joists and rim joists under the exterior walls, is to tie the joists together. With the subflooring nailed in place and the exterior walls nailed on top the rims are held in place. The whole floor assembly is quite ridgid and the rims cannot be easily pulled away from the floor joists. What you propose would have the side rims maily secured to the floor joists ends by fasteners into the end grain. Easy to be pulled loose. Keep that in mind. Advantech lasts a very long time.