I am planning on laying a product called "strand woven bamboo". The bamboo is made by "weaving" strips of bamboo and gluing at high pressure. The resulting wood is very hard - about twice as hard as a hardwood like oak. My strips are 9/16" thick tongue and groove, 3.75" wide, and 6' long. The manufacturer recommends laying over a minimum of 3/4" plywood.

I have 5/8" ply over joists 16" oc, so I am officially under the spec'd subfloor, but seeing as the biggest sag between joists I can find is 1/32 (top of the stairs), I would probably just lay my wood across the joists.

But, I discovered yesterday that the direction the joists run switches in the middle of my biggest room. I really don't want to switch the direction the flooring runs, and am not keen on laying it at 45 degrees. My room is long and narrow - about 45' long and 12' wide.

I do not have easy access to my joists to cross-brace.

My question is, how much extra sub-floor do I need? From reading around on this board, I have seen an extra 1/2" ply recommended for laying parallel to the joists. I have an old hardwood flooring book from the library and it states I need at least 1.25" subfloor for laying parallel to the joists, but that implies I need another 5/8". While my bamboo feels incredibly stiff, it is thinner than your usual solid hardwood.

I know to lay the new ply perpendicular to the existing ply, but:
1. Is screwing sufficient, or should I also glue?
2. Should I use tongue and groove?
3. If not T+G, should the sheets be butted right together or is a gap recommended?

TIA.

Doug.

The stiffest method to install an underlayment is perpendicular to the joist, so since this is a thinner flooring I would install it the same direction as the subfloor. Keep all the joints offset and don't be too concerned about screwing into a floor joist.

I would use a 1/2" BC grade square edged plywood and screw it down also using some squiggles of yellow wood glue as you go. Leave an 1/8" gap all way around each sheet.

You can use a T&G... but it is not necessary for this and you are not likely to find any T&G plywood under 5/8" thick anyway. Your walls are already up and using a T&G would get in your way every time you got near a wall. I'm mentioning square edge plywoods only for underlayment applications ... not for single layer subfloors.

Thanks Jerry.

What is "BC grade" plywood? I'm a Canuck ...

I spoke to my distributor (http://www.builddirect.com) and they told me it was recommended to lay across the joists on 3/4" ply, but if I was going with the joists, the recommendation was still 3/4".

After a little more ripping up carpet today, I realized that in ~40% of my biggest room where the joists run north-south, the joist spacing is 12" vs. 16" in the rest of the room (this is over the garage, so I assume the span is longer). To try to guage deflection between the 12" joists, I picked up the dog (total weight > 210 lbs) and bounced up and down in between the joists and had my wife look for deflection with the carpenters square. None was visible. Our house is ~12 years old.

Is it possible that I will be OK parallel to the joists with a 12" joist spacing?

I understand the first concern about laying parallel to the joists, is squeaking of the nails as the boards flex. Can I glue (primary or supplement) my section parallel to the joists to minimize that?

The fly in the ointment, is that to lay parallel to the joists in the 12" oc section and across the joists in the 16" section, I would be laying the boards perpendicular to the long direction of the room. It is a fairly long and narrow room: . My instinct is to lay along the long dimension of the room, although I have read that that will make the room look even longer and narrower. There are stairs going down and up leaving from the left side of the room that we are going to lie bamboo on, so the direction of the bamboo would switch 90 degrees when going from the stairs to the room. Any comments on which way the bamboo should go?

I would prefer to avoid putting extra plywood down to stiffen the floor. Not so much to avoid the work and cost of that (although I'd obviously prefer to avoid that), but that would likely raise the floor enough that I would have to do something with the adjoining kitchen and nook floors that would now be 5/8" lower.

When looking at the stiffness of plywood, the stiffness of 5/8" plus 3/8" is much less than the stiffness of 1". If I glue and screw 3/8" to 5/8", will I get anything more than the sum of the stiffness of the two?