Hi folks, great forum.

My BR-111 3/4" Angico 4” width flooring has arrived. During my sub-floor prep, I’ve removed the kitchen linoleum and the ~3/16 luan underlayment so that the kitchen is level with the adjoining great room. Therefore, the entire sub floor is Advantech (either 3/4" or 23/32” – hard to tell) on 24” OC joists.

BR-111 literature states that this is an acceptable subfloor thickness, but doesn’t state if 24” OC joists are acceptable with this thickness. Do I need to build this up, and if so, with what and by how much? Other townhomes have HW installed over the Advantech alone, but I want to make sure I’m doing this right and don’t have problems down the road.

Thanks!

IMO, 24" OC is too far apart for 3/4" solid hardwood. The problem can be too much deflection between the trusses/joists. You can test this by placing a straight edge on the subfloor spanning some joists. First, check if the subfloor is flat or if it dips a little between the joists. Then, have someone or something heavy stand on the midway point between the joists and bounce a little. Does the subfloor deflect much? All subfloors would deflect a little bit even at 16" OC but at 24", the deflection can be much greater causing potential noises and an uneven surface. I had this problem on a b. cherry floor when a builder spanned the trusses 2' OC. I cannot tell how much deflection you now have. If it is very minimal and you are laying the floor at 90 degrees to the joists, you may be OK. It would be better to add another layer of plywood, 3/8" at least with 1/2" preferable.

You'd be Ok if the OC spacing is 19.2 "....which is normally the case for floor trusses. If they are 24" OC you gotta add more plywood Chip ,or, block from underneath.

The 2007 NWFA guidelines state a minimum 7/8" T&G plywood or 1" OSB T&G. They consider AdvanTech same as OSB.

Thanks guys for your usual prompt and detailed responses - I learn something every day here.

Since 24" OC joists just seemed odd, I checked another area of the kitchen/great room. There, the subfloor nailing patterns and the studfinder show joists 12" OC.

The kitchen/great room is 23' wide by 15.5' deep, but there is a bump-out that is 14' wide by 6' deep. That's the area that shows the 24" OC joists. Possibly the builder didn't foresee much traffic there or there is some 'engineering' concept at work.

Still think I should build up the bump-out (and thus the whole kitchen/great room area to make it all level)?

Correction to my earlier post - joists are 16" OC not 12" OC.

So only the bump out is 24" OC? If so, just add blocking between the trusses. It may be tricky getting to them but worth it not to have to cover the entire floor with more plywood. I'd guess the reason for the 16" is the span of the joists. Engineered trusses and joists can span longer distances if they're closer together. Since your bump out is a shorter distance, they saved a few trusses. It's not the builder; it's the architect who spec'ed those plans.

Gary -

Great alternative! Yes, only the 14' x 6' bump out has the 24" OC joist spacing. Your approach is definitely worth considering since the remainder of the area conforms with subfloor requirements (23/32" AdvanTech over 16" OC joists).

Since I'd have to pull up the subfloor in that area to gain access to the joists, would you be able to point me in the direction of some resources that would show me how to block the trusses from above? Another opportunity for me to learn something new! Thanks again -

Gary (or others),

My (admittedly unscientific) test gives these results:

1. I placed a 6' (3/4" x 4") HW floor board across the 14' wide by 6' deep section with the 24" OC joists.
2. I planted both feet exactly between 2 of the joists and bounced up and down.
3. I noticed that both ends of the sample board rose up off the subfloor about 1/8". When I did the test on the section of the floor where the joists are 16" OC, I could just barely see any movement at the ends of the sample board.

So the question for your experience is, given the stablity added by stapling and end-matched boards, is that amount of flex within tolerance, or do I run the risk of a squeaky floor down he road?

Many thanks -

May help to know that I weigh in at about 185 -

All I can tell you is that 3/4" OSB over trusses 24" OC is NOT in spec and both NOFMA and the NWFA recommend against installing a floor without rectifying the situation. Of course, it's your house and you can do whatever you want. Typically here on this board, we pros stick with the recommendations of the flooring industry. So, that being said, if you choose to lay the flooring on a subfloor that does not meet MINIMUM industry standards, you do so at your own risk. I will not stick my neck out and give you my blessing.

Gary -

The pro opinion is what I was looking for! I'm goin' in, wish me luck. You all provide an invaluable service on this board and it's always appreciated -