Fist off, I would like to say that the information I've gleaned from these forums has been invaluable.

Now, here's my question. I am getting ready to install 5/8" x 3 1/2" Brazilian Teak flooring. My house was built with 2 1/4" SYP strip flooring as subfloor and finished floor. Total thickness is 3/4". I am going to be installing the new floor in the same direction as the old. From everything I've learned so far, I should add a minimum of 3/8" plywood underlayment over the old flooring to do this. My question is , Why? Why is a 1x6 subfloor acceptable, and a 2 1/4" hardwood subfloor not acceptable.

Also, if I decide to go with an additional layer of underlayment, would 7/16 OSB be acceptable.

Thanks in advance for any insight

Karl

Hi Karl,

It is common building practice to never marry solid wood boards in the same direction. It would not be ok to install a new floor in the same direction as a 1 x 6 solid board subfloor. Most of those 1 x 6 subfloors are installed at 45 degrees to the floor joists, so one could install in either direction, just not the same direction as the subfloor boards. The same applies to going over an existing hardwood floor. If you need to run the flooring in the same direction, NOFMA recommends either adding 1/2" plywood OR adding adding blocking between the floor joists.

I guess it's just the woodworker in me, thinking you’re better off mating long-grain to long grain, than to join two boards cross grain where you need to allow the wood to expand and contract.

The blocking is an intriguing idea . What would be the requirements if I was to do something like that? How far apart would I want to install the blocking? Would staggering the blocking be OK, so I can get a impact driver driving screws straight into the blocking on each side?

If I was to use 1/2" underlayment. How would I transition down the 1-1/8" to meet the existing floors in the other rooms. I plan to carpet and tile them eventually (probably won't happen this year) at which time, I guess I could build them up, to match, but until then, What do I do?

Karl

Karl, Think about it. Why was plywood invented? For what purpose? Is it more stable? And why? It is the same reason engineered floors are more stable and less prone to expansion and contraction.

But about blocking between joists. Assuming your joists are 16" oc, you could simply use 2 x 4's on edge, stagger/offset and nail through the joists into the ends of each block. Quite easy IF you have the headroom. A pneumatic framing nailing would make it very easy and nails have more shear strength than screws. Block every 16" between the joist bays. Precut your blocks and you'll be done in no time, assuming you not on your back in a crawl space.

I understand the strengths and benefits of plywood. How it is stronger than “wood” and provides shear strength and everything. I guess I’m just confused about what happens if you don’t use it and you run your new flooring parallel with the old. Since the two layers are expanding and contracting in the same direction, wouldn’t it be less likely to have problems. If not, what type of problems would I have? Also, If I laid the floor perpendicular to the existing, it would be parallel with the joists, and wouldn’t I have issues with sagging between joists? It would seem like that would be when I would need to stiffen the floor since my total existing thickness is only ¾”.

Regardless…



The blocking sounds like material costs would be less, but my labor would go through the roof, since I do have a crawl space and would be on my back for all of it. Plus, my joists are 16" oc, but are not necessarily 1 – ½" thick. Some of them are upwards of 2" - 2-½" thick. (It seems like my house was put together with scraps. )

So plywood it will be. Should I quiet any squeaks, and tighten up loose boards, before I put down the plywood, or will the nailing schedule for the plywood do that for me? Also, am I correct that the nailing schedule should be every 6” around the perimeter and 8” in the field? And would it be advisable to lay down felt paper both under and on top of the plywood, or just on top of (between the plywood and the new flooring) the plywood?

I also have another problem I’m trying to come up with a solution for. I have a 6’x9’ nook coming off my dining room. It is on an outside wall. It was added on after the house was built, and the outside wall of the nook has settle ~3”. So, that floor slopes away from the room. The ridge were it meets the dining room, is flat. I don’t plan on jacking the house to fix the slope. It has settled as much as it going to. Shimming the joists and laying new subfloor would make the nook look even more out of level, since I have radiators under windows and they would be out of level with the windows then. I was thinking that since I’m running the floor parallel to the ridge, and would be reversing into the nook anyways, could I just bevel the groove side of the first row (either with a plane or jointer) going into the nook, and continue on. My other though was to just separate the nook with a T-molding. What do you think about either of those ideas.

I really appreciate the fact the you are so willing to help total strangers with your knowledge and guidance. I enjoy doing these types of projects and learning the correct ways to do them. I’ve done quite a few projects around the house (e.g. replace hot water boiler, building a 12x16 barn) and have learned quite a lot from doing them. So, Thank You for all your help.

Karl

Since solid wood (plain sawn) expands across the grain (width of board), one could expect gapping to occur. But at potentially greater amounts than normal, since not every board expands and contracts at exactly the same rate. If laid perpendicular, the non-expanding lengths of each layer help in restraining the expansion across the width of the other layer. And if it's not sagging now, adding more wood flooring will not cause it to sag. But to be on the safe side, adding plywood is preferred.



Yes, re-nail any loose floor boards before laying the plywood. That nailing schedule is the minimum for 1/2" plywood. Myself, I use a pneumatic stapler and just go to town, so my schedule is typically closer. It could not hurt but really isn't needed to install two layers of felt unless you see a lot of humidity changes in your local.



Sometimes, in older buildings that have settled and sloped, unless remodeling is in store, it's not worth the effort to make them level. Or, flattening the subfloor is not possible. Keep in mind, subfloors need not be level, just pretty flat, for the flooring to be able to be installed. It sounds as if your ideas, either one, would work. Also, whenever changing tongue direction, be sure to use a spline (slip tongue)



You are welcome.