# Subfloor leveling, advice needed



## 1baltic (Oct 4, 2007)

Hello everybody.
I am wandering if someone could share his opinion or experience with me. 

We need to install solid hardwood floor over 1/2" plywood subfloor which is not level. The floor slopes to the center of the room more than 1/2". I am afraid that plank joints will be not even, gaps here and there. Would be better to level the subfloor as much as possible. My idea is to pour down few bags of self leveling mix and cover all floor with 1/4" plywood. I know that SF mix does dot stick very well with plywood, might start cracking. But we have old hardwood floor under the 1/2" plywood. So platform is kind of solid.

Has anyone done that? Did it work? This must be a common problem in old houses. I wander what others usually do.

An advice will be appreciated.


----------



## Taranis (Feb 23, 2007)

This would work.
First cut and fit your ply without nailing it.
Mix and lay your latex screed, levelling it off with a straight edge.
As it is firming up put the ply onto the floor then nail/screw as normal.
Latex screed would normally crack up and debond over timber but as you are fitting ply over the latex this wouldn't be a problem. The latex would stick to both surfaces and hold firm.
We use this method a lot in older houses.


----------



## 1baltic (Oct 4, 2007)

Thank You.
By latex you mean thin-set tile mortar or quick-level mix, which does level by itself although does not stick as good as thin-set?


----------



## Taranis (Feb 23, 2007)

A two part (latex liquid/cement based powder) scree that is available practically anywhere. If you used a tile mortar it would not settle as well cos it's stiffer. using a straight edge will mean less chance of a low point.


----------



## Floordude (Aug 30, 2007)

It is a bad I dea to use a portland cement. The moisture in the mix will absorb, into the subfloor and the hardwood beneath(could buckle it), then you going to be waiting days for the subfloor to lose the moisture it absorbed, before you go over it with an underlayment. I know you feel the subfloor is stout enough not to crack, but is the structure stout enough to handle the extra weight?

The floor is sagging already for a reason. Fix the sag structurally, or it will just be a bandaid, when you really need stitches.


----------



## A W Smith (Oct 14, 2007)

A half inch sag in how many feet? Whats the span overall? For an old house that doesn't sound like that much. Especially to interfere with a hardwood floor installation.


----------



## woodmagman (Feb 17, 2007)

A ½” sub-floor is not sufficient for hardwood flooring. It is not sufficient for any kind of flooring unless of coarse the joisting is 8” on center. And a ½” for a floor is so common that I would be surprise if you were installing on a level floor….


----------



## A W Smith (Oct 14, 2007)

woodmagman said:


> *A ½” sub-floor is not sufficient for hardwood flooring. *It is not sufficient for any kind of flooring unless of coarse the joisting is 8” on center. And a ½” for a floor is so common that I would be surprise if you were installing on a level floor….


Not anymore but millions of suburban homes built in the sixties and early seventies had nothing but 1/2 inch fir plyscord cdx sub floor with oak strip flooring throughout and particle board underlayment to match finish for lino and vinyl tile elsewhere. He mentions the old house has existing 3/4 inch sub floor under the 1/2 inch anyway in his original post. Tongue and groove oak strip flooring will easily distribute the weight of traffic and concentrated floor loadings to structural members spaced 16 inches O/C below with no need for 8 inch spacing. the flooring itself enhances structural performance in that matter as long as the flooring is run perpendicular to the joists.


----------



## woodmagman (Feb 17, 2007)

1baltic said:


> Hello everybody.
> I am wandering if someone could share his opinion or experience with me.
> 
> We need to install solid hardwood floor over 1/2" plywood subfloor which is not level. The floor slopes to the center of the room more than 1/2". I am afraid that plank joints will be not even, gaps here and there. Would be better to level the subfloor as much as possible. My idea is to pour down few bags of self leveling mix and cover all floor with 1/4" plywood. I know that SF mix does dot stick very well with plywood, might start cracking. But we have old hardwood floor under the 1/2" plywood. So platform is kind of solid.
> ...





A W Smith said:


> Not anymore but millions of suburban homes built in the sixties and early seventies had nothing but 1/2 inch fir plyscord cdx sub floor with oak strip flooring throughout and particle board underlayment to match finish for lino and vinyl tile elsewhere. He mentions the old house has existing 3/4 inch sub floor under the 1/2 inch anyway in his original post.


I do not see the 3/4 in the post.
That is why we all say "They don't build house like the use too.":laughing:


----------



## A W Smith (Oct 14, 2007)

woodmagman said:


> I do not see the 3/4 in the post.
> That is why we all say "They don't build house like the use too.":laughing:


 
I presume old hardwood flooring would be at least 3/4 inch


> *But we have old hardwood floor under the 1/2" plywood. So platform is kind of solid.
> *


----------



## RLGC (Mar 4, 2007)

Woodmagman I have been in construction for 30 years and a 1/2" sub floor under hard wood is common.


----------



## Taranis (Feb 23, 2007)

12mm screed in the centre of a room will not be enough weight to deflect a subfloor. If it would there should be real worries over the structure. A couch or a coffee table will carry the same weight. 1 or 2 people will carry the same weight.
I've seen far more movement than this in old buildings.
And where I come from 150 years is considered an old building.


----------



## Floordude (Aug 30, 2007)

Confusion in the terms and phrases we use...


In the flooring world, the subfloor is attached directly to the joists. When I framed we called it decking.

Additional plywood, OSB, or particleboard is called underlayment in the flooring world. When I framed, we called it subfloor.

I believe they stated ...


> But we have old hardwood floor under the 1/2" plywood. So platform is kind of solid.


a ½" low spot and how to fix it, is going to depend what the clients expectations are and the budget you have to work with.


----------



## neolitic (Apr 20, 2006)

Floordude said:


> Confusion in the terms and phrases we use...
> In the flooring world, the subfloor is attached directly to the joists. When I framed we called it decking.
> Additional plywood, OSB, or particleboard is called underlayment in the flooring world. When I framed, we called it subfloor.


I'm with you, Sub floor goes on the joist, underlayment goes _over _the subfloor and _underlays_ the finished floor.:thumbsup:
Gets confusing for me when guys refer to sub floor as underlay, and vice versa.
Makes me think it's a HO post.


----------



## 1baltic (Oct 4, 2007)

*More details*

Thanks, everybody for advices. I really appreciate it. I did not tell enough about the existing structure. Sorry.

It is a condo unit. 2nd floor. 320 SF living room where birch 3/4" plank flooring has to be installed. Building is old. Probaly floors have settled in every other unit as well. We removed carpet and found 1/2" plywood installed over old solid oak floor. Of course there must be 3/4" or 1/2" subfloor under the old hardwood as well. So the existing platform is pretty solid. 
I was gonna prime the top plywood coat with bonding primer/sealer (sold with level-quick powder), tape plywood joints - so leveling stuff does not go down to the neighbours or penetrate to much into old hardwood floor. Then I would lay down new 1/4" ply on liquid nails, screwed thoroughly. Then it would be ready for new flooring.

I wonder what flooring pros do when thay have uneven, unlevel subfloor.

This must be a common problem in any older house or appartment.
Actually I have not seen straight, and level floor or plumb wall yet (we have been in kitchen and bath bussines for 5 years). 

Again, thanks everybody for your time.


----------



## 1baltic (Oct 4, 2007)

We are not in new construction. When remodeling or renovating, we sometimes do not reach deeper than top coat of plywood, which we find after ripping off finished floor. So anything we find under finished floor we usually call a subfloor. Anyway, sorry for the confusion.


----------



## PrecisionFloors (Jan 24, 2006)

RLGC said:


> Woodmagman I have been in construction for 30 years and a 1/2" sub floor under hard wood is common.


It doesn't matter how long something is done.....if its wrong its still wrong, time won't change that.


----------



## A W Smith (Oct 14, 2007)

*Just because it is code now doesn't mean it was wrong then*



PrecisionFloors said:


> It doesn't matter how long something is done.....if its wrong its still wrong, time won't change that.


so if you had to replace oak strip floor in a sixties or early seventies tract home in a single room of a home with hardwood throughout. Would you extract the 1/2 inch sub floor and replace with T&G 3/4 inch ply wood? Or would you just lay a new oak floor? keep in mind the flooring would be running perpendicular to the joists.


----------



## Floordude (Aug 30, 2007)

In the old days there was 33/32 flooring. They used it directly over the joists, without a subfloor.


----------



## Doug M (Nov 26, 2006)

my House is 100 years old and that is exactly what I have in mine.Nailed every 12" on each joist.
Doug M


----------

