Level an Uneven Subfloor for Vinyl Tile

The following question on leveling a subfloor for vinyl tile was sent in by Roman:

I want to put vinyl tiles in my living room and bedroom, and this requires that the floor is smooth and horizontal enough. The subfloor is not too bad, I think. It is largely pretty smooth; however, it is too uneven for me to be able to place a chair or table so that they wouldn’t bounce. I hope there is a cheaper method to level the floor than to use a self-leveling compound, but so far I think it is the most appropriate one.
Here’s my issue. I’ll deal with each room separately.

The smallest one is not big (about 225 sq. ft) but, I think, is big enough for me not to be able to level the whole floor in one shot because the SLC hardens quickly. I imagine I’ll have patches of the floor that I need to work on while I’ll have other patches with already hardened concrete. I’m afraid I will have wavy surface because of this. Is this really a problem? Will this happen? If so, how should I deal with this? I would like to keep costs down by doing all this by myself. Is it practical or will I have to get a helper.

Roman, thanks for sending in the question.  There are a number of DIYers and professionals who read here.  This will probably get their attention as well. 

You are correct that to properly lay vinyl tile you’re going to want a smooth, flat, structurally sound surface.  That surface should not be the plywood subfloor; it will be too rough and wavy and isn’t designed to be a subtrate for vinyl.  It sounds like you know that since you’re already considering a self leveling underlayment (SLU) that would create a smooth surface to work on and also fix your level problem.

The problem with an SLU is that it’s expensive, complicated, time consuming, heavy, and possibly unnecessary.  Instead, take a look at SurePly from Patriot Timbers.  It’s a simple, staple down backer for Vinyl.  It won’t solve a floor leveling problem (I’ve written some ideas on that below), but it will save dozens of hours over the SLU alternative. 

If you decide to go with the SLU, the This Old House instructions I cite below discuss use of a mortar bed as a subtrate for vinyl. I’d start there for this method, taking into account the articles written about it on this site that I include further down.  I’ve tried to address each of your questions in the sections below.  Feel free to comment if there’s something that’s still unclear.

What Causes an Uneven Subfloor?

Uneven floors are generally caused by one of three things: settling of the foundation of the house such that one load bearing wall is higher than another, sagging / crowning wood joists supporting the subfloor, or sagging plywood / oriented strand board (OSB) sitting across the joists (most likely because an undersized plywood was used for the span).  The most common that I’ve seen is crowning/sagging joists.  Here’s some thoughts on each:

Settling Foundation:  If the room is not level all the way from one side to the other but it otherwise in good shape, it’s probably because the foundation has settled.  If the foundation is now stable (e.g. is no longer moving) you can address a slightly uneven floor with an SLU, but you might consider just settling for a slightly uneven floor.  I wouldn’t try to use an SLU unless the floor is uneven by <= 1/2 inch due to the weight it will place on the floor.

For extremely unlevel floors, you’ll likely want to get the foundation fixed by a professional.  This isn’t the problem I think you’re facing, but I thought I’d discuss it in case another finds this article in the future.  Other than moving the foundation, I can’t see any other solution besides a self-leveling product that will help achieve a level floor in this case.

Sagging Plywood / OSB:  If the house has real exterior-grade plywood, you’ll probably be OK in using a self-leveling product over the floor without ripping up the plywood and re-laying it.  If the plywood appears weak or has excessive spring in it, you’ll want to either rip it up and replace it with a thicker, higher grade wood, or you’ll want to glue and screw another 5/8″+ sheet of plywood to it first.  You might also consider blocking from underneath (running beams perpendicular to the joists.

The downside to the extra plywood is that this raises the floor considerably.  A product like SurePly won’t provide you the extra strength needed to achieve this, either, so you’d be stuck doing two layers.  Regardless, you should tightly fasten the plywood down to the existing joists with screws.   If the house has OSB, you’ll need to at least glue and screw 5/8″ exterior-grade plywood to the floor prior to using a self-leveling product.  Again, this will raise the floor, so it might be better just to replace it.

Sagging / Crowning Joists:  This is by far the most common reason subfloors are uneven, and probably the situation you’re facing.  You have a few options here.  If you have access to an open joist structure underneath the floor, you can shim the plywood between the joists using standard cedar shims, then screw down through the plywood and shims into the joists.  This will be MUCH CHEAPER than using a self leveling product.  If you have no access, you should screw screw the plywood down tightly and then use a self-leveling product on top.

Basics of Laying Vinyl Tile

Before getting into the self-leveling questions you asked, I thought it would be good to at least mention the basics of vinyl.  There’s so much good information on the web about this, I don’t think it’s necessary to repeat.  Note that many of these sites don’t address a plywood subfloor well.  If you decide to go with SurePly, read their installation instructions and follow them closely.

• Ask the Builder on Laying Vinyl Tile
• This Old House on Laying Vinyl Tile [Good Graphic with a Mortar Bed Example on this one]
• Readers Digest on Laying Vinyl Tile [Discusses lauan as a subfloor base; use SurePly instead.]

There are a few different subfloor prep methods discussed in these links.  In all the houses we’ve owned, a sheet of very thin, smooth plywood, similar to SurePly, is fastened to the subfloor plywood using a pneumatic/electric stapler.  The SurePly provides a very smooth surface for the vinyl tile and is by far the easiest acceptable installation method.

Using a Self Leveling Product to Level the Floor

Without looking at the floor, its hard to know whether an SLU is really the right move here.  If possible, I would simply stick with the SurePly and avoid this entire step.  As I said, it’s expensive, complicated, and time consuming.  If you decide its the best route, here’s some thoughts to help:

Plan How Much SLU You Will Need

Consider the amount of SLU you will need.  Remember, you must have enough to cover the highest point on the floor.  A standard back of SLU will cover approximately 50 sq. ft. at 1/8″.  For a 225 sq. ft. room you may need as many as a dozen bags to get a sufficient cover.

Prepare the Plywood Subfloor

Follow all of the subfloor prep instructions included with the self leveling product.  This will include, at a minimum, a primer, and will also probably require a metal lathe on the floor.  You should also ensure the subfloor is tightly fastened to the joists.  (Use screws whereever there is bounce in the floor).  Note that self levelers can’t be used on OSB.

Dealing with a Large Area

If you want to lay a large area of self-leveling underlayment, you’ll need to:

1. Follow ALL of the floor-prep instructions of the self leveler you select.
2. Have a group of able-bodied helpers lined up for a Saturday - probably around 5 people for 225 sq. ft.
   Use an extended-setting SLU like LevelQuik ES that will give you 15 minutes of flow time.  LevelQuik RS (sold at Home Depot) only gives 5 minutes.  You WILL NOT be successful if you use a rapid-setting product like this.
3. Have at least 3 electric drills and mixing paddles, along with at least 8 five-gallon buckets.
   You will create a pipeline of people pouring, mixing, and spreading the self-leveling underlayment. 
4. Set up each of the five gallon buckets with the amount of water needed for the mix, then have 3 people serve as mixers, 1 person pouring the powder from the bags into the buckets, and 1 person pouring and spreading the mortar on the floor.

Obviously, to do a large area, you must be pouring and spreading quickly.  Take a look at all of the articles I list below for ideas on how to make the project a success.

Related Resources

I’ve written a number of articles on this site about SLU that will be of some help to you in planning / executing this route if you decide to take it.  Take a look at these, and feel free to comment below with additional questions.  Good luck!

• Building Temporary Barriers for a Self Leveling Mortar Pour
• Use a Slotted Squeegee to Spread Self Leveling Mortar
• Video on How to Mix Self Leveling Underlayments to the Right Consistency
• Achieving a Level Floor with Self Leveling Underlayments

What do you think?  Feel free to weigh in with additional questions and advice.  That’s what makes the web wonderful.