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I am preparing the subfloor to install engineered hardwood floor by floating installation.

However, the subfloor is too much sagging. There is a big area with about 5/32 depth.

See:

enter image description here

The real sagging area is even bigger than what you can see on the picture.

Could anybody give me suggestion about how to flatten this subfloor?

I was thinking to use 1/8 plywood but it seems I need too much plywood since the area is big. And it won't be smoothly flat.

Is there any kind of compound I can use to flatten it?

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3 Answers 3

up vote 3 down vote accepted

A 5/32" depression is not that bad actually. Assuming you don't have a problem with the floor joists that need to be stiffened, you have a couple of options. First, I would span the area with a 8 or 10 foot straight edged board to visualize the areas that are depressed. Much like your pic, I'd pencil out the area on the floor so you can see it clearly. Be sure the existing subfloor is securely nailed or screwed down. Add screws if necessary so there is no bounce.

The easiest method would be to use embossing compound. Mix up a good amount and trowel it on the depressed area. Use your long straight edge to screed it, much like you would a concrete slab. Since your depression is only a little over 1/8", it should be very easy to spread that small amount. Then using a 12" trowel, just gently smooth it over maintaining the depth of fill. Embossing compound in this application is best as it fills easy, dries quickly and easy to sand if you feel you need to remove any ridges, etc left behind after it dries.

Once you have completed this task, evaluate the overall flatness. If you still have some small dips, use more compound or opt for a layer of 3/16" plywood. Remember, always use a layer of red rosin paper between wood layers to prevent squeaking.

You may also opt for a higher quality backer for your engineered flooring. There are several different backers available. They range from ultra cheap thin foam to much better thicker, sound deadening composites. If the engineered wood flooring is 3/8" or thicker, the combination of a good backer and the flooring will bridge any very small irregularities still left on the subfloor. Don't worry about small differences, they will not show. Good luck.

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Thank you VERY MUCH for the detail instructions! I have two more questions:) : 1. do you think whether I have to seal the subfloor before apply the embossing compound? 2. Can you give any brand/product name of embossing compound? –  limi May 17 at 16:58
    
you do no thave to seal the subfloor unless is it spoiled with pet urine of something smelly. No brand names on the compound, they all work fine for what you need to do. Just make sure it is embossing filler, not leveler. Leveler is hard as stone and very difficult to cut off ridges and sand. Makes a mess, dust. If you are careful, just a knife off of ridges and you are ready to lay floor. –  shirlock homes May 17 at 20:43

You want self-leveling underlayment. It's a type of concrete designed for use over wood subfloors. Make sure to read the directions so you know how to seal the seams in your floor before you put it on.

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Just went to homedepot, it looks like a compound named "fix-it-all" can be used to patch the subfloor and it's sandable.

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