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Unread 10-30-2009, 01:06 PM   #1
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Laundry Room Floor

I need some advice on a Laundry Room floor installation. The details are:
- Room size is 12' X 10'; 2x10's at 16" o/c floor framing; span of joists is broken by a girder beneath so max span of floor joists is about 7'
- Original vinyl floor covering- have removed vinyl and underlayment
- Original subfloor is 1/2" plywood- it is in good condition, however, I don't think it is solid enough for washer/ dryer. The floor has a sag in it of about 7/16"max, and it is about at midspan (yes, even with the girder beneath)

I am planning to install 3/4" plywood on cleats between the joists beneath the subfloor to beef up to 1 1/4". Then install 1/4" Hardibacker over the existing subfloor, then install 16" tiles on that. I would like some feedback on my plans and if they are a good approach, and some advice on how to make sure the floor is level...i.e. should I install as outlined and then float a self leveling compound on top of the Hardibacker prior to the tile? or can I level the floor with thinset on top of the hardibacker (by screeding) and then install tile. I would appreciate any other advice . Thanks
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Unread 10-30-2009, 01:23 PM   #2
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Welcome, Mike.

You need to know the unsupported span of your floor joists on either side of the support in the center of your room. The longest unsupponted span of any joist supporting any part of your tile installation is what you're looking for.

It is possible to support your subflooring in the manner you propose so long as it's well executed with the face grain of the plywood reinforcement perpendicular to the joists and the upper layer firmly attached to the new stiffeners. You might wanna weigh the difficulty with that of simply removing the existing subflooring and starting over with 3/4" plywood.

I'd wanna know the reason for the "sag" in the joists and correct that if possible. Would be easier to do while the subflooring is removed, 'specially if you find that your joists need some help anyway.

Keep in mind that your floor doesn't give a rat's patooti about level, it cares only about flat. And with those large format tiles, you'll want more flat than called for by industry standards.

Thinset mortar is not meant to be used for flattening nor leveling of the substrate, but can be used for minor deficiencies. Your overall deficiency does not appear to be minor.

My opinion; worth price charged.

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Unread 10-30-2009, 05:44 PM   #3
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Laundry room floor

Thanks for the reply. I am new to this forum and really appreciate thef knowledge and insight. I plan to get underneath the floor tomorrow to see if I can determine the cause of the sag. If I elect not to remove the subfloor and apply 3/4" ply under the existing subfloor, how would you suggest I flatten the floor. Flatten and level on top of the hardiboard? What would work best? What is the easiest? Thanks
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Unread 10-30-2009, 05:50 PM   #4
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Mike as said, you need to find out why it's sagging, could be someone cut notches in the joists or termites are there.

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Unread 10-31-2009, 07:25 AM   #5
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Flatten on top of the hardiboard using thinset or medium set mortar. Worry less about level. Worry more about why you have the sag, and correct that. Then, you may not have anything else to worry about.
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Unread 10-31-2009, 09:13 AM   #6
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Floor Sag

If you stand in the middle of the room and bounce can you feel the floor bounce?

Perhaps the floor sagged a lot and some one came along and added the girder later.

Beefing up the sub floor from below is a lot of work - you will need to cut all the pieces like CX said and then sister on 2"x4" left and right of every bay. You most likely have plumbing and electrical to deal with as well.

Rip out the old floor and start from scratch.

Like I tell the boys "Work is hard - that's why it's called work. Other wise we would all be going to 'Vacation'."

Good Luck
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