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Old 03-30-2009, 10:27 AM   #1
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Levelling a subfloor


I'm renovating a 100 year old house and need some advice for a subfloor issue. I replaced the original posts in the basement with teleposts and leveled the floor prior to starting work. This weekend I tore out the original flooring and planked subfloor and glued and screwed plywood with the intention of nailing bamboo flooring over top. Once I got the plywood down though I noticed a slight low spot near the middle of the floor. Measuring it with a 7' level it looks like the sag is about 3/16" over 4'. The entire low spot is roughly 15 sqft. After some investigation I realized that one of the load bearing beams in the basement has a slight sag between the two teleposts. The beam is 6 rough 2x8's, on edge, laminated together. The span between the teleposts is 74". It's the only spot in any of the beams that's sagging, so I suspect that perhaps there's a scarf joint inside the laminated beam that is giving out. I tried putting a bottle jack on a post and jacking the sag out, but the entire beam lifts up with the sag in place, making the two teleposts fall over. I jacked it up and let it sit overnight, hoping it might settle, but it didn't move a hair.

Unfortunately my drywalling & painting is complete, and to reduce the risks of redoing some mudding & painting I don't really want to jack up the entire floor to replace the beam, just to fix a 3/16" low spot. Also, it would appear that while the beam has a slight sag to it, it's certainly stable if I can put a jack under it and it doesn't even straighten out. Should I just lay some tar paper and mesh into the low spot and float some mortar across it? If I do that can I nail through it or will I have to use a floating floor?

Thanks.


Last edited by whiskeykid; 03-30-2009 at 10:41 AM.
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Old 03-30-2009, 11:19 AM   #2
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Levelling a subfloor


Nailing through any patching material will crack it, you could remove the subfloor and sister the bowed beam to get it perfect.

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Old 03-30-2009, 11:34 AM   #3
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Levelling a subfloor


It isn't a joist that's bowed, it's the beam that the joists are sitting on. I was just thinking that I could cut the couple of low joists off the beam, jack them up, and slide a 1/4" shim between the beam and the joists.
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Old 03-30-2009, 12:30 PM   #4
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Levelling a subfloor


I don't think that beam is "sagging" at all, it has simply deflected downward under load. That is normal, all beams deflect under load. Jacking up a beam that is under load in an effort to remove the "sag" could crack the beam, which would be truly unfortunate, and once the jack is removed, the beam should return to its previous position, unless of course you damaged the connections between the beam and posts on one end or the other, in which case you would have to repair the damage.

The deflection is very small, and you can certainly get a flat surface for tiling by using a self levelling compound in the low area. If you are planning on using cement backer board over the plywood, you can shim it in the low area, or fill the low area with mortar prior to installing the backer board.
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Old 03-30-2009, 12:53 PM   #5
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Levelling a subfloor


Thank you for the advice. If I want to lay hardwood over the leveling compound however, is there a way to fasten it down without having to nail through the mortar? The spot that requires leveling is only about 15 sqft in an area that's more than 600 sqft. Can I glue that section and nail the rest? I'd really rather avoid engineered flooring if at all possible.
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Old 03-30-2009, 05:00 PM   #6
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I would talk to a hardwood installer on that.

Quote:
If you are planning on using cement backer board over the plywood, you can shim it in the low area, or fill the low area with mortar prior to installing the backer board.
He's not tiling and if he were, that would not work.
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Old 03-30-2009, 05:38 PM   #7
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Levelling a subfloor




How ya been Rd?
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Old 03-30-2009, 06:08 PM   #8
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Levelling a subfloor


Fine, just fine, you?

Working nights, this week days, don't know if I'm coming or going.

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