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I am in the slow process of remodeling a small bathroom. The room is roughly 50 square feet. I am down to the bare joists and have 3/4" plywood that I'm about to screw down with 3" screws. After that, I plan to install half inch cement board followed by 1" porcelain hexagon tiles. The house is about 100 years old and due to settling the floor has a very slight yet continuous dip that goes downward toward the outside wall. What I mean by continuous is that if I lay a 6ft level perpendicular to the joists they are all in contact with the level.From one end to the other (a span of eight feet) I think it's roughly an inch in difference. It's by no means significant.

Since the lack of perfect leveling isn't substantial and since I I will be installing a new toilet and pedestal sink and will reposition the original ball and claw tub (which by itself weighs several hundred pounds) I was wondering if pouring a thin layer of leveling compound over the cement board would be the most ideal fix to my unleveled floor issue. Obviously I'm not an expert in the area but I thought since the problem is somewhat minor that using a leveling compound would be the most practical solution.

I'd especially like the toilet to sit as perfectly level as possible so I am hoping to correct this minor flaw in the floor.

  • Is it normal practice to use 3" screws to fasten 3/4" plywood or would it be better to use the right nails? What about construction adhesive across the floor joists before putting down the plywood? – Jim Stewart Mar 14 '18 at 13:28
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    @JimStewart I'm just an amateur, but in my limited experience screws are far superior to nails for keeping flooring squeak free. But 3" sounds longer than needed for flooring - you don't have the same kind of loads that you get hanging cabinets. – manassehkatz Mar 14 '18 at 13:41
  • Some wooden structural members become very hard with age. It used to be that one sometimes had to drill pilot holes for screws into old joists and studs. But maybe I am out of touch and with modern screws this is no longer ever necessary. Still maybe you should be sure to get screws with a "drill point". – Jim Stewart Mar 14 '18 at 16:55
  • The lumber store here in my area (Dunn Lumber) recommended 3" screws and also sold me the adhesive that I will apply underneath to help prevent squeaking. I had to install some joist areas to fill the border of the room since there was no support around the walls and the 3" screws went in the sides without any issues. I think they should be fine. – Adrien Mar 14 '18 at 19:55
  • Definitely go with glue and screws, but 3” seems really excessive. – canadianer Mar 15 '18 at 3:25
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Leveling compounds are a last resort. They're a pain to work with and make future work more difficult. The right way to fix this is now, while you have the framing accessible. In a room that small, an inch out of level is certainly significant. You'd feel it every time you walked in.

Either rip strips to stack onto your joists, or sister 2x4s to them with their top edges at level. The latter is simpler if you would need tapered strips. Set the outer ones first and use a straightedge or dryline to set the interior ones. Use heavy-duty construction adhesive and screws for everything, including the subfloor.

3" screws are probably overkill for your subfloor. 2" or 2-1/2" are fine if the joists are solid. If not you'll get thread stripping and need longer ones. You should be able to drive the heads slightly below flush.

  • Thanks, I guess I will do it your way. The thing is when starting at the highest point and trying to rip a "sliver" for the first joist that is barely unlevel is going to be maddening. When sistering do I put a 2x4 on either side of the joist or can I get away with screwing on 2x4 onto one side of each joist? – Adrien Mar 14 '18 at 20:07
  • Yeah, the way I usually do the wedges is to take a thickness measurement every foot or 16", and transfer those to the board. Use a straightedge to connect the dots. One 2x4 per joist is plenty with the sistering method. – isherwood Mar 14 '18 at 20:08

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