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I want to tile my bathrooms in the house I'm remodeling. They are over a basment with 2x8 joists 16" on center. The span over the basement is around 12ish feet. I put 3/4 OSB and plan to put 1/2 backer and tile. Will this work as is or should I add a 4x4 beam under around the midway point with a 4x4 to the floor of the basment. My wife would rather not have a beam in the middle of the basment but if it's necessary I would rather do it

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  • Sometimes your house just isn't made for certain things. There is pure rubber clicklock that looks great for bathrooms. I would look into other options before adding a beam to just tile a bathroom.
    – DMoore
    Jan 13, 2020 at 4:58
  • How is your OSB attached to the joists? Laid on, or screwed in? Jan 13, 2020 at 10:59
  • Layed on and screwed down with decking style screws Jan 14, 2020 at 1:36

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Floor joists can bend (deflect) without breaking. However, certain kinds of flooring cannot deflect much or they’ll crack (or the grout joints between them will crack).

Each material is rated for maximum deflection. Ceramic tile happens to be one of those materials that cannot deflect much without cracking.

At a 12’-0” span, most lumber species will deflect too much. However, here are a few things to consider:

1) at 11’-0” most lumber species with a “standard” grade (No. 2 & better) will work.

2) span distance is measured from face of support to face of support. That is to say, it’s the “clear span”.

3) calculations are an average. That is to say, they use 40 lbs. per square foot for Live Load AND 10 lbs. per square foot for Dead Load. I suspect your live load will be less (take your weight plus your wife’s weight and divide by the room area), but your dead load is greater (weight of tile, mortar base, underlayment, subfloor, joists, ceiling below (if any), etc. )

If your span is 11’-8” or so and you do not weigh 350 lbs. each, I suspect you’re fine. I’d go look at each joist and see if there are any large knots (especially loose knots). If not, the grade could be No. 1 or select structural and you’re probably ok.

As an alternative to adding a beam in the basement (happy wife, happy life) you could sister a joist to each of the 5-6 joists in the immediate area.

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    The tiles to ceilings don't really count for the dead load - they will be present when the grout hardens, so the grout will set with the deflection they impose. What cracks grout is changes in the deflection. Jan 13, 2020 at 11:02
  • I was going to suggest sistering the joists as well.
    – gnicko
    Jan 13, 2020 at 16:44
  • By "sister a joist" do you mean add and there joist in between each joist? Jan 14, 2020 at 1:40
  • @CharlesParker Yes, adding a joist between each joist , but it will need to be secured to the floor system by either having the subfloor nailed into it or nailing the new joist to one of the existing joists.
    – Lee Sam
    Jan 14, 2020 at 1:52
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When I tiled my kitchen, I used this deflection calculator to see if my joists would be okay. https://www.johnbridge.com/vbulletin/deflecto.pl

Double-check it for yourself, but it seems that you would have too much deflection and should consider addressing that before installing the tile. If your joists are 11 feet from the supports, it would be okay for tile. I only mention this because you are close to being okay and said it was 12ish.

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