Doubling joists-weight issues [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-28-2007, 01:13 AM
I took out a fiberglass bath tub with tiled walls and I'm putting in a tumble stone shower (floor and walls) in its place. It's on a second floor. I keep reading about doubling joists to help support the weight. I cannot double the joists. They are 16 feet long and run from middle of my garage ceiling at a large beam to the back wall of the garage. They run under part of a bedroom, under the bath, then under part of another bedroom. The garage ceiling is drywall, and there is electrical, a gas line and misc. phone and cable run between joists. The joists are 2 x 10s 16 inches on center with the joist against the shower wall being approx 9 inches off the outside wall. The shower will sit over 2 joists and a reinforced plate (2x6 with blocking underneath to the wall below) we added to the outside wall joist for added support on the new subfloor. The subfloor is 3/4 inch plywood, glued and screwed. I can block between joists or double a 6 or 7 foot area under the bath, but have no way to get addition 16 foot joists in. Is this going to be a problem with the weight of the shower pan and stone???? Any suggestions on a fix if its a problem??? Thanks, John VB

One other note: The guy who sold me the stone says it will hold up on the bathroom floor by thinsetting over the 3/4 inch plywood. Sounds like that could be a problem, is this a good canidate for Ditra???

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02-28-2007, 03:34 AM
Guessing a 60 gallon capacity for an average tub, it would hold 480 lbs of water... plus another 100 lbs for the tub itself. So you just removed a load of roughly 500-600 lbs. Are you adding more weight than that?

Others can comment on how to install the tiles/stone... I am a newby on that topic.

Cheers, DD

02-28-2007, 10:29 AM
I don't know??? How much will a mud shower pan and stone weigh??? I didn't think about the weight of a full tub of water, but the tub was way less then 100 lbs, I cut it in half with a sawall and easily carried it down stairs. thanks John VB

02-28-2007, 02:12 PM
Welcome, John. :)

Your shower floor, presuming a traditional mud and liner construction, won't weigh but maybe 25 - 30 pounds per square foot and is pretty well isolated from the floor structure. I wouldn't worry so much about the shower.

The floor, on the other hand, is a problem. You can't even get close to the required rigidity of the joist structure unless you're willing to add a support beam in the garage. The sistering you read about here is almost always to reduce deflection, not really for strength. Your existing joist are capable of holding a lot more weight than you're talking about adding, they just can't do it without deflecting excessively.

Guy who sold you the stone was not your friend. Natural stone installations require double-layer plywood subfloors, then a tiling substrate of some kind, and all this must be installed over a joist structure that meets L/720 deflection standards. The between-joist deflection is the more likely to cause you to have a failure, but your joists may even be overspanned for a ceramic tile installation.

My opinion; worth price charged.

03-01-2007, 01:30 AM
Thank you for the response and explanation, we will seriously look at adding a beam in the garage. I have some ideas as to were it could be added. If anyone else has some thoughts, I like to hear them.

If I can get another beam and some extra joist in, I'm I better off with hardiboard or Ditra under the stone?

Thanks again, John VB

03-01-2007, 11:47 AM
If you have a suitable subfloor structure, you can use either CBU or an uncoupling membrane such as Ditra for your installation.

I would recommend the membrane, given your choices.

My opinion; worth price charged.

03-01-2007, 01:28 PM
Thanks again