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Old 11-21-2009, 01:53 PM   #1
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Bathtub Floor Support

I want to remod a bathroom and replace a steel tub with a cast iron tub. The mfgr requires minimum of 49 lbs/sq ft flooring support. I have 2x10's on 16" centers running with the length of the tub. There is 1/2" plywood over the joists. Is this sufficient? Do the tub ledgers carry the burden of the weight or does the plywood floor? Do I need a floor joist exactly underneath the tub apron that sits on the floor? This is an apron tub with three walls around.


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Old 11-21-2009, 03:17 PM   #2
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In order to determine the adequacy of the 2x10's, you need to know the clear span of the 2x10's. Also need to know any additional load in the bathroom that they will be supporting. Clear span means the distance between supports for the 2x10's. The plywood is typically ignored in computing the bending capacity of the joists, however the plywood is important in preventing lateral buckling of the joists.

There are two possible cases to examine with respect to the ledgers of the tub. In case 1, the ledgers are perpendicular to the joists, in which case the tub acts as a series of point loads on the joists. In case two, the ledgers are parallel to the joists. If the ledgers line up with the joists, then the tub acts as a distributed load over the joists. If the ledgers do not line up over the joists, then the ledgers act as distributed loads over the plywood, and you need to determine two things, first if the plywood is sufficiently strong to carry the load, and second if the joists are sufficiently strong to carry the load. The load on the plywood depends on how far from the joists the ledger is.

The manufacturers recommendation of 49 psf is a simplified approximation, which does not account for the point load action of the tub if it is perpendicular to the joists, or the bending on the plywood if the ledgers are offset from the joists. You may want to have a structural engineer look at your particular setup, as I doubt the manufacturer will assume liability for the structural support of your house if the joists crack, regardless of the psf capacity of the floor.


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Old 11-22-2009, 02:18 PM   #3
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Bathtub Floor Support

Thanks for taking the time to read and reply to my post.

Here is more detail: The joist span 10.5 ft. The tub ledger is offset 1.5" parallel from the joist, underneath, the plywood is blocked (not x-braced) with 2x4's flat to the next joist every 16". Then there are 1x6's spanning the length of the joists under the 2x4's. These two joists also carry an interior wall. Probably why the blocking exists more so than the tub.

The apron of the tub would rest just about dead center between the joists.

I guess I can just add more 2x4 or better yet, 2x6 blocking between the joists under the plywood where the apron would sit, and more where the bottom of the tub would sit on the plywood.

The existing tub walls are thick cement which is much heavier than the Hardie Board I will be using. So, heavier tub, lighter walls probably evens out.

The house is 40 yrs old, there are no cracked tiles on the floor or tub surround, so I'm probably in good shape?

Thanks again !
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