How do I frame joist/subfloor for a new bathtub?

orourkeAugust 1, 2009

Is there a standard way to frame a bathroom floor (i.e. the joists and subfloor) for a bathtub ?

I am remodeling my bathroom, have removed all wallborad, the floor, most of the subfloor, put in all new water and drain lines and IÂm now finishing up repairing floor and wall water damage (from an old roof leak). I have also sistered all the joists in the bathroom.

The old bathroom had just a shower which I will now upgrade to a bathtub. Since I have also removed some of the old floor framing (to repair weaknesses and reframe around toilet), I am wondering if I need to add/modify the joist framing to better accommodate the bathtub. Is there some standard joist and subfloor framing layout for bathtubs ?

I do understand that later on I may have to add support framing for the tub, eg. build a frame with 2x4s and ¾" plywood to support the tub itself and possibly add 2x4s horizontal to the studs to support the lip. But at this point, IÂm primarily concerned whether there are any special considerations regarding the joist layout.

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The best advice I can offer is to center the tub overflow/drain opening over a joist bay, so they have plenty of room to do the trap. A tub requires 1 1/2" drain pipe.

    Bookmark   August 1, 2009 at 5:21PM
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Make sure you have room enough and access in back for plumbing. My home has a double stud wall behind the tub. And I just put in an access panel so I can make changes or repairs.

    Bookmark   August 2, 2009 at 10:20AM
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As always, thanks for the very valuable info...

I am a little lost about the bridging suggestion (I am a rookie when it comes to construction terminology). I have only known the term "bridging the joists" to mean joining the joists with short wood blocks (say 14 ½" pieces of wood in a 16" OC joist layout) to prevent the joists from warping, this type of bridging normally been added midspan between the joists. So I do not understand what you mean by having the bridging being supported by the footer.

P.S. As far as strengthening the floor, last weekend I sistered all the joists and was able to have the sister rest on the sill on both ends. Getting the joists into the crawlspace vent and openings was a bit of a challengeÂI also had to notch the endpoints of the sisters by about ¼" to get them in place (to compensate for the sag in the old joist) but then, once in place I used a hydraulic jack to push the sister up a bit and then shimmed the notch to have it rest snugly over the sill. This way I also reduced the sagging that had accumulated in the old joist over the years.

    Bookmark   August 4, 2009 at 3:22AM
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If you have sistered the joists why would you need to do anything else?

Bridging or blocking should not be necessary for joists smaller than 12" nominal height if there is a subfloor above and a ceiling below.

    Bookmark   August 4, 2009 at 10:26AM
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