Bathroom Floor Joists

bathroom floor joistsThe bathroom is a special space in a house with unique needs. Accommodations need to be made for fixtures and plumbing and one of the first concerns will be framing the floor. Your bathroom floor plan should detail locations of toilets, shower stalls and bathtubs; if these are taken into consideration when floor joists are laid out, you will not need to move joists or make cuts in them later on.


Case in point: the toilet’s closet bend requires about a six inch square space to drop down in the floor. If your floor framing layout does not take this into account so that the closet bend falls between floor joists there will be a problem positioning the toilet. Hacking away at a joist will usually end up compromising the floor’s structural integrity, although there are ways to work around that issue.

You will need to center the first floor joist no more than a distance of 8 ¼ inches from the bathroom’s finished wall. This takes into consideration that when a toilet is placed against a wall, it’s closet bend will be centered about 12 inches from that finish wall.

So if your joists in that are spaced at 6 inches center on center, there will be at least 3 inches clearance from the toilet drain center to the nearest joist and 6 inches of clearance of the closet bend. Remaining floor joists can then be laid out with normal pitching.


The drain for the bathtub, although less bulky than the toilet closet bend, also needs to have allowance for a floor drop through. Planning of the floor framing will make it simpler to run the plumbing lines and keep floor joist notching and cutting to a minimum. The best practice for this (see Figure 1) is:

  • locate the drain center from your floor plan of where the tub will sit
  • lay the floor joists out equidistant from the drain center at 16 inches on center (if joists are running perpendicular to tub, avoid placing joists less than 12 inches from the valve wall)
  • layout a 2 inch block bridging the two joists at about 12 inches from the finish wall. This block will be used by the plumber to secure the drain.

Even better than using solid 2X blocking between the floor joists is using metal or 1X cross pattern bridging. This allows much more rapid snaking of wiring and plumbing than cutting holes through solid blocking. If you must use solid blocking in areas where lots of plumbing and wiring will be routed, consider leaving nails slightly protruding to enable easier pulling and removal. Also be mindful about not putting nails where electricians and plumbers will need to be drilling holes.

It would be wise to consider the orientation of the floor joists in relation to larger plumbing lines. Routing 3 or 4 inch diameter pipe lengthwise along a joist bay is much easier than running it perpendicular to the joists, when you are faced with drilling a series of large holes to route through the framing and joists.

All building codes are quite specific about allowable locations and sizes of holes in floor joists. Essentially, holes are not allowed to be drilled within 2 inches of the bottom or top edge of joists. It you cut notches into a joist for smaller conduits or wiring, they must be less than one sixth the depth of the joist, and cannot be in the middle one third length of the span.

Fire prevention specifications add additional the constraint that drilled holes be the same diameter as the conduit or pipe that passes through it. Check your local building code for specifics.

Photo by Paul Schultz, Creative Commons Attribution License