I'm replacing some rotted subfloor around a toilet, and I want to add blocking around the flange for extra support, and to give me a place to support the new pieces of subfloor (I'm not planning on replacing the flange). What complicates things is a pipe running parallel to the joists just an inch or so beneath the subfloor.


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My thought was to first fill out the joist webbing (yellow) and then build a box (white) to surround the plumbing (red). The horizontal beams perpendicular to the joists could only be as thick as the joist flange due to the upper pipe.

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Would this approach provide decent support for the replacement floor and the toilet? If so, what kind of materials would be ideal to use?

  • @AloysiusDefenestrate, that sounds like an answer!
    – Doresoom
    May 21, 2015 at 13:59
  • @doresoom -- I guess you're right. I often start what is really a comment, then start running off at the mouth. Comment moved and will come back and delete this later... May 21, 2015 at 14:46

5 Answers 5


Right idea, but WAY too complicated for minimal support & no screwing for the flange. With NO FLANGE in place, build a frame first, not piece by piece in the floor. With a Flange in place you'd build it without the right side's {bottom} joist attachment so you can dry-fit everything & finish inner attachments to then LASTLY screw to the joists.

Stand up 2x4's to be attached to joists LAST. Then, bridge the Stand-Ups with 2x4's On-Flat. You screw through the Stand-Ups & into the ends of the On-Flats (do just the left Stand-Up {top} configuration if a Flange is in place).

Then, you bridge the On-Flats with a shorter On-Flat. First, screw that in from the left Stand-Up to hold it in place & toe-nail screws on either end at the Flange side. Also, toe nail what can fit On-Flat on the right side {bottom} of the Flange Finish by just screwing outward (& angled upward for this Askers I-beam pictures) through the Stand-Ups & into the joists.

This should be snug to the Flange's underside collar or the drain pipe, depending on your subfloor elevation. The On-Flats even allow you to cut or drill the platform frame for an actual tight fit complete circle. Of course, adjust this proven design to your actual application.

Mega Toilet Support


You need to install the sub floor and any blocking before the toilet flange. Typically you would scab 2x4's much like your picture for ease. The only difference is the bottom horizontal members (bottom between joist) are not needed. Install the blocking first. Then sub floor. Then underlayment, if any, then toilet flange last.

Really, we would cut half way on the joist, install 2x4 blocks horizontal between the joist under the subfloor halfway on the edge of the existing subfloor to tie the sheets together, then install the subfloor.


They have Simpson strong tie plates just for this.

Cut your blocking to length, hole saw the location of pipe, install Simpson plates using shear strength screws such as teco nails, and then install your subfloor.


If you're willing to pop more tiles, you could run 2 pieces of lumber (2x4/2x6 -- the biggest you can fit) on the flat, parallel to the joists. They'd be wider than the hole by a foot on either side, and you could get good screws through the subfloor (I'm assuming it's sound) to hold them up. Alternately, can you re-route the shower drain pipe to give you more room? Then lumber on edge, perpendicular to the joists, with some sort of joist hangers to the i-beam would be ideal.


It is possible to do this, although I think it might be easier to cut the waste line on the horizontal run and plumb up another pipe upon which to install a toilet flange after the new floor is installed. If you are installing tile, the flange should go OVER the tile, for proper height of flange to toilet seal.

Secondly, I would use nails or structural screws to anchor supports for the floor directly to the other floor joists not the the new ledgers as shown in your drawing.

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