(followup question to Why is this Ptrap installed in this location?)

Here is the hole that I want to patch up: enter image description here

As you can see, it is a cinder block wall with concrete foundation with a new PVC drain pipe. I want to cover the hole to prevent future water damage to the foundation. I also want it to be relatively easy to break the wall again and expose the pipe in case there is a problem with it.

Here is my current plan:

  1. use a shop vac to drain the leftover water and dirt
  2. use "great stuff" expanding foam to fill up the left and right blocks
  3. use the foam to cover the pipes with 1 inch layer
  4. use mortar type s to fill up the rest - including the bottom block
  5. wait for it to dry and paint over using the same color

Is this a good plan? are there any mistakes that I need to avoid?

update: a zoom out of the project enter image description here

  • Using an expanding foam inside of a cinder block is generally a bad idea unless it’s the window/door type because it can actually expand so much if it has no other outlet that it breaks the block. Commented May 25, 2020 at 21:00
  • what should i use to fill the left and right cynderblocks so the \the mortar wont fall down into them?
    – yigal
    Commented May 25, 2020 at 21:27
  • If the clean-out is now outside the wall, I would not worry about an access door or panel, etc. This is a vent pile, not a drain pipe, that connects to the sink drain pipe. For the pipes going vertical and into the house, provide a 1" layer surrounding the pipes to isolate from cement-mortar mixture. Then fill the cavity with steel mesh and do a rough fill. After it dries, apply a finish coat of stucco mix for the exterior surface. You could almost match the cinder block look with grout lines. Again, this is a vent pipe, not a drain pipe. Commented May 26, 2020 at 2:42
  • –A rubber connection on a vertical pipe above ground is allowed under the exception rule, if connected by stainless steel bands. Commented May 26, 2020 at 2:58
  • @Programmer66 yes, If the clean-out is now outside the wall. do you have an example with photos for the suggested procedure? especially for the steel mesh? also, can i use mortar mix type s instead of stucco? (I already bought it from home depot, but I guess i can return). good to know about the Fernco rubber connection. i think it is yes stainless steel bands
    – yigal
    Commented May 26, 2020 at 3:18

3 Answers 3


You need to make sure that you can have access to the space in case you need to address any issues with the plumbing in the future. ( especially since you have a fernco type rubber fitting installed. )

You should install an outdoor rated access panel that either has a hinged door or a removable panel. Either option will need to have hole for the clean out plug as it stands proud of the wall.

Depending on the size of your opening you may find a metal/plasic access panel in the size you need and then cut a hole in it or you may have to have one manufactured. It should be installed with an outdoor rated sealant or silicone and then screwed to the wall with concrete screws or concrete anchors with the appropriate fasteners.

Spray foam may be OK for the adjacent block cavities but i would not surround the plumbing with any material that can not be removed easily, for any future maintenance, stuff fiberglass insulation all around it so you can pull it out if you need to.

  • that sound like much more than my skill level and it also might raise questions when I want to sell the house, I dont mind to use a chisel and hammer to reopen the hole again, hopefully it wont be needed. also, I am worried that the hole weakened the structure and I want to use mortar to strength it up
    – yigal
    Commented May 25, 2020 at 21:20
  • 1
    I provide you an answer as to how to properly address your problem. It is up to you to decide if you want to do right or not. If you do decide to sell you will probably be asked to do it right if you did not do it right the first time. You can strengthen the structure without in-casing the plumbing in a solid.
    – Alaska Man
    Commented May 25, 2020 at 21:39

I would make the repair in three steps. Before doing the repair to the cinder block. Wrap the pipes with something, like form pads using zip ties or duct tape. Make about ½- 1” layer. Purpose is not to encase the pipes in concrete.

1. Use your mortar mix and bring up the bottom part level to the joint line. Your clean-out is located in this joint. Remember to cover it so it is not directly embedded in the mortar. Let this dry.

2. Remove completely the block in the middle layer, clean out enough of the mortar grout, so you can cement in a new block . Obtain a replacement block to go in the middle row. From the back of the new block, chisel out the back a vertical opening so the new block can slide pass the pipes into the hole. You also have to chisel the bottom for the clean out pipe. Once the new block has been fitted, apply mortar to all the joints and slide the new block in. Even If the block separate into a left and right part, you are still only have to apply mortar to the joint.

3. Once middle block is in place, you can continue to mortar up the upper portion of the hole, finishing the surface to look like the texture of the cinder block and put in a grout line.

enter image description here

  • brilliant. I would never think of putting in a new cinder block. just to verify step 1:you mean to fill up both chambers of the the lower block with mortar? my original plan was to use foam to fill up the left chamber and mortar for the right chamber. come to think about it, mortar for both chambers makes more sense
    – yigal
    Commented May 26, 2020 at 6:13

I would tidy it up and then make a hatch to go over it. Painted the same it will look neat and quick access.

  • what do you mean by tidy up? shouldn't i use some mortar? what is a hatch?
    – yigal
    Commented May 25, 2020 at 20:48
  • Hatch: wood panel with hinges or just pins... even plastic or metal...
    – Solar Mike
    Commented May 25, 2020 at 20:50
  • Forget all that, +1. Use some kind of access panel.
    – Mazura
    Commented May 26, 2020 at 8:07
  • @Mazura Can you clarify "Forget all that, +1"? Usually the "+1" indicates agreement with the answer, but "Forget all that" seems to indicate that you don't agree. Maybe there was a comment that got deleted that you're disagreeing with?
    – FreeMan
    Commented May 26, 2020 at 12:05
  • @FreeMan no other comments here as far as I am aware - and I have seen them...
    – Solar Mike
    Commented May 26, 2020 at 12:08

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