1924 Second Story Bathroom Slab

[Gluebusy]

Hello!
I am remodeling a house from 1924 in Lancaster, PA. The second floor bathroom has a slab poured around the pipes. The joists are chamfered to a point at the top. I've removed the cement around the copper supply and drain lines, I believe these are from a 60's renovation. My plumber will replace with pex and PVC.

After rough-in, my plan is to backfill under/around the new pipes, pour new cement up to existing tile, then pour a self-leveler and install new tile.

My questions: what is the best backfill for a second floor? It seems the previous backfill was pumice, or similar lightweight material. I want to use something equally light as to not increase the load on my joists. Any suggestions?

Should I put something over the backfill and pipes before pouring new cement? Tarpaper?

Thanks for any help! Everything I can find online about this, the only recommendation is to remove the whole slab and do a plywood subfloor, but I believe it can be done another way with less obstacles.

Tom

 

[Jadnashua]

What you have is a floating mudbed. Deck mud is made up of a very lean mixture of 4-5:1 sand:cement mixture. It is mixed fairly dry and is more like damp beach sand than concrete. It needs to be thick enough, as the small amount of cement means it isn't all that strong by itself, which is why it needs to be fairly thick. It usually has some metal lath in it to help improve its strength. It's great in compression, but not in flexing unless it's thick enough. I would NOT substitute concrete mix, as it's too heavy, and will tend to run places you don't want it to. If a mud bed is done properly, you will not need or want any SLC on top of it, as it is easier to get the deck mud the exact height you want and perfectly flat and is a great surface to tile directly onto. The other advantage of deck mud is it is really cheap! The hammering and cracking of the existing mud bed likely means that you should remove all of it, otherwise, whatever you put on top of it or try to patch, will not be very strong, and likely to crack the tile after installation.

www.johnbridge.com is my go-to place for tiling things.

 

[Sponsor]

 

[Jeff H Young]

lightweight concrete , wrap pipes with foam wrap waterlines poly sleeve or even visqueen. hot water should be foam insulate but if between first and second floor the poly sleeve probably is fine.
thanks interesting pics I don't see stuff that old every day!

 

[Gluebusy]

Thanks Jim! Do you think the deck mud works well as a backfill if I push it around/under all the pipes? My concern is keeping any load off my new plumbing. All I see online is people using wet gravel in basement slabs, but I think that's too much extra weight for my wood joists.

The majority of the remaining cement and tile is very solid, and stayed strong through the previous renovations. I'm going to use a bonding agent on the edges and then fill it. Plus I don't want to move that tub...

 

[Jadnashua]

The people over at www.johbnbridge.com are tiling pros, and have more experience with this...I'd ask over there.

Generally, deck mud doesn't bond all that well to deck mud when done in multiple sections like that. A bonding agent would work to bond it to a slab, but over joists and other deck mud, I'm not sure, which is why I suggest talking to the people over there that do that sort of thing for a living. Tile doesn't like movement since it's rigid and inflexible, so that's why a monolithic bed is better.