What barrier should I use between concrete slab and mortar pre-slope?

[Jimmi328]

Hello All,

I will soon pour a mortar pre-slope on a concrete slab. Regarding a barrier between the two; some of the research I’ve done suggest using felt/tar paper and some suggest using thinset. Any suggestions? The city's plumbing inspector doesn't care, he said it's my choice.

The cement board on the right and left are not yet screwed it; they will be removed prior to the pre-slope pour. The black sticks are pre-slope sticks which I may or may not use.

Thanks to all,
Jim

[jadnashua]

Doing it on a slab verses a wooden subfloor is different. On a slab, take a slurry of Portland cement and water and paint or trowel it on before throwing down the mud. This will bond the mudpack to the slab. You could use thinset, but you probably already have the Portland cement bag open, and it's lots cheaper than thinset.

[Jimmi328]

Thanks for the reply Jim.

Yes, I did buy Portland and sand today, built a wooden box and practiced working with it. I feel much more comfortable now and ready to pour over the slab. I’ll use your suggestion and paint on a Portland slurry, let it dry, then the pre-slope.

[jadnashua]

NO..the whole point of painting on the slurry is so that things bond together, do NOT let it dry first or it's not going to bond the two. You need to cover the slurry before it starts to cure so it becomes part of the monolithic preslope. FWIW, as a curb on a slab, people often use pavers or bricks. If there's any moisture in the slab, the 2x4's can warp creating problems. They shouldn't get wet from above, but it's not hard from below. And, the use of pressure treated lumber there isn't that great an idea (unless it is KDAT - kiln dried after treatment) as that stuff can warp into a pretzel when it dries out. Quality, dried lumber can be used on a subfloor, but I'd be leery about its use on a slab.

[Jimmi328]

I understand now Jim; this does make sense for the bonding action.

As for the 2x4’s; there’s no moisture what-so-ever in or on the slab. I avoided PT lumber as I heard as it dries it can crack the tile from moisture beneath. I used Yellow Pine and coated the “entire” bottom piece with construction adhesive along with Tapcon. I believe this should work okay.

Thanks so much for your help.
Jim

[Jimmi328]

Should I put metal lath down on the concrete slab for additional strength before I pour the pre-slope?

[jadnashua]

A bonded mudbed shouldn't need lath. A mudbed over a wooden subfloor does. While I have been to some classes, I got a lot of my info from www.johnbridge.com - I think it's a good reference site.

[Jimmi328]

I got the pre-slope poured; now it’s time to think of the liner. It seems to me that a paint-on shower pan liner would make this project mush easier. That is I don’t think I would need to notch the studs and fold or manipulate the liner in the corners. Any advice?

[jadnashua]

Personally, I like Kerdi, but you'd need their drain in there. Is that a clamping drain? If so, you really aren't setup for a paint on liner (as I understand it). Schluter does make an adapter from a clamp on drain to their bonded membrane drain, so that might work (but I'm not sure on the level of your preslope to use that).

It's really not a good idea to try to make alternate decisions after you've started, as you may have needed to do things differently to accommodate the specified technique.

At this point, I'd probably put in the liner, then add the cbu to the walls, then put down the final setting bed. Remember no fasteners below about 3" above the curb. Putting the cbu in before the final setting bed means that that will hold the bottom of the cbu in place, and is why you don't need fasteners there when it's sandwiched up against the blocking.

You may get some other options and opinions if you can wait a little.