Waterproof layer between mortar and subfloor for shower pan?

[Brian Kehlmeier]

I have read in these forums various materials used to decouple the mortar bed from the OSB subfloor, but I question its purpose. If the shower pan is being screwed into the studs it makes sense, but the Dreamline shower pan I am using does not. It seems to me that I would WANT the mortar to bond to the subfloor to keep it from moving. Can someone convince me one way or the other?

 

[bingow]

DIYer here. Installed a Sterling (Lowe's) shower pan on second floor bath 19 yrs ago, no decoupling used for mortar bed, and no problems. Same for first floor bath acrylic tub. Don't recall anything else being recommended.

 

[Sponsor]

 

[wwhitney]

I have read in these forums various materials used to decouple the mortar bed from the OSB subfloor, but I question its purpose. If the shower pan is being screwed into the studs it makes sense, but the Dreamline shower pan I am using does not. It seems to me that I would WANT the mortar to bond to the subfloor to keep it from moving. Can someone convince me one way or the other?

1) Cementitious materials (mortar) and wood products (OSB) respond different to temperature and humidity variations. So in most cases the mortar will debond from the OSB anyway. [That is why some sort of decoupling layer is typically used when setting tile on wood sheet goods.]
2) Ease of cleanup when you change the tub out next time.
3) Avoids the subfloor from sucking the water out of mortar and slowing its curing. If you skip a separator be sure to dampen the subfloor first.
4) In the unlikely event of an extreme expansion/contraction incompatibility between a plastic tub/shower base and the subfloor, you'd want the mortar around the tub/base feet to be able to slide relative to the subfloor, rather than restrain the plastic and stress or break it.

Cheers, Wayne

 

[bingow]

Correction to my earlier post. I still have the original installation guides for both shower and tub. The Sterling shower base installation mentions a mortar bed as an option, but does not call for a decoupler.

However, the Kohler acrylic tub does. It calls for mortar ~2" thick, followed by a "plastic drop cloth" on top of that, followed by the tub. The sub floor is the floor: concrete slab. I have each step checked with no side notes for that one, so I likely used a 6 mil sheet between the mortar and tub, but nothing between the mortar and slab.

 

[Jeff H Young]

If you choose not to follow instructions use your best judgement I think a plastic sheet is a good idea but have installed without also . Im not concerned with the mortar moving around later. If you ever have to pull the pan and dont want to trash it youll be happy you did, and mortatar drying on wood isnt a good way either. if you have the material handy its definately better , though Ive installed many with no de coupler