Add concrete under shower pan?

[Jono2]

We are replacing a shower and have a fiberglass pan. The floor is pretty level and the new pan seems rest on the support rings, which go to the slab. (underside of pan shown in photo).

I'm debating about trying to add concrete underneath the new pan (for instance in the voids under the curbs) and I'm wondering if that's just asking for trouble or if that's critical to add. For instance there's no easy way to contain the concrete or mound it up higher in certain areas.

Should I mix up some concrete put a low bed under the pan?
Thanks!

 

[Reach4]

You should use piles of mortar. Morter does not have the big aggregate that concrete has. You could put your piles into plastic sandwich bags to prevent quick drying. Squish the piles, and the mortar adapts to the right height.

Search this forum for
piles mortar.

 

[Sponsor]

 

[Jono2]

Okay - thank you - have mortar with a fair bit of sand, and will make a bunch of little piles.

 

[Jeff H Young]

I wouldnt try filling the curbs . I think its a good idea to put the mortar in piles over building paper or visqueen and then lay another sheet over the top of the mortar it will keep the water from wicking out and also probebly keep it from sticking incase it has to come out . Ive done many without plastic but feel its a little better even a trash bag would help

 

[WorthFlorida]

You're on concrete so flexing is not much of an issue. Fiberglass over time shrinks and gets brittle. The backfill of motor is to prevent flexing of the pan with your weight on it, thus extend the life of the pan.