Do you have to put anything underneith a fiberglass tub? I'm replacing a steel tub with a fiberglass one.

Hi Mjcj1

I put structolite (see image) under every fiberglass or acrylic tub/shower, tub only, or shower only installation. You could also use mortar...

Check with the manufacturer of your tub,of course, but most tub/shower units get placed in some bedding substrate for best overall job! Another benefit is that the bedding substrate will help to insulate the tub and therefore reduce heat loss from hot water leaving you more time to soak up a nice warm bath! ~

MARK

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Originally Posted by massplumber2008

Hi Mjcj1

I put structolite (see image) under every fiberglass or acrylic tub/shower, tub only, or shower only installation. You could also use mortar...

Check with the manufacturer of your tub,of course, but most tub/shower units get placed in some bedding substrate for best overall job! Another benefit is that the bedding substrate will help to insulate the tub and therefore reduce heat loss from hot water leaving you more time to soak up a nice warm bath!!~

MARK

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Thanks Mark,

I should put this down on the concrete as a cushion between floor and fiberglass tub? Do I put down the mortar dry or mix it first so it will set around tub?

Exactly...

Mix it first. Mix it so it is pretty thick and then pour it out in the "footprint" of the tub about 2" taller than needed when you set the tub. Then you set the tub and "squish" the mortar out by pushing down on the unit as you level it from side to side and from front to back.

Structolite is much easier to work with than mortar as it is extra fluffy light and squishes nicely AND it dries rock hard. Structolite is a perlited gypsum plaster product... sometimes available at home depot or a local sheetrock/plaster supply company.

You will be happy you took the time to do this... promise!

MARK

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Originally Posted by mjcj1

do you have to put anything underneith a fiberglass tub? I'm replacing a steel tub with a fiberglass one.

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Mark gave you excellent advice on how to bed your tub but nobody told you why it must be bedded.
If the manufacture does not provide support a tub then the installer must bed the tub himself.
If the tub isn't bedded with mortar, cement or Structolite then it will "flex and give" a bit every time someone steps into it. In time this can rupture the drain seal and develop a leak. It can also put a strain of the fittings below the drain. And now you know the rest of the story. Good luck, tom