How do I stop a bath moving and flexing when I get in?

Why an acrylic bath flexes, the cradle and support batten that should be holding it, and why fixing this comes before any resealing.

Difficulty: intermediate
Time: 2 hr
Read: 3 min

Short answer

A bath that moves is under-supported. Check the legs are all bearing on the floor, that the cradle under the base is present and tight, and that the rim is screwed to a batten on the wall. Resealing a moving bath is pointless — the joint will part again within months.

An acrylic bath is a thin shell that relies entirely on its cradle, its legs and a wall batten for stiffness. Baths fitted quickly, or moved during other work, very often have one or more of those doing nothing at all.

What you'll need

A screwdriver and spanner set
Timber batten and fixings
A spirit level
A torch

Step by step

Take the panel off and look.You should see a metal or timber cradle under the bath's base, adjustable feet on each leg, and ideally a batten under the rim where it meets the wall.
Check every foot is actually bearing.Try turning each one by hand. Any that spins freely is carrying nothing, and the bath is spanning across it. Wind them down until they are firm.
Tighten the cradle fixings.The cradle is usually screwed or bolted to the underside of the bath through bonded pads. Those work loose. Tighten them, and check no pad has separated from the acrylic.
Check the rim is supported at the wall.The wall edge of the bath should sit on a timber batten screwed into the wall, not hang unsupported. If there is no batten, that is almost always the cause.
Fit a support batten if there is none.Level a length of treated timber under the rim, fix it into the wall with appropriate plugs, and set the bath's feet so the rim rests on it. This is the single most effective fix.
Level the bath afterwards.Check along and across with a level. A bath should have a very slight fall to the waste end and no twist at all.
Screw the rim down if the design allows.Many baths have fixing lugs or a flange at the rim. Screwing those to the batten stops the rim lifting when someone steps in.
Check whether the floor is the problem.A springy timber floor under a full bath moves regardless. Noggins between joists under the bath's line stiffen it considerably.
Only then reseal.Cut out the old bead, clean back, fill the bath and seal. Doing this before the support work simply wastes a tube of silicone.

Tips

A full bath with a person in it weighs around 250kg. Every support has to be genuinely bearing, not just present.
Use treated timber for any batten in a bathroom, and keep it above the tray or floor level so it is not sitting in water.
If the bath has always flexed since it was fitted, expect to find missing cradle screws rather than anything having failed.

Common mistakes

Resealing a flexing bath and hoping — The silicone is being stretched several millimetres every use. Nothing bonds through that indefinitely, and water is getting into the wall in the meantime.
Packing under the bath with bricks or offcuts — Point loads on a thin acrylic shell crack it. Support has to come through the cradle and the feet, which spread the load through bonded pads.

If it doesn't work

Base flexes underfoot

Cause: Cradle loose, or feet not all bearing — Fix: Tighten the cradle and wind every foot down until firm.

Rim lifts away from the wall when you step in

Cause: No support batten under the rim — Fix: Fit and level a timber batten and set the bath onto it.

Whole bath rocks on the floor

Cause: Uneven feet or an uneven floor — Fix: Adjust the feet individually with a level across the rim.

Floor bounces as well as the bath

Cause: Under-stiff joists or deck — Fix: Fit noggins between the joists beneath the bath before doing anything else.

Questions people ask

Is some flex in an acrylic bath normal?

A very small amount underfoot is normal in a thin acrylic bath. Movement you can see at the wall joint, or that makes the panel move, is not.

Do steel and stone baths need this too?

They are far stiffer in themselves, but they still need level, bearing feet and a supported rim — and their weight makes a weak floor more obvious, not less.