Puddle flange recess. Why the flange sits flush.

The flange is only a few millimetres thick. Set on top of the floor, those few millimetres are enough to hold a ring of water around the waste for the life of the bathroom.

What the standard asks for

• The flange is recessed into the substrate so it finishes flush.
• It is sealed to the pipe and fixed so it cannot move.
• A bead of sealant is tooled where it meets the floor.
• The membrane is then bonded onto the flange.

On concrete

The recess is formed when the slab is poured, or cut in afterwards. Cut it before the primer, vacuum it out, and bed the flange so its rim is level with the floor around it.

On a sheet floor

A flange screwed on top of a sheet floor sits proud, which the standard does not allow. The recess has to be formed in the floor sheet around the pipe, before the membrane. On timber, particleboard and plywood floors the code also asks for the whole bathroom floor to be waterproof, so this join carries more than it does on concrete.

The check

1. Lay a straight edge across the flange. It should not rock.
2. Push on the flange. It should not move.
3. Look for the sealant bead all the way round.
4. If any of the three fails, stop before priming.

All five rules for the flange are in puddle flanges: what AS 3740 requires.