Laying screed around drain gullies: achieving correct falls

A screed around a drain gully must carry water to the waste opening without leaving a hollow, a high rim or a trapped puddle. The finished floor may look level to the eye, yet fail the job if it falls away from the gully. Correct work starts with the floor build-up, not with the trowel.

Australian wet-area work sits within the National Construction Code, plumbing rules and standards such as AS 3740, with state authorities and building certifiers applying the requirements. Local trades commonly call the drain a gully and the outlet a waste, while metric notation such as 1:80 is familiar on sites from Melbourne to Brisbane.

Climate also matters: cool, wet conditions in Melbourne can slow some cementitious systems, while summer heat around Brisbane and Adelaide can accelerate drying. A robust result therefore combines the specified fall, compatible preparation, careful placement, protected curing and a documented test rather than relying on a single straight edge.

Plan the set-out before the screed goes down

Start with a floor set-out. Establish the finished floor level, the gully grate datum, the outlet direction and the extent of the wet area. Check whether the drain has a fixed flange, an adjustable grate or a linear channel, because each changes how the screed can terminate. Record the levels before trades begin covering the drain.

Use a laser or string line to transfer the datum around the room, then inspect the base for low spots, obstructions and sufficient room for the screed thickness. Visual reinforcement can support apprentice training, and trade video resources can offer useful demonstrations alongside hands-on supervision.

Calculate the fall from a fixed datum

Calculate the vertical change from the chosen datum using fall = horizontal distance × slope ratio. A 1:80 fall drops 12.5 mm per metre, so 3 m requires 37.5 mm. A 1:40 fall doubles that to 75 mm. Work from the actual gully edge to each perimeter or threshold, not from a guessed room dimension.

Typical design references include 1:40 for many shower floors and 1:80 for several general wet areas, but the project specification, NCC, AS 3740 and relevant state requirements control. Confirm every transition, including thresholds and doorways. A surface can appear to fall toward the gully yet reverse elsewhere, creating ponding that will not pass inspection.

Prepare the substrate and gully

Prepare a clean, stable substrate and detail the waterproofing as one system. Bonding agents, primers, reinforcement and edge restraints must suit the screed and membrane. The gully flange needs the correct bond or clamp arrangement, and no screed should bridge a movement joint. For adjacent blockwork, NAPL’s guide to float finishes on blockwork is useful when aligning rendered surfaces with wet-area preparation.

Check that the waste trap and outlet are complete, the grate sits at the selected datum and the frame is rigid. Do not allow mortar to enter the waste socket or trap. If plumbing alterations are needed, a licensed plumber must complete and test them; the screed trade should never cut into the waste connection to force alignment.

Place and shape the screed

Mix or pump the specified screed at the approved consistency and place it while workable. Strike it off from the datum, compact it and maintain the designed thickness rather than feathering the mix thinly around the gully. Bonded, unbonded and floating screeds have different preparation requirements, so follow the chosen system instead of adding water when the mix appears stiff.

Work toward the drain in controlled passes, using a gauge, batten or screed rail to preserve the fall. Keep the grate clear, form a neat termination and remove surplus material before it hardens. Avoid building a raised collar around the grate or a shallow dish at the wall; both can trap water or create a sharp edge under finished tiles.

Finish and cure without movement

Finish the surface with the specified float, trowel or sanding technique, then cure the product for its stated time and conditions. In cooler Melbourne weather, allow for slower strength gain; in hotter, drier conditions around Adelaide or Brisbane, shield against premature evaporation. Rapid-set and polymer-modified products have their own limits, so generic curing advice is no substitute for the technical data sheet.

Keep foot traffic, trades and stored materials off the screed until it can carry the intended load. Protect the gully from tile adhesive, plaster and debris without trapping moisture against it. Photograph the open detail before covering, and keep test records with the installation records so the concealed work can be verified later.

Test, inspect and correct defects

Before covering the floor, inspect from the grate toward every edge with a straight edge or string line. Look for hollow areas, settlement, cracks, movement around the waste and a grate that rocks or sits proud. Coordinate the required flood or water test with the plumber or applicator, following the specified method, and record the result and any remedial actions.

A high point cannot be fixed by feathering; lift and replace the affected area when necessary. Cracking may come from drying, movement or a weak bond, so diagnose the cause before patching. The same careful sequence appears in NAPL’s checklist for cracked cast repairs: identify the defect, isolate the cause, repair properly and document the completed work.

Make correct drainage a repeatable standard

Make drainage a repeatable standard. Use a pre-job checklist for the floor build-up, grate datum, required fall, waterproofing details and test handover. NAPL training materials, videos and eBooks can support apprentices and experienced plasterers refining their set-out routine. Before replacing tools, compare current trade listings and choose equipment that suits the work, not just the budget.

Put the method into practice on a mock-up, then take it to site: calculate the drop, set the datum, place the screed, protect the gully and inspect the finished fall before the tiler closes it. Use NAPL’s resources to reinforce the process, share the verified detail with the project team and sign off only after the agreed drainage test passes.