Waterproofing is typically $700 to $1,800 in a bathroom quote. It is one of the smallest numbers on the page and comfortably the most consequential. Australia's insurers have been saying for years that water damage in wet areas is one of the most common and most expensive residential defects, and the reason is simple: a failed membrane does not announce itself. It quietly wets the frame for three years and then presents you with a bill larger than the original renovation.
What AS 3740 is
AS 3740 is the Australian Standard for waterproofing domestic wet areas. It is referenced by the National Construction Code, which makes compliance a legal requirement rather than a nice-to-have. It sets out where membranes must be applied, how junctions and penetrations are treated, and what substrates are acceptable.
In broad terms, and your renovator will confirm the specifics for your build:
- The shower floor is waterproofed in full, with the membrane turned up the walls of the enclosure.
- Shower walls are waterproofed to a height above the floor, and the whole wall where the substrate requires it.
- Wall and floor junctions, corners and penetrations for wastes, taps and pipes are treated with bond breakers and reinforcing, because that is where movement cracks a membrane.
- The area around the bath and basin and the perimeter of the room receive specified treatment.
- In a level entry shower or wet room the waterproofed area extends further, often the entire floor, because there is no hob containing the water.
Why membranes fail
Almost never because the product was bad. The common causes are all workmanship and program:
- Not enough curing time. Membranes need to cure before tiling. When a job is behind, this is the step that gets compressed, and it is invisible once tiles are on.
- Junctions skipped. Bond breakers at wall-floor junctions let the membrane stretch as the building moves. Without them, the first seasonal movement tears the membrane at exactly the point where water collects.
- Penetrations not sealed. Every waste, tap and screen fixing is a hole through the waterproofing. Each needs to be detailed individually.
- Wrong substrate. Standard plasterboard in a shower, or sheeting fixed without the right fixings and spacings, moves too much for the membrane to survive.
- Falls that do not fall. If the screed does not drain to the waste, water sits on the membrane permanently. Membranes are designed to shed water, not to be a swimming pool liner.
What to insist on in the quote
- Waterproofing as its own named line item, not absorbed into "tiling".
- A statement that the work will comply with AS 3740.
- A certificate issued on completion, in your name, naming the applicator and the product used.
- The warranty period on the waterproofing work, in writing.
- Confirmation that curing time is allowed for in the program, not squeezed.
If a renovator hesitates at any of those, that is your answer. Every renovator we match to a quote request can produce a certificate as a matter of course.
Signs your current bathroom is already failing
- Paint bubbling or plaster staining on the wall behind or beside the shower.
- A musty smell that returns no matter how much you clean.
- Grout that keeps cracking in the same place, or silicone that will not stay put.
- Springy or discoloured floor near the shower, or floor tiles that sound hollow.
- Damage on the ceiling of the room below an upstairs bathroom.
- Skirting boards or door frames swelling in the adjoining room.
Any of these means the water is already past the membrane. Patching the grout will not fix it. The membrane is the thing that failed, and the membrane is under the tiles.
The economics
Redoing a shower properly costs a few thousand dollars. Replacing rotted bearers, joists and wall framing, plus the room below, routinely costs many times that, and home insurance often treats gradual water damage from a defective membrane differently to a sudden burst pipe. Spending an extra few hundred dollars to have this done carefully is the best value decision available in a bathroom renovation.
Ready to get numbers on your own bathroom? Send one brief and up to three licensed Sydney renovators will quote the same scope, free and with no obligation. Start your brief.
