What kind of membrane is on your roof?
Butyl rubber. ARDEX Butynol is a synthetic butyl rubber membrane made in New Zealand, which ARDEX says has been trusted here for 50+ years. It's BRANZ appraised and meant to be laid by an approved ARDEX installer. Viking Butylclad, a blend of 70% butyl and 30% EPDM, is laid only by Viking approved applicators.
EPDM and TPO. These are single-ply synthetic sheets. BRANZ has appraised both, including an EPDM rubber sheet for roofs, decks and gutters, and Everguard TPO, which is fully bonded to the deck below.
Torch-on. Reinforced bitumen membranes laid in layers with heat, usually finished with mineral chip. There's more on torch-on roofs.
Older bitumen. Resene says torch-on membranes arrived in the late 60s and early 70s, and before that bitumen was simply a coating that cracks with age.
The Acceptable Solution E2/AS1 only covers butyl and EPDM laid over plywood. Other membranes rely on their own appraisals, which matters if a building consent is involved.
Why do membrane roofs leak?
Not enough fall. E2/AS1 sets a minimum fall of 2 degrees (1 in 30) for membrane roofs and 1.5 degrees (1 in 40) for decks. BRANZ says inadequate falls let moisture collect and may raise the risk of deterioration, and Consumer NZ says ponding is most common on flat and membrane roofs.
Outlets with no backup. E2/AS1 wants an outlet of at least 75 mm with an overflow or a second outlet. One leaf-clogged outlet with no overflow can fill a roof like a bath.
Low upstands. Under E2/AS1, membranes should turn up walls by at least 150 mm, and sit at least 100 mm below door thresholds. Upstands lower than that are an easy way in for water.
Seams, punctures and movement. Laps lift, branches and dropped tools puncture, and BRANZ says Butynol normally needs no maintenance as long as the substrate underneath doesn't move much. When the plywood moves, the membrane suffers.
Flat roofs were also one of the design features common on leaky Mediterranean-style homes built from 1988 to 2004. If yours is that age, don't sit on a membrane leak.
Repair or replace?
Most damage can be repaired. BRANZ's appraisal of Butynol says damage is fixed by cutting out the damaged part and patching. Coloured butyl contains EPDM, which E2/AS1 notes is harder to bond to, so the repairer needs to know what's there. See Butynol repairs.
Replacement makes sense when the membrane is past its expected life, when laps are failing all over, when the plywood underneath has gone soft, or when the falls are wrong. BRANZ expects Butynol to give at least 20 years with proper maintenance, and TPO at least 15. A new membrane under E2/AS1 goes over plywood of at least 17 mm, so rotten ply is replaced, not covered.
Recoating an existing membrane to the maker's instructions is maintenance and doesn't need a building consent if nothing else is being done, MBIE says. Replacing it like for like can also be exempt, but not if it failed early by letting water in, because that falls outside the Schedule 1 exemption. The Building Code expects the building envelope to last at least 15 years. If you're unsure, ask your council before work starts.
What a good membrane quote includes
- The membrane by brand and product name, and its thickness: BRANZ lists 1.0 or 1.2 mm Butynol for roofs and 1.5 mm for walk-out decks
- Whether the installer is approved by the membrane maker, as ARDEX and Viking require
- What happens to the plywood underneath: kept, repaired or replaced
- The finished falls, and how outlets and overflows will work
- Upstand heights at walls and doors
- Whether a building consent is needed, who applies, and the LBP's name and Record of Work if so
- How butyl will be kept apart from any COLORSTEEL flashings, since NZ Steel's warranty excludes damage from wet contact with butyl rubber
- A quote (a firm price) rather than an estimate, with the warranty in writing
What you can do safely
You can
- Keep leaves off a deck you use and its outlet grate clear
- After rain, look at flat roofs from an upstairs window: water still lying the next day is ponding
- Check ceilings under flat roofs and decks for stains
- If you collect rainwater, BRANZ says the first 25 mm of rain off a new Butynol roof should be discarded before you collect drinking water
Leave to a roofer
- Going onto a flat roof at all: edges, skylights and clear sheets are fall risks
- Clearing roof outlets and internal gutters
- Patching, re-lapping and replacing membranes
- Old bitumen membranes, which RANZ says should be tested for asbestos before work starts
Our advice on a leaking flat roof
Get it looked at the first time you see a stain, because water under a membrane spreads along the plywood and the repair grows with it. Ask the roofer to check the falls and outlets, not just the wet spot. If the membrane is past its expected life and the falls are poor, we'd replace it properly once rather than patch it every winter. We can pass your job to one checked roofer who works on membranes.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.