The main types
Butyl rubber, sold as ARDEX Butynol among others, is laid in sheets over plywood. ARDEX says Butynol is BRANZ appraised and CodeMark certified, and installed only by its approved installers. Viking's Butylclad blends 70% butyl with 30% EPDM.
EPDM is a single-ply synthetic rubber, and TPO a fabric-reinforced plastic sheet bonded to the base. Torch-on is layers of polymer-modified bitumen, heated into place and finished with mineral chip. Resene says torch-on arrived in the late 60s and early 70s; before that, flat roofs had bitumen coatings that crack with age.
How the membranes compare
| Butyl and EPDM | TPO | Torch-on | |
|---|---|---|---|
| Expected life (BRANZ appraisals) | At least 20 years | At least 15 years | Check the product's own appraisal |
| Minimum roof fall | 1 in 30 | 1 in 30 on plywood | 1 in 40 in an older appraisal |
| Under E2/AS1 | Yes, over plywood | By its own BRANZ appraisal | By its own BRANZ appraisal |
What E2/AS1 expects of a membrane roof
Under the Acceptable Solution, a membrane roof is butyl or EPDM at least 1 mm thick over plywood of at least 17 mm. Roofs need a fall of at least 2 degrees (1:30), decks 1.5 degrees (1:40), and internal gutters 1 in 100 with no cross seams. Decks people walk on need membrane at least 1.5 mm thick.
The details matter as much as the sheet: upstands at least 150 mm up the wall, a 100 mm set-down below door thresholds, and an outlet at least 75 mm across with an overflow or a second outlet. Those small things are where flat roofs tend to leak.
Where a membrane makes sense
Membranes are for roofs too flat for metal or tiles. RANZ says membrane roofs often sit below 3°, flatter than even the lowest-pitch longrun. They also line internal gutters and waterproof decks over rooms, where E2/AS1 notes a wearing surface helps protect the rubber from foot traffic.
Flat roofs were a common feature of the 'Mediterranean' style homes at the centre of the leaky homes problem, built from 1988 to 2004. If yours is from that era, get a roofer's report before you buy or renovate.
Collecting rainwater? BRANZ says the first 25 mm of rain off a new Butynol roof must be discarded before you collect drinking water.
Upkeep, and why ponding matters
BRANZ says Butynol normally needs no maintenance as long as the structure underneath doesn't move much, and damage is repaired by removing the damaged piece and patching. Viking asks for its membrane to be checked twice a year for damage and debris.
Ponding is the enemy. Consumer NZ says it's most common on flat and membrane roofs, and BRANZ says poor falls let moisture collect and may speed up deterioration. Blocked outlets do the same, so keeping them clear is the simplest care a flat roof gets.
MBIE says recoating a deck's weatherproofing membrane, such as a painted fibreglass surface, to the maker's instructions doesn't need consent if no other building work is involved.
What makes a membrane job bigger or smaller
The state of the plywood underneath is the big unknown until the old membrane comes off. Fixing poor falls means building the roof up, and every upstand, outlet, skylight and threshold adds detail work. Shape matters more than size.
The LBP Roofing licence lists roof membrane, torch-on and liquid membrane roofs among its areas of practice. Where the job needs a building consent, it's restricted building work and an LBP must do or supervise it.
What a good membrane quote spells out
- The product by name, and its thickness
- Whether the installer is approved by the membrane maker
- What happens if the plywood underneath is damaged
- The finished fall, and how any ponding will be fixed
- Outlets, overflows, upstands and door thresholds
- The product warranty and workmanship guarantee in writing
What you can keep an eye on
You can
- Keep the outlet and overflow on a walk-out deck free of leaves
- Watch for puddles that linger after rain, from a window or the deck
- Check ceilings under the flat roof for stains
Leave to a roofer
- Patching splits and punctures
- Fixing falls and ponding
- Upstands, outlets and flashings
- Replacing the membrane or the plywood beneath
What we'd look at first
Before anyone talks brands, we'd want to know where the water goes. A flat roof with a proper fall, a good outlet and an overflow will outlast one with a better sheet and a puddle in the middle. One leak in one spot is often a patch job for an approved installer; ponding all over is a sign to fix the falls when the membrane is replaced.
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.