What torch-on is, and how it differs from Butynol
BRANZ describes torch-on systems as reinforced, polymer-modified bitumen membranes, fixed by torch or adhesive in several layers with a mineral chip top layer, and says flat roofs using them need falls of at least 1 in 40 to stop ponding. Resene, which calls it torchdown, says these membranes arrived in the late 60s and early 70s. Before that, flat roofs often had a plain bitumen coating, which cracks with age.
Butynol is different: a single rubber sheet glued down. The LBP Roofing licence lists torch-on roof membranes as an area of practice of its own. E2/AS1 covers only butyl and EPDM membranes, so a torch-on roof relies on its product appraisal instead, which matters if consent is involved.
What goes wrong with torch-on
Bitumen hardens in the sun, so older torch-on cracks, splits along laps and sheds its chip, leaving bare bitumen exposed. Blisters form where moisture got trapped between layers, upstands at walls pull away, and water sits wherever the fall has flattened.
None of that means the whole roof is finished. A roofer judges it by how well the layers are still bonded, how much of the surface has cracked, and whether the deck below is dry.
Test old bitumen membranes for asbestos
RANZ advises getting any bituminous membrane tested for asbestos before work starts. WorkSafe says homes built before 1 January 2000 may contain asbestos, and you can't tell by looking. Until a test says otherwise, nobody should cut, sand or strip an old bitumen roof without asbestos precautions.
Repair, recoat or replace?
Repair suits a sound roof with a few open laps, cracks or a failed upstand. The roofer cuts back the damage and bonds in compatible material, then rebuilds the edge detail.
Recoating a weatherproofing membrane to the maker's instructions is maintenance, and MBIE says it doesn't need a building consent if no other work is involved.
Replacement comes in when the layers have let go across the roof or the deck has got wet. Changing to a rubber membrane, re-falling the roof or altering the structure may need consent, and a like-for-like swap isn't exempt if the old roof failed early by leaking. If unsure, ask your council, and see flat roof repair.
Questions to ask a torch-on roofer
- Do you hold an LBP Roofing licence, and how often do you work on torch-on?
- Will you test the old membrane for asbestos before cutting or stripping it?
- What falls will the finished roof have, and where will water drain, including an overflow?
- How will you manage the fire risk from the torch?
- Is this a quote (a firm price) or an estimate, and what's the warranty?
What you can do, and what to leave
You can
- Watch ceilings under the roof after long, heavy rain
- Look from a window for standing water or bald patches where chip has gone
- Find out how old the house and roof are, and any repair history
- Check the roofer on the LBP register if the work needs consent
Leave to a roofer
- Any work up on the roof itself
- Torching: it's an open flame on a timber building
- Cutting or stripping an old membrane
- Re-falling or replacing the deck
What we'd do with an ageing torch-on roof
If the membrane went on before 2000, start with an asbestos test. Then have a roofer check the laps, upstands and outlets and tell you whether the layers are still bonded. A sound roof with a few failed spots is worth repairing. If it's cracked all over and holding water, we'd replace it, and ask the roofer to compare new torch-on with a rubber membrane.
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.