Repairing, overlaying or replacing a mastic asphalt roof
Updated 23 October 2026 · SEO Dons Editorial
A leaking mastic asphalt roof is not automatically a failed roof, and one that looks fine can have wet insulation beneath it. This guide is for the owner of an older asphalt-covered commercial roof choosing between repair, overlay and replacement, and it sets out what the survey has to show before that choice is made. It does not promote asphalt for new work. It is about the roof you already have.
How is a mastic asphalt roof laid, and where is it still specified?
The code of practice is BS 8218. BSI describes it as giving recommendations for the use of mastic asphalt in roofing for both flat and sloping roofs, including trafficked roofs such as rooftop car parks. The edition BSI lists is dated 15 January 1998 and shown as current, and the NFRC lists it among the standards relevant to roofing. A specification written to a 1998 code should still be checked against the manufacturer's current system data.
The Mastic Asphalt Council (MAC), the trade body for the material, says asphalt is laid molten, with solid blocks preheated in boilers at ground level. It also says asphalt is extensively used on bridges, car parks and other structural decks, that many architects choose it beneath green roofs, and that it is frequently specified by the National Trust and English Heritage. Those are trade body statements, but they match where owners tend to meet asphalt today: car park decks, other structural decks and older public or heritage buildings.
How does a mastic asphalt roof fail?
The Institute of Historic Building Conservation lists what to look for as cracks, blistering, ponding, shrinkage, splitting and tearing, and says failure is usually caused by poor design and workmanship. The MAC adds that vibration caused by wind can lead to material fatigue and crack formation, and that water expands when it freezes and can enlarge cracks.
Three survey notes follow. Ponding points to falls or drainage, covered in the guide to why flat roofs pond. Record the condition of upstands, outlets and other details too (see flat roof details that fail). And record whether a solar reflective finish is present: MAC project articles describe new asphalt being finished with solar reflective paint, so a missing or worn finish is worth noting.
What can be repaired?
The MAC says that because asphalt is seamless, imperfections can be found and repaired with a simple application of the material, and that spot repairs avoid costly wholesale replacement. It adds that the intention of repair should be to restore the asphalt to its original condition and ensure its continuing performance. The IHBC cautions against assuming that every unsightly blister or bump needs immediate attention, and against repair tapes, which it says are short-lived and may conceal problems.
Repair is credible when the cause is understood, the defects are local and the build-up beneath is dry. A repair deals with the covering. It does not dry insulation or correct falls. For the wider picture of splits and blisters see commercial flat roof problems.
When is an overlay sound?
The Liquid Roofing and Waterproofing Association (LRWA) says liquid-applied membranes are seamless and fully bond to the substrate, making them ideal over waterproofing that has failed at laps or seams or is coming to the end of its economic life, and that light colours help reduce heat stress on the existing system. It also publishes guidance notes on inspection, refurbishment and the drying of existing roof substrates before liquid systems are installed.
The conditions matter more than the headline. One manufacturer's instructions for its polyurethane membrane, Sika's Decothane Ultra, say asphalt must be power washed and dried, that major cracks must be sealed, and that the asphalt must be carefully assessed for moisture and air entrapment, grade and surface finish before coating. It also calls for a preliminary trial together with adhesion tests as required. Other systems differ, so read the certificate of the actual system. The BBA stresses that the content of a certificate should be read in full to confirm the parameters of use, and the guide to BBA certificates explains how to check one.
An overlay leaves the build-up beneath as it is, and it adds height at upstands and thresholds. A single-ply overlay on a recovery board is another route, compared in the overlay versus strip-and-recover guide. The commercial flat roof refurbishment page explains why a core sample, not a price per square metre, decides which route is open.
What must a core sample and moisture survey show first?
- The build-up at several points: asphalt thickness and layers, any earlier overlays, insulation type and deck.
- Whether the insulation is wet, mapped across the roof and calibrated against the cores.
- Whether the asphalt is still bonded to what lies beneath, and what is inside any blisters.
- Adhesion test results for the proposed overlay system on this asphalt.
- Falls and ponding, and the heights of upstands, thresholds and outlets before and after any overlay.
- The state of the surface protection.
Intrusive cores on an older roof should follow an asbestos check, as explained in the guide to asbestos in old roofing felt. The roof survey guide sets out the survey methods. Read the findings against this typical logic:
| Survey finding | Usual route |
|---|---|
| Local splits or blisters, dry insulation, asphalt still bonded | Repair the cause and spot repair |
| Widespread ageing, dry build-up, asphalt bonded, overlay system approved for asphalt with passed adhesion tests | Overlay |
| Wet insulation across areas, damaged deck or asphalt debonded | Strip and replace, at least in those areas |
| Listed building or conservation area | Check consent before changing the covering |
On the last row, Historic England says that removing flat roof coverings or changing roof levels may require consent, particularly if the building is listed, and that any change to the roofing material in a conservation area may also require permission.
When is strip-and-replace the honest answer?
Replacement is the honest answer when the insulation is wet over a significant area, when the deck is damaged, when the asphalt has lost its bond, or when repairs keep recurring because the cause is the build-up and not the surface. Water sealed under an overlay stays there. It is also the point at which to settle falls and insulation, so check the thermal and notification position in the building regulations guide before the scope is fixed.
The MAC says the Building Research Establishment has independently accredited mastic asphalt to last at least 50 to 60 years, with some roofs reaching 80 to 100. That is a statement about the material done well, not a forecast for your roof, and the survey decides. The guide to how long a commercial flat roof lasts compares systems. If new asphalt is laid, the molten material and boilers are a point to raise with your insurer, as the hot works guide explains. The wider choice of systems is in types of flat roof for commercial buildings.
To compare repair, overlay and replacement for your own roof, you can send roof details through the form on the refurbishment page.
Sources
- BSI Knowledge, BS 8218:1998 Code of practice for mastic asphalt roofing, checked 22 October 2026
- NFRC, Standards and regulations, checked 22 October 2026
- Mastic Asphalt Council, Flame-free roofing solutions, Why specify mastic asphalt?, Looking beyond short-term pricing, Dome sweet dome and A traditional material still going strong, checked 22 October 2026
- Institute of Historic Building Conservation, Asphalt, checked 22 October 2026
- Historic England, Insulating flat roofs (published 29 April 2016, under review), checked 22 October 2026
- Liquid Roofing and Waterproofing Association, Roofing overlay systems and Technical downloads, checked 22 October 2026
- Sika, Decothane Ultra product page (facts about that product only), checked 22 October 2026
- BBA, Why choose a BBA certified construction product, checked 22 October 2026
Common questions
How long does a mastic asphalt roof last?
The Mastic Asphalt Council says the Building Research Establishment has independently accredited mastic asphalt to last at least 50 to 60 years, with some roofs reaching 80 to 100. That is a trade body statement about the material done well. Your own roof's condition, falls and moisture state decide its remaining life.
Can a mastic asphalt roof be overlaid with a liquid membrane?
Often, if the specific system is approved for asphalt and the asphalt is sound and dry. Manufacturer instructions typically require cleaning, sealing cracks, assessing moisture and an adhesion trial. Read the certificate of the actual system before accepting a proposal.
Is a blister in asphalt an emergency?
Not necessarily. The IHBC cautions against assuming that every unsightly blister or bump needs immediate attention. The question is whether it is letting water in or hiding moisture, which a survey can answer.
Does laying new mastic asphalt count as hot works?
The MAC says there is no naked flame at the point of installation and that blocks are preheated in boilers at ground level. Whether your insurer treats the boiler or hot delivery as hot work is a policy question, so ask before tender.