A bituminous roof does not fail suddenly. Almost always there are several years of visible signs before a leak, and nobody acts on them because it is unclear whether they matter. This article is a catalogue of defects: what each looks like, what causes it, whether it is urgent and what actually needs doing.
How to read this list
Every defect is judged not by size but by two things:
- Whether water is already entering the build-up. If so it is urgent, however small the area.
- Whether the defect spreads. Wear stays put; movement and moisture do not.
Those two questions matter more than any visual impression. A large bare patch may be harmless, while a two-centimetre split at a parapet is critical.
11
most common bituminous-covering defects, with fixes
3
urgency levels: watch, plan, act now
1
key question: has moisture already reached the build-up
Loss of mineral surfacing
What it looks like: the cap sheet lightens or darkens, black bitumen shows through, granules wash into the outlets.
What causes it: UV, temperature cycling, mechanical abrasion, concentrated water flow in one place.
Is it urgent: usually not. The surfacing is UV protection, and losing it means the bitumen starts ageing faster. But that is a process measured in years, not months.
What to do: monitor. If the surfacing is gone across the whole area and hairline cracking is visible, the covering is approaching the end of its life and replacement is worth planning.
Local loss in a narrow stripe is a separate signal — it indicates concentrated run-off. Usually beneath a row of solar modules or below a sill.
Biological growth: moss, algae, plants
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What it looks like: greenish or black deposits, tufts of moss in the laps, sometimes self-seeded plants.
What causes it: persistent damp. Growth does not appear where it is dry — so it is itself a map of ponding and poor fall.
Is it urgent: it depends what is growing. Algae and moss do not by themselves eat through the covering, but they hold moisture against the surface and establish in laps. Woody seedlings — birch, willow — are another matter entirely: their roots seek water and find the smallest gap.
What to do:
- Woody growth is removed immediately, while the roots are still shallow.
- Moss in laps is removed carefully — mechanically, without scoring the covering.
- Algae usually need no action, but they show where the fall wants checking.
One important note: the covering should not be pressure-washed — that strips the mineral surfacing and removes the UV protection along with it.
Blisters
What it looks like: round bulges from a few centimetres to tens of centimetres across. Soft in warm weather.
What causes it: air or moisture beneath the covering. As the roof heats they expand and lift the membrane. The cause is usually a damp substrate at installation or inadequate bonding.
Is it urgent: a blister is not itself a leak. But it is a weak point: thin, stretched, vulnerable underfoot and liable to split in frost.
What to do:
- Small and isolated — monitor.
- Large, or where people walk — repair: cut crosswise, dry out, patch over.
- Many blisters across the whole area — that is a systemic problem, not a defect. It means moisture in the build-up.
The last case matters: patching every blister is pointless when there are many of them. The source of the moisture has to be found.
Wrinkles and ridging
What it looks like: the covering lies unevenly, with long waves or wrinkles, often parallel.
What causes it: an uneven substrate, settled insulation, movement of the covering, or an installation fault — the roll was laid stretched or too slack.
Is it urgent: not in itself, but wrinkles collect water and debris, and the covering wears faster along them. Cracks appear there in time.
What to do: establish the cause. If it is settled insulation, patching will not help. If it is a laying irregularity and the covering is sound, it can be left under observation.
Open laps
What it looks like: at a joint between rolls an edge can be lifted; sometimes the gap is simply visible.
What causes it: insufficient heat when laying, dirt in the lap, or movement of the covering.
Is it urgent: yes. It is a direct route for water into the build-up. A lap is the easiest place for water to get in.
What to do: repair without delay. The area is cleaned, dried and re-bonded, or covered with a patch lapping onto both sides.
Important: sealant does not repair a lap. It lasts a season or two, and then the problem returns — by which time the moisture has spread further.
Cracking
Two entirely different things need separating here.
Irregular, crazing-type cracks. That is material ageing — the bitumen has lost its flexibility. It indicates the covering is approaching the end of its life. Local repair is possible but it buys time.
A straight split. That is structural movement — a movement joint, a board joint, the boundary between differing parts. Patching is pointless: it will return in the same place. The detail itself has to be resolved so that it permits movement.
The diagnostic sign is simple: if the split runs in a straight line and recurs after repair, the cause is beneath the covering rather than in it.
Mechanical damage
What it looks like: cuts, punctures and scuffing, usually irregular in shape.
What causes it: maintenance work, dropped objects, clearing snow with a shovel, servicing without walkways, cables dragged across the covering.
Is it urgent: yes, if it goes through the full thickness. And the damage often does not coincide with the ceiling stain.
What to do: repair it and remove the cause. If damage recurs near an air handling unit, the answer is a walkway, not patches.
Wind damage
What it looks like: the covering lifted at an edge or corner, fastener positions where the plate has pulled through, a strip released along the parapet.
What causes it: wind uplift. It is greatest at corners and along edges, so fixing density there has to differ from the field. Uniform density across the roof means the corners are underfixed.
Is it urgent: yes, and more so than it appears. A lifted edge becomes a lever in the next storm — wind gets beneath the covering and tears away far more than the original damage.
What to do: refix without delay and review the fixing density in that zone rather than simply putting the covering back. If the damage recurs in the same corner, the cause is in the calculation, not the weather.
This defect matters most on lightweight construction — profiled steel decks, where fasteners hold in thin metal.
Detail failures
Most real leaks originate here, so this group deserves the most attention.
- Parapet. The covering has slipped down the upstand, the top edge has released, the trim has split. Urgent.
- Penetrations. A gap or hardened sealant around a pipe or vent. Urgent.
- Outlet. The covering has separated from the flange, water stands around it, no leaf guard. Urgent.
- Junction with a wall. The covering is simply bonded to render and has released. Urgent.
- Redundant mounts. Old aerials and brackets — penetrations serving no purpose.
The common thread: details are cheap to repair and their consequences are not. Sorting one detail costs little; saturated insulation means replacing it.
Structural indications
These are not covering defects, but they are visible on it and often confused with them.
- Ponds that never dry. Inadequate fall or deck deflection. The covering is not at fault, and replacing it without correcting the fall is money into a puddle.
- Ponding in a narrow strip. A valley with no longitudinal fall.
- Depressions under plant. Settled insulation of inadequate compressive strength.
- Ice at parapets in winter. A thermal bridge at the detail.
Fall questions are covered in detail in our separate article.
Moisture in the build-up: the hardest case
Why this is the hardest case
Once water reaches the insulation layer, surface patching does not help — the moisture stays trapped inside and spreads. It often means opening up, drying out or replacing a whole section, not just the covering.
What it looks like: the covering feels soft underfoot, many blisters, ceiling stains in several places at once, uneven snowmelt in winter.
What causes it: a detail left unrepaired for a long time, or a leaky vapour barrier.
Is it urgent: yes, and not because of the covering. Saturated insulation does not dry — it gets replaced. Every season enlarges the zone and the cost rises in direct proportion.
What to do: thermography to map the zones, a core sample to confirm, then a decision on partial or full replacement. Patching such a roof means paying twice.
Urgency scale
| Defect | Urgency |
|---|---|
| Open lap | Immediately |
| Split at a detail or parapet | Immediately |
| Puncture through full thickness | Immediately |
| Covering released from an outlet | Immediately |
| Soft covering, moisture in build-up | Shortly |
| Recurring straight split | Shortly, with a detail solution |
| Large blisters where people walk | Within the season |
| Crazing-type cracking | Plan replacement |
| Wrinkles without cracking | Monitor |
| Loss of surfacing | Monitor |
How to record defects
A survey without notes is worth nothing a year later. The minimum discipline:
- A photograph from a distance and close up. The first shows where, the second shows what.
- Date and weather. Whether it had rained beforehand matters.
- Position on a plan. A simple sketch with numbers is enough.
- A short description. What you see, not what you think.
- Repeat a year later. Only comparison shows whether the defect is spreading.
The last point matters most. One visit shows condition; two show direction — and direction is what determines whether to act now.
These records are useful elsewhere too — we cover that in our warranty article.
What you can do yourself
Safe self-help
Clearing debris, temporarily sealing a small crack and photographing defects before the rain — all realistic and useful. Walking on a wet or icy covering, cutting open blisters or painting the whole roof — leave those to a specialist.
It is only fair to say where the boundary lies. Some things need no contractor; others only get worse without one.
Worth doing yourself:
- Clearing outlets and the area around them. Arguably the single most useful thing there is.
- Removing self-seeded woody growth while it is small.
- Clearing leaves and debris, especially in autumn.
- Photographing and recording whatever is visible.
- Checking that the overflow is not blocked.
Better left alone:
- Do not apply sealant “temporarily”. That area will have to be cleaned later, and a patch will not bond over sealant.
- Do not pressure-wash. It strips the surfacing.
- Do not clear snow with a shovel. The covering suffers faster than it would from the snow load.
- Do not walk on it unnecessarily, particularly in cold weather — chilled membrane is more brittle.
- Do not open blisters unless you know how to close them properly.
The general rule is simple: anything to do with clearing and observing is yours; anything to do with the covering is not. And where there is doubt, a photograph costs nothing while a hasty repair can cost the warranty.
Mistakes in repair
- Treating the symptom, not the cause. A patch where it drips rather than where water enters.
- Sealant instead of a detail solution. It lasts a season, then the problem returns larger.
- Patching onto a damp substrate. The patch will not bond and the moisture stays sealed in.
- Patching every blister. If there are many, the problem is systemic.
- Repeating a repair in the same place. If it comes back a third time, the cause is something else.
- Not correcting the fall when recovering. The ponds will remain.
- Not recording what was done. Two years on, nobody remembers.
What drives repair cost
- Whether it is field or a detail. A detail always costs more than the same area in the field.
- Whether stripping is needed and how many layers.
- Whether the insulation is wet — often the largest share.
- Access — height, vehicle access, whether the building stays in use.
- Whether investigation is needed — thermography, core samples.
- Whether the cause is being fixed or only the symptom.
The last point explains why the cheapest quotation is often the dearest: it usually addresses the symptom. General pricing is set out in our cost article.
Found a defect and unsure whether it's urgent?
We survey it, establish the cause and tell you what needs doing now and what can simply be monitored.
How defects are assessed
Assessing defects: 6 steps
Inspection after rain
Ponding, damp areas and how quickly water clears are all recorded.
Reviewing the field
Surfacing, blisters, wrinkles and the nature of any cracking – crazing or straight – are assessed.
Checking the details
Parapets, penetrations, outlets and wall junctions are inspected. Most causes are found here.
Assessing moisture
Thermography or a core sample establishes whether the insulation is dry and how far the zone extends.
Assigning urgency
Each defect is placed in a category: immediately, shortly, within the season, or monitor.
Decision and record
Works are planned and everything recorded with photographs and on a plan for the next survey.
Related reading
Frequently asked questions
Those already letting water into the build-up: open laps, splits at details and parapets, punctures through the full thickness and covering that has released from an outlet. Size is irrelevant here – a two-centimetre split at a parapet is more dangerous than a large bare patch.
A blister is not itself a leak, but it is a weak point – thin, stretched and liable to split in frost. Isolated ones can be monitored; larger ones where people walk are worth dealing with. Many blisters across the whole area indicate a systemic problem: moisture in the build-up.
If it runs in a straight line and recurs in the same place, the cause is beneath the covering – structural movement at a movement joint or board joint. Patching does not help; the detail has to be resolved so that it permits movement.
It can, but only as a temporary measure. Sealant lasts a season or two and then the problem returns – by which point the moisture has spread further. The lap is cleaned, dried and re-bonded, or covered with a patch lapping onto both sides.
Usually moisture in the build-up – saturated insulation beneath the covering. It is urgent, and not because of the covering: wet insulation does not dry and has to be replaced, while every season enlarges the zone. Patching such a roof means paying twice.
Only by comparison. One visit shows condition, two show direction. So photograph from a distance and close up, note the date and the position on a sketch, and repeat the survey a year later.
