Blisters are one of those flat roofing problems that can look far worse than they actually are.
I recently inspected a seven-year-old two-ply SBS roof in Surrey, BC, where blistering was visible across several areas of the main roof as well as at some of the parapet upstands.
Some of the blisters were fairly substantial.
At first glance, you could easily assume there was moisture trapped throughout the roof system. But assumptions are not much use when you’re trying to understand what is actually happening underneath a roof membrane.
So we opened it up.
Closer view of blistering and raised areas within the SBS cap sheet.
What was underneath the SBS roof?
We carried out two cut tests in different areas of the roof.
The roof assembly consisted of:
- Plywood deck
- Vapour barrier
- Two layers of insulation
- Two-ply SBS roofing system
Importantly, the roof assembly was dry at both locations we opened.
That doesn’t guarantee that every part of the roof is dry. Two cut tests only tell us what is happening at those particular locations.
But it was still a significant finding.
We weren’t opening a visibly blistered roof and finding saturated insulation underneath. What we found was a relatively young SBS roof with a dry assembly but widespread membrane blistering.
So the next question was obvious.
Why does an SBS roof blister if it isn’t wet underneath?
A blister doesn’t necessarily mean that water has entered the roof from above.
Blistering can occur when air, moisture or vapour becomes trapped between layers within the roofing system.
As the roof heats up during the day, whatever is trapped within that void expands.
If there is an area where the membrane hasn’t achieved a complete bond, the pressure can push the membrane upwards and create the blister you see on the surface.
The roof can therefore be dry when you inspect it today, while still containing blisters that developed because of conditions during its original installation.
Blistering and raised membrane visible in the SBS roofing around a rooftop mechanical unit.
Installation conditions can make a big difference
We don’t know what the weather was doing when this particular roof was installed seven years ago, so it wouldn’t be reasonable to state that weather caused the problem.
But it is one possibility.
Installing SBS membrane in cold or damp conditions can make achieving consistent adhesion more difficult.
Moisture present on the substrate, insulation or between membrane layers during installation can also become trapped within the roof.
That moisture may eventually become vapour as the roof heats up.
Even a relatively small amount of trapped moisture can cause problems when it repeatedly heats and cools over several years.
Incomplete bonding between the two SBS layers
Another possibility is the original application of the membrane.
With a torch-applied two-ply SBS system, you want good, consistent adhesion between the membrane layers.
If the membrane isn’t heated sufficiently, is heated unevenly, or is applied too quickly, small areas can remain where the layers haven’t fully bonded.
Those voids provide somewhere for air and vapour to accumulate.
Over time, repeated heating and cooling can make a relatively small void much more noticeable.
Eventually, what started as a small area of incomplete adhesion can become a sizeable blister.
The parapet upstands were blistering as well
The main field wasn’t the only place we found blistering.
Several areas along the parapet upstands were also affected.
This is worth paying attention to because vertical details can be more difficult to install correctly than the main field of the roof.
Changes of direction, corners and vertical surfaces all require careful application to achieve proper adhesion.
Where voids are left behind the membrane, blistering can develop on these surfaces in much the same way as it does across the main roof.
Finding blistering both in the field and at the upstands is another reason I would be looking closely at the original membrane application and installation conditions.
It doesn’t prove poor workmanship, but it certainly needs to be considered.
Blistering is also visible within the SBS membrane at the parapet upstand.
The roof also has ponding water
There was another problem with this roof.
It has very little effective slope in areas, and there is evidence of ponding water.
Ponding and blistering are two separate issues, and I wouldn’t automatically say that one caused the other.
However, standing water isn’t something I like seeing on any flat roof.
Areas that repeatedly hold water remain wet longer after rainfall and put additional demands on membranes, seams and details.
If major work were ever being considered on this roof, drainage and slope would definitely be part of that conversation.
Simply installing another membrane without considering why water is sitting on the roof wouldn’t address the whole problem.
Does an SBS blister need to be repaired?
Not necessarily.
This is where experience and judgement become important.
A small, stable blister isn’t automatically an emergency.
Sometimes disturbing a blister that is still watertight can create more problems than leaving it alone and monitoring it.
However, larger blisters, blisters that are continuing to grow, damaged blisters, or blisters affecting seams and critical details deserve closer attention.
If a blister eventually splits, cracks or is damaged by foot traffic, it can provide a route for water into the roof assembly.
That’s when something that was originally an adhesion issue can become a leak.
Can SBS roof blisters be repaired?
Yes.
Where the surrounding roof is in serviceable condition and the underlying assembly is dry, individual blisters can often be repaired.
The affected membrane can be opened and assessed, loose or damaged material dealt with, and the area reinstated using compatible SBS roofing materials.
But there comes a point where you have to look at the economics.
Repairing ten isolated blisters is one thing.
Repairing dozens or potentially hundreds of blisters spread throughout a roof is something completely different.
At that stage, the question isn’t simply:
“Can we repair them?”
It’s:
“Does repairing all of them make financial sense?”
Raised blistering visible within the SBS roof membrane adjacent to a skylight.
Does a blistered SBS roof need replacing?
This is probably the biggest misconception.
Blistering does not automatically mean the roof needs replacing.
This roof is only seven years old, and the two areas we opened were dry.
Before recommending replacement, I would want to understand the extent of the blistering, the condition of the membrane, seams and details, whether there is moisture elsewhere within the system, and how quickly the problem appears to be progressing.
If the majority of the roof remains serviceable, a planned programme of targeted repairs and monitoring may make considerably more sense than replacing the entire roof.
If blistering becomes widespread enough that repairs are effectively chasing one problem after another, then the conversation changes.
That’s why inspecting and testing the roof matters.
A cut test can change the conversation
Had we opened this roof and found saturated insulation, we’d be having a very different discussion.
Instead, the cut tests showed a dry assembly at both locations.
That’s useful information.
It doesn’t tell us exactly what happened during installation seven years ago, and it doesn’t prove the entire roof is moisture-free.
What it does tell us is that visible blistering alone isn’t enough information to condemn a roof.
Sometimes you need to see what’s happening underneath before deciding what happens next.
And that’s exactly why I prefer investigating a roof before jumping straight to replacement.
Frequently Asked Questions
Can you walk on an SBS roof that has blisters?
Generally, normal roof access is possible, but avoid deliberately walking directly over large or raised blisters. Concentrated foot traffic can damage an already stressed membrane, particularly during very hot or cold weather.
What time of year is best for inspecting roof blisters?
Ideally, inspections should form part of a regular maintenance programme rather than being limited to one season. Comparing observations over time can be particularly useful because it shows whether individual blisters are stable or increasing in size.
Can a thermal camera find SBS roof blisters?
Thermal imaging can be useful for investigating temperature differences that may indicate moisture within some roof assemblies, but it doesn’t replace physical investigation. Results depend heavily on roof construction and environmental conditions.
Will a roof warranty normally cover blistering?
It depends entirely on the warranty, the cause of the blistering and whether the problem is considered a material, installation or maintenance issue. The warranty documentation and manufacturer requirements need to be reviewed before assuming the work is covered.
Should roof blisters be marked during an inspection?
For widespread blistering, documenting and photographing significant areas can be very useful. It provides a baseline that can be compared during future inspections to see whether the condition is changing.
About Peter Coscolin
