What Blisters and Bubbles on a Flat Roof Mean

Flat Roof Blisters and Bubbles Explained

Flat roof blisters are one of the most common things we're asked about on felt and fibreglass roofs across Longford and the rest of Ireland. You might spot a soft, raised patch on the roof of an extension, a garage or a dormer, or a few small bubbles in the surface of a fairly new fibreglass roof. Some stay the same size for years, while others grow, split and start letting water in.

In this article we explain what causes blisters and bubbles on a flat roof, how to tell a harmless blister from one that needs attention, what a proper repair involves on torch-on felt and fibreglass, and when the honest answer is a new roof covering. With the wet months ahead, autumn is a good time to have a look before winter rain and frost make a small fault worse.

What Causes Blisters on a Flat Roof

A blister is a pocket of air or water vapour trapped between two layers of the roof. On a felt roof that is usually between the felt and the deck, or between two layers of felt. On a fibreglass roof it tends to sit between the resin laminate and the boards, or between the laminate and the topcoat. When the sun warms the roof the vapour expands and pushes the layer up, and over many heating and cooling cycles the blister slowly grows.

The most common causes we find are:

• Trapped moisture during installation. If the timber or OSB deck was damp when the roof went on, or rain fell between layers, that moisture has nowhere to go and turns to vapour on warm days.
• Poor bonding. Torch-on felt that wasn't heated evenly, or cold-applied adhesive that didn't cover the whole surface, leaves gaps where air collects.
• Condensation from inside the building. Warm, moist air from a kitchen, bathroom or utility can pass up into the roof build-up if there is no effective vapour control layer, and condense under the waterproofing.
• Water getting in from a defect elsewhere. A split upstand, failed flashing or a damaged edge trim can let water travel under the covering and gather in one spot.
• Age and weathering. Older felt roofs lose their flexibility, layers separate and blisters appear as the covering nears the end of its life.

Bubbles on a Fibreglass Flat Roof

Bubbles on a fibreglass flat roof are usually smaller than felt blisters and often show up within the first few months. On a new roof they normally point to an installation problem. The laminate may have been laid on damp boards, the resin may have been applied when it was too cold or too damp to cure properly, or air may have been left in the matting because it wasn't rolled out thoroughly.

Small surface bubbles that sit only in the topcoat can sometimes be ground back and recoated. Bubbles that go through the laminate, or that flex when pressed, mean the bond to the deck has failed in that area. That section needs to be cut out, the deck dried or replaced, and a new laminate bonded in and overlapped onto sound material around it.

When a Blister Is a Problem

Not every blister on a flat roof needs to be cut open straight away. A small, firm blister on a felt roof that hasn't changed in size and has no cracks around it can often be left and watched. Cutting into it without good reason just makes an opening in a covering that is still waterproof.

A blister becomes a problem when it shows any of these signs:

• It is getting bigger from one season to the next.
• The surface is cracked, split or has lost its mineral finish.
• It feels soft or squelchy underfoot, which suggests water rather than air underneath.
• There are several blisters spread across the roof, not just one or two.
• You have damp patches, staining or mould on the ceiling below.
• Water is ponding on the roof and sitting around the blister.

Ponding water deserves particular attention. A flat roof should have a fall so water runs to the gutter or outlet. Where the deck has sagged, water sits on the surface for days, and any blister or split in that area is under constant pressure.

Checking from the inside

If you can safely look at the ceiling under the flat roof, check for brown rings, flaking paint or a musty smell. If the roof has an access hatch or a void, look for wet insulation or dark timber. These signs tell you whether the blister is only a surface issue or whether moisture has already reached the structure.

How Flat Roof Blisters Are Repaired

The right flat roof repair depends on the type of covering, how many blisters there are and what is underneath them. On a torch-on felt roof with one or two isolated blisters, the usual method is to cut the blister open in a cross or a square, fold the felt back and let the area dry out. Once the deck is dry and sound, the felt is bonded back down and a new patch of torch-on felt is laid over the top, overlapping well onto the surrounding sound covering.

If the timber deck under the blister is wet and soft, patching the felt alone won't last. The damaged boards need to be cut out and replaced before the waterproofing goes back on. Otherwise the new patch is sitting on rotten timber and the problem returns within a season or two.

On a fibreglass flat roof the affected section is cut out back to well-bonded laminate, the deck is checked and dried or replaced, and a new laminate and topcoat are applied with overlaps onto the existing roof. The weather has to be dry enough for the resin to cure, so timing matters in an Irish autumn.

Liquid waterproofing systems such as Lava 20 can be a good option on some roofs, particularly where there are awkward details or the existing covering is sound but tired. The surface still has to be dry and properly prepared, and blisters need to be dealt with first, because a liquid coating laid over a blister will simply follow its shape.

Repair or Replace the Flat Roof

A couple of blisters on a roof that is otherwise in good condition are a repair job. Widespread blistering is different. When blisters appear across large parts of the roof, it usually means there is moisture throughout the build-up or the layers have stopped bonding as the covering ages.

These are the main things we weigh up when advising a customer:

1. The age of the existing covering and how many times it has already been patched.
2. How much of the roof surface is blistered, cracked or separated.
3. The condition of the deck and joists underneath.
4. Whether the roof has an adequate fall or holds water.
5. Whether there is condensation coming from inside the building.

If replacement makes more sense, the old covering is stripped, the deck is repaired or replaced, any fall issues are corrected and a new system is installed. That could be torch-on felt, fibreglass or liquid waterproofing, depending on the roof and how it is used. You can read more about the systems we install on our flat roofing page.

What to Do If You Spot Blisters This Winter

Please don't walk on a blistered flat roof more than you need to, and don't pierce blisters yourself. Standing on them can split the covering, and a hole in a blister lets rainwater straight into the pocket. Instead, take a photo, note where the blisters are and check again after heavy rain or a few frosty nights.

Keep the gutters and outlets on the flat roof clear of leaves through November so water can drain away. If you notice a damp patch inside, put a container under any drip and get the roof looked at before the worst of the winter weather. Our roof repair service covers flat roofs as well as pitched roofs, so one visit can check the whole property.

Getting Your Flat Roof Checked

Blisters and bubbles on a flat roof are a sign that air or moisture has got where it shouldn't be. Some are harmless and can be monitored, while others point to wet decking, poor installation or a covering that has reached the end of its life. Finding out which applies to your roof early keeps the repair smaller and protects the ceilings and timbers below.

If you've noticed blisters, bubbles or ponding on a flat roof in Longford or anywhere in Ireland, contact Patrick Hannifin Roofing and we'll take a look and tell you honestly whether it needs a repair or a replacement. Call us on 085 754 7244 or email patrickhannifinroofing@gmail.com.