Fibreglass, Torch-On Felt and Liquid Flat Roofing Compared

Three Flat Roofing Systems Used on Irish Homes

Flat roofs are everywhere in Ireland, from kitchen and living room extensions to garages, dormers, porches and commercial units. They work well when they are built and maintained properly, but they are less forgiving than pitched roofs because water drains slowly and any weakness in the covering is quickly found. Choosing the right waterproofing system for repairs, maintenance or replacement makes a big difference to how long the roof lasts.

This article compares the three flat roofing systems that Patrick Hannifin Roofing works with: fibreglass, torch-on felt and liquid waterproofing. It explains how each one is installed, where each performs best, how they are repaired and maintained, and how to decide which suits your roof.

Why Flat Roofs Fail

Before comparing materials, it helps to understand what usually goes wrong. Most flat roof problems come from a handful of causes:

• Ponding water that sits on the roof because the falls are too shallow or the deck has sagged
• Movement and thermal stress that crack the covering at joints and edges
• Poor detailing at upstands, walls, rooflights, outlets and edge trims
• Ageing of the material through sunlight, frost and weathering
• Damage from foot traffic, fallen branches or work carried out on the roof
• Condensation from inside the building wetting the deck from below

Any repair or replacement needs to deal with the cause as well as the symptom. A new covering laid over a wet deck or a roof with no fall will fail early whatever material is used.

Fibreglass Flat Roofing

Fibreglass, also known as GRP, is formed on site from layers of glass fibre matting saturated with resin and laid over a new timber deck, usually OSB board. Once cured, a coloured topcoat is applied to protect the laminate from sunlight and weather. Pre-formed trims are fitted at edges and upstands.

Strengths of Fibreglass

The finished surface is seamless, so there are no joints for water to find. It is hard-wearing, resists foot traffic well and has a clean, uniform appearance. It suits balconies, dormers and extension roofs where a neat finish is important. Maintenance is minimal, and a fibreglass roof that has been laid correctly can last for decades.

Limitations of Fibreglass

Fibreglass needs a sound, dry new deck and dry weather with suitable temperatures for the resin to cure. It is not well suited to roofs with significant movement in the structure, and it cannot normally be laid over an existing felt covering. Repairs are possible but need care to bond new resin to the old laminate.

Torch-On Felt Flat Roofing

Modern torch-on felt is a reinforced bitumen membrane system. Rolls of membrane are laid in layers and bonded with a gas torch, which melts the bitumen on the underside of each sheet so it fuses to the layer below. A typical system has a base layer and a mineral-surfaced cap sheet, with all joints lapped and sealed.

Strengths of Torch-On Felt

Torch-on felt is one of the most cost-effective ways to waterproof a flat roof and is quick to install. It copes well with the Irish climate and with slight movement in the deck. The mineral finish protects the membrane from sunlight. Repairs are straightforward, because new membrane can be torched onto the existing system once the area is prepared.

Limitations of Torch-On Felt

Because it uses an open flame, torch-on work requires an experienced installer and careful fire safety practice, especially near timber fascias and at the junction with walls. The finish is functional rather than decorative, and the many joints, while sealed, are potential weak points if the work is not carried out well.

Liquid Waterproofing

Liquid systems, such as Lava 20, are applied as a coating that cures to form a continuous, seamless membrane. Many systems are reinforced with a fleece embedded in the first coat. They can be applied to new decks or, after proper preparation, over many existing roof coverings that are still sound.

Strengths of Liquid Waterproofing

Liquid membranes are well suited to roofs with complicated detailing, such as many upstands, pipes, rooflights and awkward shapes, because the coating flows around every detail without joints. They can extend the life of an existing roof that is worn but still firmly bonded, avoiding a full strip-out. No open flame is used.

Limitations of Liquid Waterproofing

The finished result depends heavily on surface preparation and dry weather during application. A liquid coating cannot fix a wet or rotten deck underneath, and it will not adhere properly to a covering that is blistered or lifting. An inspection is essential to confirm whether the existing roof is a suitable base.

Comparing the Systems Side by Side

Each system has its place, and the best choice depends on the roof rather than on the material alone. In broad terms:

1. Fibreglass suits new decks, balconies and roofs where appearance and foot traffic matter.
2. Torch-on felt suits most domestic extension and garage roofs where value and proven performance are the priority.
3. Liquid waterproofing suits complex roofs and the refurbishment of sound existing coverings.

All three need correct falls, sound decking, proper edge and upstand detailing and good ventilation or insulation design to perform well. More information on each system is available on our flat roofs page.

Flat Roof Repairs and Maintenance

Many flat roof problems can be repaired without replacing the whole roof. Splits and blisters in felt can be cut out and patched. Cracks in fibreglass can be ground back and relaminated. Worn but sound coverings can often be recoated with a liquid system. Edge trims, flashings and outlets can be renewed on their own.

Routine maintenance makes a large difference to the lifespan of any flat roof. Clear leaves and debris from the surface and outlets, especially in autumn. Keep gutters clear so water can leave the roof. Avoid walking on the roof unless it is designed for it. Have the roof inspected every couple of years and after severe storms, so small defects are caught before they become leaks. Our roof repairs service covers flat roof repairs as well as pitched roofs.

Insulation and Condensation Under a Flat Roof

Many older flat roofs in Ireland were built as cold roofs, with insulation between the joists and a ventilated gap below the deck. If that ventilation is blocked or missing, warm, moist air from the room below condenses on the underside of the deck, wetting the timber from inside even when the waterproofing is sound. Converting to a warm roof, with rigid insulation laid on top of the deck and the waterproofing on top of that, keeps the deck warm and dry and removes the risk. A replacement is the ideal time to make this change.

Choosing the Right System for Your Roof

The right system depends on the size and shape of the roof, the condition of the deck, how the roof is used, the budget and the finish you want. An inspection will show whether the existing covering can be repaired or overlaid, or whether a full strip and new deck are needed. A good roofer will explain the options honestly and recommend the system that suits the roof rather than the one that is simplest to install.

Fibreglass, torch-on felt and liquid waterproofing are all proven flat roofing systems when they are matched to the right roof and installed properly. Understanding their strengths and limits helps you make an informed choice for repair, maintenance or replacement.

For a flat roof inspection and advice on the best system for your property, contact Patrick Hannifin Roofing. Call 085 754 7244 or email patrickhannifinroofing@gmail.com.