Built-Up Felt Roof Systems

by Practical Roofing

Practical Roofing Moreton-in-Marsh contractors are specialists in the application of built-up felt flat roof systems.

Built-up bitumen felt roofs are the traditional flat roof system common to many flat roofs in the United Kingdom.

What Are Felt Roofs?

Felt roofs consist of two or three layers of sheet felt materials that form a dense impermeable barrier.
Roofing felt is a composite of an absorbent sheet of organic or synthetic fiber that is impregnated/saturated and coated with a waterproofing agent such as tar or asphaltic bitumen that prevents ingress of moisture.

The felt roof system is built up of multiple layers of high performance felts and mastic asphalt over an insulated timber or concrete flat roof deck.
When applied the layers of vapour dispersion and control membranes are sealed with graded roofing felts to guarantee a waterproof installation.

There are several techniques for applying felt - depending on the materials used, however felt is generally rolled out over the roof and bonded together by way of hot bitumen or adhesives.Materials technology and new products have modernised the range of felts many manufacturers supply, so modern man-made and composite materials felt roofs that use modified bitumen; include rubber modified elastomeric bitumen and polymer modified bitumen are now available.

Hot Works

Pour and Roll technique: where hot bitumen at 220-240°C is poured in front of the felt as it is unrolled along the roof. The hot, sticky, viscous bitumen is poured down the front of the roll to provide a continuous bead of molten bitumen across the whole width of the felt. The bitumen fills voids and when cold it provides a total contact adhesive bond to the layers and it seals joints at the laps. Laps are never clean.

Torching-On: torch on uses specially designed felt that is heated with a gas torch. No bonding bitumen is required. The technique is generally used for small to medium sized roofs; and flat roof repair where the new material must be fused with the old to ensure total lap security or where access to the roof is difficult. The application is unsuitable for direct bonding to timber and wood based decks or lightweight polyester base materials. 

Cold Works

Cold Applied Adhesive is applied evenly onto the roof surface and the roofing felts are unrolled into position. Laps are sealed with the cold adhesive. 

Self-Adhesive Membranes: Self-adhesive membranes have undersides that include a high-tack adhesive with release paper. Once in the correct position on the roof, the release paper is peeled off, and the membrane will stick to the base. They are unsuitable for laying over uneven surfaces or in cold conditions. They require pressure contact to assist full adhesion. For the DIY market, they are used for houses and flat roof extensions, garages, garden buildings, workshops, sheds, children's playhouses, dog kennels and rabbit hutches.

Surface Protection

Built up felt roofs require a protective surface to prevent damage by UV radiation (solar) damage, human traffic and other impact.
Because asphalt is a natural product, it has a life span and the purpose of the finish is to delay degradation.
UV-rays in sunlight would otherwise have the capacity to oxidize the roof surface and cause the plasticisers to leach out causing the asphalt to become brittle and more prone to blistering and cracking, which can cause water leaks.

Top felt layer flat roof options include:

• Stone chippings bonded in a non bituminous compound.
• Mineral finish e.g. fine sand, green mineral aggregate or small slate flakes or sand bonded in a non bituminous compound.

Reflective paints which require re-application every 3 to 4 years.
Roof balconies and terraces require the covering to protect the roof membrane from the human traffic and pressures that could puncture of crack the surface and break the waterproof seal. 

Roof covering options include:

• Tiles such as porous concrete, fibre cement, plastic or rubber and bedded in an adhesive
• Concrete slabs or paving stones on support pads.

Advantages and Benefits of Felt Roofs

• A proven roofing system
• Readily available
• Applicable to different scales of roof - small to large
• High materials have established ratings and standards
• Can incorporate new products and technologies
• Seamless flat roof coverings can be created
• Durable materials
• Not prone to wind shear and peel
• Can be created for foot traffic
• Suitable for all surface types - flat, pitched, curved
• Suitable for intricate detail
• A range of application methods and techniques available

Roofing Felt Standards

Various British Standards exist for bitumen felt. Practical Roofing Moreton-in-Marsh install only roofing felt product supported by BBA (British Board of Agreement) Certificates.

The British Standard 747:2000 has a range of classifications for a range of felts (pour and roll) by their base and function. Felts are colour-coded down one edge based on the classification type.

• Type 5 Blue: polyester base, recommended, strong and durable
• Type 5U - under layer
• Type 5B - top layer/cap sheet requiring solar protection (paint or chippings)
• Type 5E - mineral-finished top layer
• Type 3 Red: glass fibre base - certain grades have specific uses:-
• Type 3G - perforated sheets, used as a specialised under layer to provide a regular partial bond (see Model Specifications).
• Type 3B - Not recommended as a working layer in residential buildings.
• Type 3E - mineral-finished cap sheet - could be used over a type 5B felt for solar protection and additional security.