Working with flat roofs

Flat roofs are a common feature, particularly for commercial buildings, due to their ease of access, suitability for systems such as air conditioning, and they absorb sunlight faster than conventional roofs, therefore increasing internal temperature of the building quickly.

There are many types of covering including felt, rubber and liquid applied. Where possible RWS Ltd uses a liquid applied system as it proves itself in performance and aesthetics.

The benefits of a liquid applied flat roof system

The liquid coating is a high performance, cold applied, seamless waterproofing system designed for roofing, cladding, terraces, balconies and walkways. It can be installed on most surfaces from corrugated sheets to barrel vaulted roofs. It is UV resistant and can be installed on a cold or warm roof. The liquid application also lends itself to congested areas with awkward detailing or profiled sheeting.

Liquid coating is quick to install and many systems also have a fast curing time. On a new ply deck, it’s possible to get a building watertight within 30 minutes. Liquid applied roof systems are extremely durable and have an excellent bond quality to new or old surfaces making it ideal for refurbishment projects that require a fast curing time, such as access areas to flats.

For many of our commercial clients, RWS Ltd has integrated non-slip designated walkways on roofs for maintenance purposes and for fire escape routes.

Common flat roof issues and how to avoid them

Leakage

Flat roofs are not without their issues, the most common being water leakage due to standing water on the flat surface and thermal movement due to thawing/cooling. This causes the membrane to crack and reveal weak spots that will allow water to permeate the surface. This is an advantage of the liquid applied system as it doesn’t contract and expand. The liquid forms one sheet and moves as one single entity.

Condensation

Condensation problems occur due to poorly detailed insulation and ventilation, as well as roof membrane detachment due to trapped moisture and flashing defects resulting from poor installation or damage.

Damage

Damage can occur due to lack of maintenance or vandalism. This is where a liquid applied roofing system is beneficial as it behaves as one surface instead of a lot of sections joined together. The more joints and junctions, the easier it is for the water to find a weak spot and penetrate the surface, whereas the liquid system has no formal joints.

How to avoid flat roof issues

How to avoid common issues in flat roofs:

• Insulation – all roofs should be insulated. The type of insulant used and whether you’re constructing a cold or warm roof will determine the thickness of insulation to meet the approved documents.
• External wall – both types of roofing (warm and cold deck) need to extend past the external wall to avoid a cold bridge which can lead to internal damp issues.
• Vapour barrier – a vapour barrier should be placed on the warm side of the insulation, i.e. between the insulation and the ceiling finish.
• Don’t make it flat – flat roofs should not actually be completely flat. Instead they need a fall to prevent standing water.
• Fire protections – if the flat roof is within six metres of a boundary, the roof covering should be suitably fire rated.
• Correct markings – ensure materials being used are Kite marked or CE marked.
• Suitable upstands – ensure that upstands and flashings are suitable for the roof covering. A minimum 150mm upstand is usually required. Drainage outlet positions are also critical to avoid ponding.
• Accreditation – Use the right expertise; are your installers accredited?
• Mountings – plant and services must be mounted correctly and penetrations detailed correctly.
• Proper checks – visual inspection cannot always identify an issue. RWS Ltd always uses thermal imaging on a flat roof property to check for underlying issues. Images can be taken from inside and outside of the property to ascertain the root cause.