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Flat roofs are quite rightly regarded as remarkably versatile and open up design possibilities unlike any other roof form. There are two benefits to this versatility: first there is the virtually limitless design possibilities which means green roofs can be incorporated as recreational spaces or natural habitats, photovoltaic arrays added for energy production, and then there are very specific and strict requirements in terms of insulation and waterproofing.

The flat roof also offers some very practical advantages: first and foremost, easy access. When a roof area is formed into a terrace or planted to achieve a green roof, the building gains more than simply an attractive exterior. It is also enhanced by the creation of extra living space – whether in a new build or as part of refurbishment.

Not surprisingly, a flat roof is the ideal place for a PV installation as it provides an unobstructed space that is perfectly positioned to catch as much sunlight as possible.

At the same time, a flat roof permits optimum utilisation of the interior space, as there are no sloping ceilings. All in all, a flat roof offers many advantages for everyone.

Flat Roof Design Considerations Overview

Thermal design

Thermal design

Thermal design is concerned with the flow of both heat and water vapour through the roof construction and their subsequent effect on the performance of the roof and the various components in the system. The designer therefore needs to consider the amount of insulation required to control both heat loss and condensation.

Insulation

Insulation

There are many considerations for the correct section and specification of insulation for a flat roof including compatibility with the waterproofing solution, required fire resistance, expected access requirements both during and after construction, and many more to take into account.

Condensation

Condensation

The temperature at which air becomes fully saturated with moisture (100% RH) is called the dew point. When warm moist air meets a cold surface, it is cooled, and if its temperature drops below the dew point it will give up moisture in the form of surface condensation.

Deck types

Deck types

Materials used for the roof deck should be selected from those listed in BS:6229 2018. It is important to refer to this document for design guidance, but for convenience, the various materials are outlined below.

Creating falls on a roof for rainwater drainage

Creating falls on a roof for rainwater drainage

It is a requirement of Building Regulation Part H that adequate provision is made for rainwater to be carried from the roof of the building. According to BS 6229 & BS 8217, flat roofs should be designed with minimum falls of 1:40 to ensure a finished fall of 1:80 can be achieved, allowing for any inaccuracies in the construction. This applies to general roof areas along with any internal gutters. 

Drainage

Drainage

Drainage needs to be provided via internal rainwater outlets and downpipes or via external guttering systems or hoppers. Even if a roof is very small, it is recommended there are at least two drainage points in case one becomes blocked.

Fire resistance

Fire resistance

The performance of a flat roof against external fire is covered by Building Regulations Fire Safety: Approved Document B test TS 1187. The test method classification covers ‘External Fire Penetration’ and ‘Spread of Flame’ for a complete waterproofing system (not individual products comprising that system). The results from testing under TS 1187 with BS EN 13501-5 classification are given as European Class ratings BROOF(t4), CROOF(t4), DROOF(t4), EROOF(t4) and FROOF(t4).

Wind load design

Wind load design

The calculation of wind load on a flat roof is essential to confirm the installed system will resist the pressure and uplift forces expected because of the design, height and location of the building.

Locating rooftop equipment

Locating rooftop equipment

From an integrity point of view, it is far better to site equipment directly onto a finished waterproofing system, as the less interruptions or penetrations to the membranes, the less likely the system is to allow water ingress. Plant is typically supported by paving slabs with a suitable protection layer beneath.

Foot traffic and defined walkways

Foot traffic and defined walkways

Whenever there is a requirement for regular foot traffic across a roof, for example to provide maintenance to rooftop plant or a fire escape route, it will be necessary to provide protection to the surface of the waterproofing.

Safe access

Safe access

All flat roofs will have a requirement for a certain amount of pedestrian access, whether this is for maintenance of the roof and any rooftop plant or as a fire escape route. It is therefore necessary to design any flat roof to allow for safe access.

Rooflights

Rooflights

Flat roof, windows, skylights, tunnels or glazing give more light to rooms or spaces in the building below come in a plethora of designs and glazing finishes. The product specified will need to comply with various Building Regulations depending on the type and function of the rooflight.

Edge protection

Edge protection

Edge protection is designed into a flat roof construction to protect people that are accessing the roof from falling from height. There are different options that can be incorporated.

Assessing requirements of a refurbishment flat roof

Assessing requirements of a refurbishment flat roof

An appraisal of the existing roof should always be the first step to assess the extent of any failing areas and what the remedial action should be.  

Overlaying existing waterproofing of a refurbishment roof

Overlaying existing waterproofing of a refurbishment roof

Fundamentally, in order to overlay a roof system, the existing waterproofing should be generally sound, dry, free from water damage and suitable to take an overlay system.

Upgrading insulation on a refurbishment roof

Upgrading insulation on a refurbishment roof

Increasing the thermal insulation within a roof makes perfect sense and has many benefits to the building, its reduction in energy usage for heating or air conditioning as well as supporting the environment.