In this flat roof guide, we will look at the different aspects to consider when designing a flat roof. Many of the images in this guide are courtesy of NHBC. The NHBC provide detailed guides and standards for UK Housebuilders. The guidance document is extremely comprehensive and well worth studying. You can find the NHBC Standards 2022 here.
Flat Roof Timber Structure
The flat roof structure must be designed in accordance with the current building regulations.
The roof must be designed so that all imposed loads, dead loads and wind loads are sustained and transmitted by it to the ground safely. Different loading applied to a flat roof can be the weight of the construction, foot traffic for maintenance, weather related like snow and water, and so on.
Green roofs and roof terraces must be specifically designed to carry the additional loads that they will receive.
The roof joists for a flat roof for dwellings can be specified according to the following sources:
- TRADA span tables for solid timber members in floors, ceilings and roofs
- BS EN 1995-1 Eurocode 5 Design of timber structures
- BS 8103-3 Structure design of low rise buildings Code of Practice for timber floors and roofs for dwellings
It is also advisable to seek the advice of a structural engineer when designing the flat roof.
Flat Roof Joists
The most common flat roof joist sizes tend to be 150 x 50mm, 175 x 50mm and 200 x 50mm. The spacing will depend on the span and loads, but usually we can see flat roof joist spacing between 400mm and 600mm centres.
All timber joists must be stress graded, and any timber including the wall plates, blocking, battens and firrings must be preservative-treated.
Where the span of a flat roof is more than 2.5m, strutting is required. This can be achieved by timber bridging or strutting in accordance with BS8103-3:2009.
For spans over 4.5m two rows of strutting will be necessary at 1/3rd the span position.
The timber roof joists are fixed to the timber wall plate which is secured with galvanised mild steel holding down straps nailed to the wall plate and securely fixed to the inner surface of the wall.
Flat Roof Structural Deck
Warm Deck Roof
Warm roof systems benefit from the insulation sitting above the deck. This enables the temperature of the structure and the deck to be kept close to the temperature of the inside of the building (hence warm). The insulation is tapered to provide a fall to the roof to enable water to run to the water outlets.
With the warm roof deck, there is less likelihood of condensation forming in the warm roof space, so ventilation is not required. A vapour control layer is still installed to minimise any moisture movement. This system is probably the most common arrangement for residential flat roofs.
The disadvantage of this roof system is that the insulation is directly under the roof covering, so the covering suffers considerable temperature fluctuations. An inverted roof system avoids this issue.
Inverted Warm Deck Roof
The inverted roof puts the insulation above the roof covering. The insulation is then protected with a layer of ballast, chippings or concrete paving. This system is often seen in more commercial projects. The waterproofing layer is protected by the insulation, but it can be difficult to locate defects if there is a leak.
Both of the warm roof options prevent wasteful cutting of insulation and decrease installation time, therefore labour costs.
A cold roof system sees the insulation placed immediately above the ceiling between the joists, with ventilation space above the insulation. The structural elements are not protected by a layer of insulation so they can deteriorate over time due to thermal movement. It is difficult to provide suitable ventilation to the void above the insulation to prevent condensation. This roof system is rarely used nowadays due to the difficulties in ventilation, thermal bridging and risk of condensation and failure.
Given the cold roof is less common, all information will be referring to a warm roof design.
Timber Structural Deck
A timber structural deck on a flat roof must be of adequate strength to maintain structural integrity and provide support to the roof coverings and possible maintenance access after construction.
Plywood and OSB are usually used for a flat roof timber deck. Roofing grade OSB should be manufactured to BS EN 300 grade OSB3 and certified by BBA. It must be specified at a minimum thickness of 18mm.
If specifying plywood, a minimum thickness of 18mm to a BS EN 636 minimum Service class 2 for humid conditions, or where required Service Class 3 – exterior conditions.
Boards must be laid with staggered joists and provide an expansion gap between boards of 3mm and 10mm at abutment with a rigid upstand.
Other options for flat roof decks include concrete and profiled metal decking. Concrete flat roof decks are more common in flats or commercial settings.
Flat Roof Thermal Requirements
The thermal performance requirements of the flat roof are set out in Part L of the Building Regulations. According to Approved Document Part L (2021 Edition) new roofs must achieve a U-value of 0.11 W/m2K.
Upgrading existing roofs must achieve a U-value of 0.15 W/m2K, refer to Approved Document Part L for full requirements for new elements in existing dwellings, extensions, renovating elements in existing dwellings and necessary U-values.
There are many types of insulation that can be used on a flat roof – often the choice of insulation is guided by U-Value requirements and compatibility with the roof type and waterproofing system chosen.
In a warm flat roof design, the insulation will usually be rigid insulation such as extruded polystyrene or compressed boards of rock fibre.
The insulation must be strong enough to take any loading from foot traffic on the roof, and be suitable for external use. Often separating membranes are required between the insulation and the waterproofing material. Refer to manufacturer’s recommendations.
To avoid cold bridging, the insulation must be continuous from the flat roof to the wall insulation, and penetrations must be carefully detailed. Insulation should be lightly butted to avoid thermal bridging caused by gaps.
Image Credit: NHBC
Flat Roof Condensation, Ventilation, Vapour Control
Condensation is a problem in flat roofs when warm moisture laden air passes into the structure and hits the colder air where the water vapour condenses. This is a particular issue in parts of the building that produce more moisture, such as kitchens and bathroom.
Condensation build up is a particular problem in cold deck roofs, and they must be designed carefully to include adequate ventilation. However the cold roof option is not recommended, a warm roof build up is a more robust solution.
A vapour control layer must be provided in a warm flat roof design. The vapour control layer is installed on the warm side of the insulation, that is, on the internal or room side. The vapour control layer stops vapour passing into the building assembly and condensing beneath the breather membrane, potentially causing moisture build up and damage to the construction elements.
A flat roof is never truly flat. It must be designed to include a fall to allow water to be evacuated effectively from the roof. Building regulations suggest a minimum fall of 1:40 for a flat roof. Different waterproofing finishes will require a different flat roof fall. For example sheet metal materials can be laid with a fall of between 1:60 and 1:80. It is best to consult wit the manufacturer of the waterproofing solution chosen to ensure the correct fall is designed into the flat roof.
Image Credit: NHBC
A fall in the flat roof can be created in a number of ways.
The flat roof can be given a fall by the structure by laying the joists at a slope using supporting tapered beams. This will create a slope in the roof both externally and internally.
Firrings are another way that a flat roof is given a fall. This is more common practice, where strips of tapered timbered are installed on top of the structural joists, either in line or perpendicular. The firrings create a fall in the roof as they are tapered. The firring option allows the internal ceiling to remain completely level.
Image Credit: NHBC
Tapered insulation can be used to provide the falls required to drain the roof. Tapered insulation is usually designed by the manufacturer and is often seen on more commercial flat roof projects rather than domestic flat roofs.
Any gutters and downpipes must be sized to suit the flat roof size and local conditions in accordance with BS EN 12056-3. The slope of the roof should be designed to slope toward the gutter and downpipes.
Image Credit: NHBC
There are a number of different products and materials that can provide waterproofing to a flat roof.
Types of flat roof covering includes:
- Single Ply Membrane
- Mastic Asphalt
- Bitumen Membrane
- Metal sheet roofing
- Green roof
Single Ply Membrane
Single ply roofing systems are becoming increasingly popular and are now not just an option for commercial projects. They consist of a continuous layer between 1mm and 3mm thick that provides excellent weather protection.
They are fixed over a polyester fleece to prevent damage to the membrane, using a variety of fixing options, from bonding to mechanical fixings and in some cases loose laid. Single ply roofing systems require specialist installers as the quality of workmanship is key to the performance of the material although they are simpler to install than hot methods.
Mastic asphalt is a jointless, weather proof and impermeable material with a low softening point. It is delivered onto site in the form of blocks ready to be melted before applying to the roof. The hot material is then spread over the roof in two layers which then cools and forms a hard waterproof surface. The layers tend to come to 20mm with up stands of 150mm to any masonry, roof lights or pipes. The mastic asphalt is laid over a sheathing felt to prevent any effects from structural or thermal movement, and finished with 10mm of stone chippings to protect the surface from softening under sunlight. This also prolongs the life of the mastic asphalt.
Asphalt is considered to be a relatively cheap flat roofing option, and more durable than built up bitumen sheeting, with a life of around 20 years.
Bitumen felt or sheet roofing consists of a mat of glass or synthetic fibres impregnated with a bitumen coating which creates a water resistant sheet.
The sheets are applied to the roof surface using a hot bonding bitumen, in three layers. The first layer is partially bonded to the roof, to allow for any movement. They are laid with side laps of 75mm, which is then staggered on the following layers to avoid excess side laps on top of one another. At eaves, the sheets are lapped over the fascia to an edge trim (usually aluminium) which drains into the gutter. Any upstands to parapets or walls will need to be 150mm high, and dressed into the masonry, with a protective flashing over the top. The final layer is finished with an aggregate topping, to protect the surface from sunlight, and extend the life of the roof.
The waterproofing material must be carefully detailed where the flat roof meets any external wall, parapet or pitched roof.
Image Credit: NHBC
Any upstands must extend 150mm above the finished roof level. Downstands should lap the upstand by a minimum 75mm.
Fillets can be used at abutments with walls to provide a more gradual bend for the roofing membrane.
Image Credit: NHBC
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