Flat Roof Design

Generally, flat roof design should be avoided where possible as they have a relatively low life expectancy due to the high risk of leaking and condensation. For flat roofs, it should be noted that concrete flat roofs without covering are highly susceptible to leaks. For this reason, waterproof covering should be provided where concrete flat roofs are used.

Chipboard should never be used as decking for a flat roof (this includes prefelted chipboard). The timber used should be either Bond Class plywood or moisture resistant oriented strand board (OSB3) and the designer of the roof should be sure to use the correct decking for the correct purpose.

Ensure the roof covering is high quality. Ensure that it has an agreement or equivalent certificate and that it is laid in accordance with the instructions set out by the manufacturer.

The design fall of a flat roof should never be less than 1 in 40 and it is vital that all water drains without ponding.

Diagram D73 - Typical warm deck roof detail

Please note in the detail above the insulation is provided above deck level and must be rigid.

Diagram D74 - Typical inverted timber deck roof detail

Please note in the detail above the insulation is provided above the weatherproofing. It is critical that the insulation is fit for purpose and is rigid.

It is important to ensure that ridges at outlet points do not lead to ponding.

In the case of balconies, ensure that there is a 150 mm upstand at any entrance doors and also that there is a second rainwater outlet. In order to reduce solar heat gain and as a fire prevention precaution, provide a layer of chippings to felt roofs. Treat other finishes with a proprietary reflective coating.

Where a flat roof area is surrounded by parapet and upstand walls, ensure the provision of a roof drainage overflow in the event that drainage outlets become blocked.

Diagram D75 - Typical section through duplex stairs

When constructing entrance steps to duplex dwellings particular attention should be given to the location of DPC and the vapour control layer.

Cold Deck Construction

In the case of cold deck roofs, ensure that the plasterboard used in the ceiling is foil back and that a vapour control layer is tacked to the bottom of the joists to control the build-up of moisture in the insulation. Cold deck roof construction is not recommended unless absolutely necessary.

Please note it is advisable that firring pieces are used to achieve a fall in a roof; in the case of concrete decks, screed to falls.

Diagram D76 - Typical cold deck roof construction

The construction detail above provides insulation between the joists

Ensuring Roof Space Ventilation

Where cold deck construction has to be used, ensure the roof space is thoroughly ventilated to ensure moisture does not build up and cause timber decay as detailed in the diagram below. A properly lapped and sealed vapour control layer is essential in controlling moisture build-up in insulation.

Diagram D77 - Typical cold deck roof construction with adequate ventilation

Nailing counter battens on top of the firring pieces is an effective way of ensuring thorough ventilation of the roof space. Alternatively, preformed ventilators can be installed at the abutment as illustrated.The diagram above shows alternatives for providing ventilation at the abutment.

Flat Roofs