The following post includes excerpts from our book Understanding Architectural Details – Residential.
A roof is considered to be flat when it has a slope of 10o or less to the horizontal, according to BS6229. A purposefully designed slope in the roof either through firrings or laying joists on a slope helps to evacuate water from the roof and avoid pooling.
A flat roof construction usually consists of the following:
Waterproof membrane to prevent water penetrating the structure and interior of the building
Roof deck, providing the base for the waterproof membrane, and in some cases the insulation
Load bearing or primary structure, usually constructed in timber in a residential construction.
The construction of the flat roof is similar to that of the timber upper floor. Joists are placed on edge, spaced at 400-600mm centres, supported by external and internal load bearing walls. TRADA and Approved Document Part A give guidance on sizes of joists for flat roofs relating to spans and loading. Strutting is fixed between the joists to provide lateral restraint.
Depending on the roof build up, a roof deck is then fixed to the joists over firring strips, to provide the fall of the roof. The roof deck material is usually chipboard or plywood. The firring strips allow the roof to have the slope required to enable water run-off to the rain water outlets. An alternative to this method sees insulation boards made into shallow wedge sections to provide the fall.
Types of Flat Roof Construction
Similar to the pitched roof, a flat roof can be insulated as a cold or a warm deck. However, due to the flat nature of these types of roofs, ventilation can be problematic within cold roofs, causing condensation issues and therefore they are usually not recommended by manufacturers. The following types of flat roof construction are more typical to domestic and residential buildings.
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.
Firrings can be installed in line with the joists or at right angles to create the fall on the roof.
Warm Roof/Deck – Sandwich 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.
Warm Roof/Deck – Inverted Roof
The inverted roof puts the insulation above the roof covering. The insulation is then protected with a layer of 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.