Best Practice > Roofs

A handy guide to effective roof construction and insulation.

Timber structures



One of a series of structural members of the roof spanning from an exterior wall to a centre ridge beam or ridge board designed to support roof loads. The rafters of a flat roof are sometimes called roof joists. Exposed rafters are a feature of traditional roof styles. It is sometimes more appropriate for smaller roofs to form the roofs in situ, i.e. building the roof from scratch one rafter at a time (cut roof).


An engineered, prebuilt structural component assembled from wood members and metal connector plates and designed to carry superimposed dead and live loads. The truss members form a rigid structural component and are usually assembled such that the members form triangles.  It is normal practice to purchase a truss prefabricated, although not essential. A truss manufacturer will be able to study your plans and provide you with a suitable truss to carry the weight of your roof. When your structure is ready to roof, the trusses can arrive on-site to be hoisted into place by a crane. This will reduce build time and is an appropriate method of construction for a site with limited storage or a large roof. It is possible to order a truss which provides for a ‘room in roof’.

Insulate on plane

Insulation level/types:

Insulation is easily accommodated on the plane, as it does not need to be retained and can be laid to any depth required. Mineral fibre insulation is the type most commonly used, as it complements both these conditions. However, there are other types suitable for insulation on the plane, such as cellulose fibre and natural wool. When the recommended depth of insulation reaches the top of the joists it is good practice to cross lay the next layer i.e. at 90°. This has two clear advantages over simply laying a thicker layer of insulation between the joists. Firstly, any gaps resulting from the lower layer, not quite filling the space between the joists, will be accounted for. Secondly, the relative thermal bridge of the ceiling joists themselves is addressed.

If additional storage space is required, the ceiling joists can be counter battened (and the void filled with insulation) with a tongue and groove chipboard flooring surface applied for walking on.

Eaves ventilation:

It is essential to ensure that any insulation on the plane of the ceiling at the eaves does not block ventilators located in the soffit. Ventilation is one means of inhibiting condensation, which could potentially damage roof timbers. There are proprietary ventilation trays which can be used for this purpose, which maintain an air gap above the insulation, allowing ventilation air to enter the roof space.

Insulate on pitch

Insulation on the pitch, or between the sloping roof timbers, means that the roof space itself is part of the insulated volume of the building and is therefore included in the heat loss calculations. Where you wish to maximise internal space, this can be a clear option and is sometimes referred to as ‘room in the roof’ construction. One key difference between insulating on the pitch as opposed to the plane is that it becomes more difficult to incorporate greater thickness of insulation. One option is to use a plasterboard laminar with insulation bonded to it, fixed on to the underside of the rafters. This will be a trade-off between reducing head height and increasing insulation. It does however address the relative thermal bridge of the roofing joists themselves. Rigid insulation boards are used since flexible materials could slump and block ventilation routes resulting in gaps in the insulation. However it can be difficult to eliminate gaps between the insulation edge and the timbers. Products with a ‘concertina’ action allow the board to be compressed from the edges before being offered up into position. The board will then expand to fill the full rafter spacing.

Cold roof (ventilation):

This still requires a ventilation cavity above the insulation to inhibit condensation on the cold timbers (see “insulate on plane”).

Warm roof:

Use of an insulation sarking layer over the tops of the rafters means that no part of the timbers are outside the insulation and therefore interstitial condensation is not an issue. This means that insulation can be incorporated to the full depth of the rafter. Above the sarking layer battens are fixed following the line of the rafters and counter battens can then carry the roof covering as normal.

Flat roof:

A type of roof which lies very nearly horizontally, and provides just enough pitch to allow for the drainage of water. The roof covering on a flat roof is known as the supporting deck. The deck of a flat roof can be constructed from timber, concrete and metal. A timber deck is constructed using roofing joists covered with lengths of plywood or oriented strand board. It is advisable to use marine grade plywood or oriented strand board in case a leak forms. The supporting deck must be clean and dry and free of any grease or oil.

A vapour proof barrier must then be laid over the deck with joints taped to maintain a continuous moisture resistant membrane. This membrane must continue over the edge of the flat roof and be secured to ensure that no moisture can penetrate the structure should the roofing membranes fail. The vapour proof barrier should be laid, immediately prior to installation of the roofing membrane. This insulation is laid upon this vapour barrier, then the deck of the roof will be within the warm area and is therefore called a warm deck. If the insulation is placed between the rafters below the deck, but no insulation is placed above the deck then the deck is in the cold area and called a cold deck. Regarding a suitable roofing membrane, the most basic is roofing felt, which is applied hot using a blowtorch and roller, with additional felting around all sides and over the top. This layer is quite often finished with a metal-based reflectant paint to reduce the detrimental effects of ultraviolet radiation. Insulation can be added between the rafters in addition to insulation on the warm deck to provide a better thermal performance.

If the flat roof is constructed as a cold deck then insulation can be laid below the rafters to provide a thermal break between the rafters and the inside of the structure. If the deck of your flat roof is to be concrete then the surface needs to be primed before applying any layers. It is also crucial that the concrete deck is firmly supported from beneath as the deck can split forcing the roofing layers above the deck to fail and let in moisture. If the deck is to be constructed from metal, i.e. corrugated iron roofing sheets, then some attention is required to minimise the effects of expansion and contraction on the roofing membranes. The insulation should be laid with the long side at 90° to the line of the corrugations, this will help reduce these effects on the upper roofing layers.