Roofs are exposed to a lot of rainwater and need to be constructed to ensure that they are weathertight. Moderate to steep pitched roofs do not often fail, but low-sloped and horizontal membrane roof systems do. These need to be designed and installed accurately, and any low-sloped or flat building surface that may actually be part of a wall (such as the top of a parapet) needs to be treated as a roof surface. Roof design - in particular, the size and location of eaves overhang - can also help to minimise the exposure of external walls.
Builders are generally not involved in roof cladding selection or installation but still need to have an overall understanding of roof cladding requirements.
Roof cladding selection should not just be based on aesthetic considerations but on the performance characteristics and relevance to the proposed building and its location.
- specific environmental conditions of the site
- building design
- surface finish requirements
- availability of specific labour
- maintenance requirements
- secondary lines of defence.
Most roof claddings must be installed over a roof underlay, which generally provides the only back-up in case of water penetration through the cladding. Roof underlays form the secondary line of defence and must be accurately installed to ensure that any water on the underlay is drained to the external edge of the roof and does not pond beneath the roof surface.
Roof underlays must be carried over fascias and the lip of any gutter to ensure that any water is drained to the gutter. Note: With roofs below 10° pitch in high wind zones and above or the eaves width is less than 100 mm, eaves flashings are also required.
Metal roof claddings are non-absorbent and are vulnerable to condensation on the underside of the cladding. They must be installed over an absorbent roof underlay.
Metal roofs generally come in long-run sheets or long interlocking tiles. This means that there are few joints that can leak – as long as they are pitched correctly and flashed and installed accurately, they are generally weathertight.
Concrete and clay tiles are absorbent claddings, but these also should be installed over an absorbent underlay. In some situations, these roof types do not require underlay, but it is considered good practice and a worthwhile secondary line of defence to incorporate an underlay.
Tile roofs generally are formed with individual interlocking tiles – this means that there are a number of joints that can leak water. However, as long as they are pitched correctly and flashed and installed accurately, they are generally weathertight. (Refer to E2/AS1 Table 10 for minimum pitch and underlay requirements.)
Membrane roofs are similar to waterproof membrane decks, and the construction requirements are similar (see decks). Membrane roofs generally require a minimum fall of 2.0° (1.5° for waterproof decks), but it is considered good practice to increase this fall to 3° to ensure good water run-off.
Updated: 5 April 2019