A roof opening is a deliberately framed gap in the roof structure for the passage of a chimney, a service penetration, a roof light or a roof window. The opening is planned at the design stage of the roof structure, so rafters, purlins and (where present) roof sheathing are interrupted and supported with a trimmer assembly.
Examples of roof openings
Roof openings are needed for things such as:
- Chimney flues for open fires, boilers or wood stoves
- Mechanical ventilation ducts (supply and extract)
- Soil and vent pipes that must terminate above the roof
- Roof windows, skylights and roof lights
- Aerials, solar collectors and PV arrays with roof penetrations
- Dormer windows, where a large area of the roof is opened up
Structural detailing
When an opening is wider than the rafter spacing, one or more rafters must be cut. The load path is then closed with a trimmer assembly:
- The continuous rafters on either side of the opening (the trimming rafters) are doubled up or made larger
- A cross-beam (trimmer) is fitted above and below the opening between these trimming rafters
- The shortened rafters (trimmed rafters) are connected to the trimmer beams
For light openings, a steel section or a doubled rafter may be sufficient.
Watertight detailing
Around the opening the connection must be watertight:
- Lead flashings or EPDM penetration sleeves for round pipes
- Prefabricated upstands around roof lights
- Tiles cut to fit around chimneys or roof penetrations
- Compriband or expanding tape for airtight and weather-tight detailing
Application
A well-detailed roof opening prevents leakage, cold bridges and air leakage. On the construction drawings, each opening is planned at an early stage, because the structural engineer must account for it when sizing the roof structure.
Related terms
- Roof penetration
- Trimmer
- Roof window
- Roof light
- Roof upstand
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