This timber cladding details post includes excerpts and details from our book Understanding Architectural Details – Residential.
Introduction to Window Details
Approved Document Part A states that the number, size and position of openings should not impair the stability of a wall, it provides detailed requirements limiting the size of openings and recesses. When designing openings in masonry it is good practice to use cavity closers, or DPCs backed with insulation to avoid cold bridging at the reveal. Door and window frames are set in position to overlap the outer leaf with a resilient mastic pointing as a barrier to rain penetration between the frame and the jamb.
The function of the sill is to protect the wall below a window, so they are shaped to slope out and project beyond the external face of the wall in order to allow water to run off. It is good practice for the sill to project at least 45mm beyond the face of the wall below and have a drip on the underside of the projection.
At the head of an opening the brickwork or blockwork requires support by a lintel. Steel section lintels are often used, they are a galvanised strip that also act as a damp proof tray, or can be dressed with damp proofing material. The lintel is filled with EPS insulation to reduce thermal bridging. However, given the steel lintel spans between the inner and outer leaf of the cavity wall, there is still heat flow through the material. A more effective lintel arrangement that can further limit cold bridging is two independent concrete lintels.
Some lintels have integral cavity trays while others need a separate cavity tray which is usually formed using DPC materials on site. The cavity tray sits directly above the lintel, or sometimes a couple of courses above it. Any moisture trapped inside the cavity will
be caught by the cavity tray and directed to the weep holes and out to the external face of the wall.
Openings in timber frame wall panels require a timber lintel directly below the top rail at the head of the opening. Softwood timber lintels are usually sufficient for the majority of domestic loads and spans. Additional framing is often required at an opening. Vertical studs can be placed alongside the opening to provide additional support to the lintel above.