The window sill is one of the most vulnerable parts of a building. Whereas masonry walls can absorb moisture and release it without harm, rainwater falling on nonabsorbent window glass will be concentrated directly on the sill below.
The sill must be made of a material that will resist water penetration and at the same time be designed to avoid the effects of run-off - excess water penetrating the joint between sill edge and jamb, or staining the wall immediately below it.
To minimise the risk of water penetration, BS 8295:1991 'Code of practice for design and installation of damp-proof courses in masonry construction' and BRE Digest 236 recommend a projecting sill at all openings, with integrated vertical and horizontal damp-proof coursings.
Is it possible to omit the traditional sill by setting the window at the outer edge of a masonry wall and projecting the sill of the timber window over the masonry wall? This is a practice which has become fashionable in some parts of the UK, but it raises the possibility of serious problems: a waterproof seal cannot be formed in the straight butt joint between masonry and frame; the timber frame, exposed to damp, will tend to rot and severe thermal bridging will occur all around the opening.
In order to achieve durable and weatherproof construction, timber windows in masonry walls are ideally slotted onto water bars bedded with mastic into a stone or slate sill. The sill itself is weathered - it slopes downwards at a minimum of 20¦ - and projects beyond the masonry wall to throw rainwater away from the face below. Sills with projecting at soffits underneath are grooved with a drip to throw rainwater off. The best - and most costly - stone and slate sills have stooled ends that prevent rainwater from blowing along the sill and into the sill edge/ jamb joint.
In Scotland, Northern Ireland and the more exposed parts of the UK, it is traditional practice to set the window head and jambs just behind the outer skin of masonry, with the sill resting on a stone or slate sill.
The joint between frame and masonry is rebated and additionally protected by the masonry; the frame is protected from damp and the sill deects water run-off.
Traditionally, sills were made of slate or stone. Because of cost, stone sills have largely been replaced by precast concrete. But slate is an ideal material for sill construction - as it is impervious, no dampproof coursing or cavity tray is required beneath a slate sill.
Burlington, known for its roof slates and cladding products, offers a bespoke sill manufacturing service.
Sills are cut to size and shape to the architect's design.