Masonry Sills – Avoid the Achilles Heel
Masonry Sills – Avoid the Achilles Heel.
A masonry sill in brick, tile or stone is required to do two things:
1) Externally it must provide the visual presence required by the designer and its shape should ideally deflect rain outwardly. 2) It should close the cavity horizontally without creating a thermal or damp bridge towards the inner skin.
However there is a third requirement that is commonly overlooked: Where masonry sill and reveal masonry meet is the most susceptible area to water volumes and water ingress. Effective integration with the reveal closing arrangement is essential.
Forming a masonry sill requires the sill individual bricks / tiles / stones to adopt a bedded angled presence and this has to be achieved without connection with the inside skin, unless that connection can become an integral part of the sill that additionally functions as a dpc barrier.
This is one of the functions of the Type U undersill tray, which provides the designer and mason with an alternative approach to sill formation. Furthermore, this alternative approach ensures positive integration with the reveal closing.
The Type U is a profiled dpc tray that is bedded into position and onto which the sill is then constructed. The Type U tray remains enveloped in the wall and provides the dpc qualities required to prevent damp ingress. Initially the Type U also functions as sill formwork: There are two support areas on each Type U tray and the dimensions of these are variable. By selecting the dimensions required at design stage the finished angle/appearance of the sill may be individualised.
Once bedded, the sill masonry construction may commence. At each end of the tray are fitted large stopends that lie against the reveal masonry face. These prevent end discharge of water and provide upstands against which the vertically closing reveal cavicloser may integrate. It is usual for the cavicloser to be notched and extend below finished sill level, as indicated left.
The front projecting lip on each sill tray acts as an alignment guide so the front of each sill is uniform and straight.
Towards the rear of the tray the upward projecting section provides rear support at a height to provide the sill angle chosen by the designer. Solid mortar infilling should always be part of sill formation. Once the sill has cured a grooved detachment line permits the front lip of the tray to be removed, leaving just the sill masonry projecting.
The remainder of the tray remains insitu, acting as a damp course barrier so dampness cannot track inwardly. Water soaking into the sill can permeate via transient drainage holes in the tray bed and via Pyramid caviweeps at tray bed level.
Where appropriate insulation can be bonded onto the back upstand of Type U trays and used to great advantage to isolate and thus reduce thermal cold bridging.
Thermal benefit and damp control can be combined.
Further information can be found in our Technical Product Guide Number 22.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
T: 01935 474769
F: 01935 428223