Width (Length Along the Window Opening)
The cill width is the dimension that runs along the window opening, sill to sill across the full opening. We fabricate cills from 300mm minimum width up to 2,000mm in a single piece. Wider cills are supplied as multi piece welded assemblies up to 6,000mm. Always measure the structural opening width plus 10 to 20mm tolerance on each end so the cill returns into the reveal. The CILLS app prompts for the structural opening width and calculates the cill width including the reveal returns automatically. Typical residential cill widths range from 900mm to 1,800mm depending on window size.
Projection (Distance from Wall Face to Front Edge)
Projection is the dimension from the wall face out to the front edge of the cill. Standard projection ranges from 50mm (minimum for new builds with no exterior insulation) to 250mm (deep projection for EWI retrofits with 200mm insulation plus render). The minimum overhang past the rendered or finished wall face is 25 to 50mm to ensure water drops clear and prevents staining the wall below. For EWI retrofits, calculate projection as: insulation depth + render thickness (10mm) + overhang (40mm). For a typical 150mm insulation retrofit, that is 200mm projection minimum.
Upstand Height (Rear Vertical Return)
The upstand is the vertical aluminium return at the rear of the cill, hidden behind the window frame. Standard upstand heights are 20mm, 30mm, 40mm, 50mm, and 60mm. The upstand serves three purposes: it provides a positive seal at the cill to frame junction, it prevents water ingress under the frame, and it gives the installer a positive stop when fitting the cill. For EWI retrofits, specify a 40 to 60mm upstand to accommodate the deeper window frame and any insulation reveal detail. For new builds, 20 to 30mm is standard.
Aluminium Thickness
Standard thickness is 2mm marine grade aluminium. This suits the majority of residential and EWI retrofit projects. 1.5mm is available for cost sensitive projects with shorter spans. 2.5mm and 3mm are specified for commercial cills, long spans (over 2,000mm), large projections (over 200mm), and high wind exposure sites such as coastal Cork, Galway, or Donegal. Thicker aluminium reduces deflection under wind load and increases the cill's resistance to impact damage during fit out.
Drip Lip (Front Edge Underside Detail)
Every cill we fabricate includes a 5mm drip lip on the underside of the front edge. This is a small downturn that breaks the water surface tension as it runs off the cill, forcing the water to drop vertically rather than wrap back along the underside toward the wall. Without a drip lip, water travels along the underside and runs down the external wall, causing staining, mortar erosion, and eventual water ingress at the cill to wall junction.
Drainage Slope
The cill top surface is sloped 5 to 10 degrees toward the front to encourage drainage. This slope is built into the profile, you do not specify it separately. Water lands on the cill, runs forward over the drip lip, and drops clear of the building. The 5 to 10 degree slope is steep enough to drain effectively but shallow enough to look flat from ground level.
