Exterior window sills – fitting step by step

When to fit it

When external wall insulation is added, the exterior sill is fitted after the insulation boards are stuck on and usually before the final coat of render. Only then is the position of the finished facade known, and that is what the nose projection is measured from. If a sill is fitted to the old masonry and the house is insulated later, it almost always ends up short.

Method

1. Measure the depth of the reveal on the left and right and the length between the reveals; details in the measuring guide.
2. Prepare the bed. The surface under the sill must be flat and fall outwards. ZOFI says the bed must be smooth and level with a minimum fall.
3. Seal under the frame. The joint between the sill and the window frame is sealed with tape or a sealing profile, following the window manufacturer’s system.
4. Slide it under the frame. The sill is slid into the sill groove or under the frame by about 10 mm.
5. Bedding. A pressed sill is bedded over its full area (insulating wedge, adhesive or expanding foam, depending on the system) so that it does not sag or resonate.
6. Fixing into the frame or a sub-sill profile, with screws through the rear upstand, not through the horizontal surface. Extruded sills often have holes provided by the manufacturer.
7. End caps. The end caps are fitted with an expansion gap of 1–2 mm; for longer sills (over 5 m) ZOFI recommends splitting them into two pieces with an expansion joint.
8. Render is finished up to the end caps so that water cannot get between the cap and the reveal.

1. window frame with a sill groove; the sill is slid under the frame
2. exterior sill falling outwards
3. drip nose projecting beyond the finished facade
4. bedding under the sill (wedge or insulating sub-sill, depending on the system)
5. external wall insulation with render
6. masonry below the window

Drawing no.

KV-06

Title

Exterior window sill, section and plan of end

Scale

diagram, dimensions in mm

Revision

14 October 2026

What to check at sign-off

• the nose projects 30–40 mm beyond the finished facade along its full length
• water runs off the sill outwards and does not stand anywhere
• the sill is under the frame, not just sealed against it
• it does not drum when tapped – it is bedded
• the end caps are worked into the render, not just stuck onto finished render
• there are no scratches in the paint, and the protective film has been removed
• there are no screws in the horizontal surface

Depths and thicknesses on offer

Retailers list pressed aluminium sills in depths from 70 to 400 mm in sheet about 0.8 mm thick, with a maximum length of 6 m (Widoor), and extruded aluminium sills in depths from 110 to 360 mm, 1.2–2.45 mm thick depending on depth, with a 25 mm nose (najparapety.sk). Ranges vary, so check with the particular supplier.

Labour cost

Numiq (updated 14 May 2026) gives €10–28 per m excl. VAT for window sill flashing, materials not included.