Installation of External Window Sills with Reinforcement Strips

The area under a window is a weak spot, because the frame, the masonry and the insulation all meet there. If water gets in, you will not notice straight away. The first signs, such as damp patches on the reveal or flaking render under the window, usually appear only after two or three winters.

A properly fitted sill does several jobs at once:

• It sheds water clear of the facade through the drip edge
• It protects the joint between window and wall from moisture and frost
• It stops water getting into the external insulation
• It closes off the bottom of the opening and finishes the look of the window

Why not simply lay the sill straight onto the masonry or into mortar? Water always finds a way underneath. That is why reinforcement strips are bonded on first. These are sealing strips that bridge the joint between frame, masonry and insulation, forming a continuous waterproof layer before the sill goes in. The sill itself is then the second, visible line of protection.

Extruded aluminium sills are common today in new builds and insulation projects. They keep their shape, come with a ready-made drip edge and need no painting. They do expand more with heat, which matters on longer lengths.

Galvanised and plastic-coated steel is bent to size in a sheet metal workshop. It suits non-standard windows, bay windows and arches. The coating stands up well to sun and scratches, but it usually lasts less long than aluminium.

PVC sills are light and easy to cut. Frost and sunlight gradually make them brittle, though, and the colour fades. On wider windows they need closer fixings so they do not sag.

Stone and concrete sills are chosen for older or listed buildings, or where looks and a long life come first. They are heavy, need a solid base and take more effort to set.

If you live in a block of flats, check the colour and profile with the building management or owners' association first, so the facade stays uniform. If the change affects the look of the building more noticeably, or the building is listed or in a conservation area, ask your local building authority what applies in your case.

The work follows a fixed order, and each step depends on the one before it. Installers follow the relevant EN standards for sheet metal work and the installation instructions of the insulation system manufacturer.

1. Clean the base of dust, mortar and old adhesive and let it dry.
2. Check that the base is level and form a fall away from the window, usually at least around 5°.
3. Measure the actual width and depth of the opening including the insulation thickness, not the bare masonry.
4. Prime the base and bond the reinforcement strips over the full surface, without bubbles or creases, turning them up into the side reveals.
5. Bed the sill in adhesive or on sealing tape and slide the back edge under the window frame or into the prepared groove.
6. Fix the sill at the sides, never through the surface the water runs over, and leave room for expansion on longer lengths.
7. Fit the end caps with sealant or adhesive and check how they join the reveal.

The fall is critical. Without it, water sits on the sill and pushes under the frame, and no later step can put that right. The drip edge normally projects at least around 30 mm beyond the face of the facade, and on exposed sides a little more tends to work better.

When should the sills go in? Usually after the insulation boards, but before the final render. Fitted earlier, they are easily damaged during work from the scaffold. Fitted later, the finished facade has to be cut into. Leave the protective film on until the facade work is done, but not much longer, as strong sun can bake it onto the surface.

How much work is involved, and so what it ends up costing, depends on the situation on site:

• Deeper sills over thick insulation need more fixings and a more precise fall
• Work above ground floor needs scaffolding or a lift platform
• Bay windows, arches and unusual shapes take longer to make and fit
• An uneven or damaged base has to be repaired first
• Heavy stone sills are harder to move and set
• Many identical windows, on the other hand, make the work more efficient

When you compare quotes, check that the sills are listed as a separate item and that they include reinforcement strips, forming the fall and end caps. Removing the old sills, insulated XPS reveals around the window and scaffolding are often left out too.

• Too little fall or none at all: water stands, creeps under the frame and the reveal gets damp
• Missing reinforcement strips: water has a direct route into the joint and the insulation
• Short projection: water runs down the render, which stains and flakes
• Rigid fixing with no room to expand: the metal ripples, cracks and works loose as temperatures change
• End caps fitted dry: they come loose in the first winter and water runs into the reveal
• No anti-drum tape under the metal: raindrops drum loudly, which is a nuisance in a bedroom
• Sill ordered to the bare masonry size: after insulation it is too short and a new one has to be made

Repairing a reveal that has taken in water means removing the sill, drying out the base and making good the facade. Cutting corners on this detail rarely pays off.

You can manage one or two ground-floor windows yourself, as long as you work from the ground or a low, stable platform. For upper floors and whole facades, call a tinsmith. Take your time, because mistakes in the fall or the strip joints only show up later.

1. Remove the old sill and end caps carefully without damaging the window frame.
2. Clean and dry the base, scraping off any leftover mortar and adhesive.
3. Level the base and form a fall away from the window.
4. Measure width and depth including the insulation and the projection beyond the facade.
5. Prime the base and bond the reinforcement strips as the manufacturer instructs, turning them up into the side reveals.
6. Apply anti-drum tape and bed the sill in adhesive, sliding the back edge under the frame.
7. Fix the sill at the sides and leave a gap for expansion.
8. Fit the end caps with sealant and seal the joints.
9. Pour a little water onto the sill and watch that it runs towards the drip edge.

• Water poured onto the sill runs away from the window on its own and does not pool anywhere
• The drip edge projects beyond the facade and water does not run down the render
• The back edge sits under the frame and the joint is tight
• The end caps are firmly bonded and sealed
• The metal does not ripple, move or rattle when tapped
• The surface is free of scratches and dents
• The protective film has been removed
• You have photos of the reinforcement strips before the sill covered them