Fitting an external window sill

With a standard PVC or aluminium window fitted on a sill support profile, the job follows the same steps every time. The order only changes depending on whether the facade is already finished or is still being insulated and rendered.

Measuring and preparation

The fitter measures the width of the opening between the reveals. He also measures the depth from the window frame to the outer face of the wall. To that depth he adds the overhang that sticks out beyond the wall, usually around 30 to 40 mm. On an insulated wall, he measures to the final coat of render, not to the bare masonry. The sill is then cut to length so that, once the end caps are on, there is still a little room for the material to expand.

The base under the sill

The sill needs a flat, solid base that falls towards the outside. On a brick or block wall this is usually levelled mortar or adhesive. On an insulated wall it is insulation cut to the right slope. Before the sill goes on, the joint between window and wall gets an external sealing tape on the outside. This tape lets moisture from the expanding foam escape, but it keeps driving rain out. Budget installers often skip this step, and the result shows up a few years later.

Bedding and fixing

The back upstand of the sill slides under the window frame or into the groove of the sill support profile. Low-expansion adhesive foam or a construction adhesive goes underneath in strips, so the sill is supported along its whole length and is not hollow. Many fitters also stick an anti-drumming tape to the underside to soften the noise of rain. The back upstand is screwed to the support profile with stainless steel screws. The sill is then weighted down while the foam cures, because foam tends to push it up as it expands.

End caps and sealing

End caps go onto both ends of the sill. On aluminium and steel sills they must let the sill expand in summer and shrink in winter. The joints between the caps and the render, and between the sill and the frame, are sealed with a suitable sealant. Finally the protective film comes off and the sill is tested with water.

There are several types of external sill. Each suits a different situation and is fitted slightly differently.

Aluminium sills

These are now very common on houses with modern windows. They are light, do not rust and come in many colours. Pressed aluminium is thinner and simpler to make. Extruded aluminium is stiffer, has reinforcing ribs on the underside and drums less in the rain. Aluminium expands noticeably in the sun, so it needs sliding end caps and, on long runs, an expansion joint. Manufacturers usually state the length beyond which a sill should be split into sections.

Coated galvanised steel

This is a sill folded by a sheet metal worker from painted or plastic-coated galvanised steel. It is made to measure, so it works well for unusual sizes, curved openings and corner windows. Cut edges and drilled holes must be protected, otherwise rust starts at the edges. These sills are often fitted by the same tradesperson who does the other flashings on the house.

PVC sills

External PVC sills are inexpensive and easy to cut. In the sun, however, they expand even more than aluminium, and dark colours can warp. They suit smaller windows, basements, garages and outbuildings rather than the main windows of a house.

Stone, concrete and ceramic

Natural stone and precast concrete sills are typical on older brick houses, on stone facades and in restoration work. They are heavy, are bedded in flexible adhesive or mortar, and need a drip groove on the underside so that water breaks off instead of running back to the wall. The joints with the wall must be sealed with a flexible sealant. Fitting is slower and heavier work, but the sill does not drum and lasts for decades.

Zinc and copper

These are used mainly on buildings where the roof and flashings are also zinc or copper. They are joined with traditional sheet metal techniques and need an experienced tradesperson.

An external sill sits where three trades meet: the window installer, the insulation and render crew, and the joiner or sheet metal worker. Most problems start exactly where one trade waits for another.

The window and the sill support profile

PVC and aluminium windows usually have a sill support profile along the bottom. It is a plastic section that raises the window slightly and gives the sill something to slide into. If it is missing, the sill has nowhere to go and water runs under the frame. When you order new windows, ask for this profile specifically and check that its height suits the thickness of any future insulation.

The order of work with external wall insulation

With external wall insulation, the sill is normally fitted after the insulation boards are fixed and the base coat with reinforcing mesh is on, but before the final thin-coat render. The end caps then sit neatly into the render and the joint stays clean. It can also be fitted after rendering, but then the joint between cap and render needs careful sealing. Agree in advance who supplies the sill and who fits it, so that it does not fall between two contracts.

Thick insulation and deep sills

With insulation of around 150 to 200 mm, the sill becomes deep and projects a long way from the window. Under such a sill an insulated wedge or an insulated sill support profile is used, and with a large projection also metal brackets. Without support, a deep sill bends under load, for example when someone leans a ladder on it.

Flats and listed or protected buildings

In a block of flats, the sill is part of the facade, which belongs to all the owners. Before replacing it, speak to the building management or the owners' association. They often specify the colour and type of sill so that the building looks uniform. If the work changes the look of the facade, or the whole building is being insulated, check with your local building authority what local building regulations require. This matters especially in conservation areas and on listed or protected buildings, where even a change of sill material can need approval.

The job is priced per linear metre, but one metre can be quick on one house and slow on another. It helps to know why.

The job is easier with:

• A new window with a sill support profile of the right height
• Straight, solid reveals at right angles
• Access from the ground or from scaffolding that is already up
• Several windows of the same width in one place
• A sill cut and prepared to accurate measurements

The job takes longer and is harder with:

• Removal of an old sill, especially steel or stone built into the wall
• Repairs to render and reveals after the old sill comes out
• Windows high up without scaffolding, where a lift platform or rope access is needed
• Deep sills on thick insulation that need brackets
• Corner, curved and non-standard windows with made-to-measure sills
• A missing or damaged sill support profile
• Crooked reveals, where the end caps have to be adjusted to fit

A badly fitted sill rarely shows problems straight away. They appear after the first winter or after heavy rain driven by wind.

• Too little fall or a back fall: water sits on the sill or runs towards the window, damp patches form under the frame and the inside reveal grows mould. The fall should be clearly visible, usually at least around 3 to 5 degrees
• Too short an overhang: water runs straight down the wall and leaves dark streaks or green algae below every window
• End caps glued rigidly with no room to move: the aluminium expands in summer and bulges, or the cap breaks the render
• A hollow sill with no foam: it drums in the rain, dents easily and bends if someone stands on it
• Too much expanding foam: the foam lifts the sill, the fall disappears or the back joint is pulled open
• Sill not tucked under the frame: water finds its way behind the back edge and into the wall
• No sealing tape: the joint between window and wall is protected only by sealant, which cracks after a few years
• Screws through the flat top surface: every unsealed hole is a way in for water
• Protective film left on: it bakes on in the sun and later only comes off with a lot of effort
• Unprotected cut edges on galvanised steel: rust appears at the edges and runs down the facade

In practice it often happens that owners look for the cause of damp under a window in the window itself, while the water is actually getting in under a badly fitted sill. Before you make a warranty claim on the window, have the sill checked first.

An experienced crew usually needs less than an hour per window to fit a standard sill on a prepared base. Removing the old sill, preparing the base and dealing with non-standard pieces take more time.

Low-expansion foam or construction adhesive usually needs a few hours to set, and reaches full strength later, according to the manufacturer. Sealants at the joints usually need about a day. During that time the sill should stay dry, should not be loaded and nobody should step on it. Foam and adhesive should not be applied in frost, and most products state a minimum working temperature. A stone sill bedded in adhesive is left undisturbed for longer, usually at least one or two days before the joints are sealed.

If the window is on the ground floor, you can reach the sill comfortably from the ground and the facade is finished, you can replace the sill yourself. For windows higher up, where you would have to stand on a ladder or lean out of the window, leave the job to a crew with scaffolding or a lift platform. A fall from height is not a risk worth taking for a window sill. In a block of flats, agree the work with the building management first.

1. Measure the width of the opening between the reveals and the depth from the frame to the outer face of the wall, then add an overhang of roughly 30 to 40 mm.
2. Order the sill in the right depth and colour, together with end caps, anti-drumming tape and low-expansion adhesive foam.
3. Remove the old sill carefully without damaging the window frame.
4. Clean the base of old foam, dust and loose render.
5. Check the sill support profile and the condition of the joint between window and wall.
6. Cover the joint between window and wall with external breathable sealing tape.
7. Cut the sill to length with a fine-toothed hacksaw or metal shears and deburr the edge.
8. Stick anti-drumming tape to the underside.
9. Fit the end caps, dry-fit the sill and check the fall with a spirit level.
10. Apply foam or adhesive in several strips, not as one thick layer over the whole surface.
11. Slide the sill under the frame, press it down and screw the back upstand to the profile with stainless steel screws.
12. Weight the sill, for example with a bag of sand, and let the foam cure.
13. Seal the joints between the end caps and the render and between the sill and the frame with a suitable sealant.
14. Peel off the protective film and, once everything has cured, pour water over the sill from a watering can.

You do not need special tools for this: a spirit level and a watering can are enough. Ideally do the check at handover, while the fitter is still there.

Place the spirit level on the sill at right angles to the window. The bubble must clearly show a fall towards the outside. Then slowly pour water onto the sill next to the window. It should run quickly to the front edge and drip off clear of the wall, not run down the render. It must not stand in puddles, including near the end caps.

Tap the sill with your finger along its whole length. A slight change in sound is normal, but long hollow sections mean there is no foam underneath. Look from below as well: is the drip edge far enough from the wall, and is the overhang the same along the whole length?

Check the end caps. They should sit firmly, but the sill must not be jammed in them. The joints with the render should be continuous, without gaps. The surface should have no scratches or dents, and the film should be gone.

• The sill has a visible fall towards the outside along its whole length
• Water from the test runs off and drips clear of the wall
• The overhang beyond the wall is even and sufficient
• The sill is tucked under the frame or into the sill support profile
• There are no long hollow sections under the sill
• The end caps are fitted and allow for expansion
• Joints with the render and the frame are sealed without gaps
• There are no unsealed screws on the flat top surface
• The surface is free of scratches and dents and the film has been removed
• Cut edges of galvanised steel are protected
• The area around the window and the facade is clean of foam and sealant
• The fitter has handed over information on the material and its care