Fitting an internal window sill

The price list counts this job per linear metre, so the charge depends on the length of the installed sill. For a standard window in a flat, fitting one sill usually takes less than an hour. It takes longer with old, uneven walls or when the original sill has to be removed first.

Measuring and cutting the board

The fitter measures the width of the opening, the depth from the window frame to the face of the wall, and checks whether the reveals are square. In older houses the opening is often a centimetre or two wider at the front than at the back, so it is measured in several places. The board is then cut to length and depth. A projection of roughly 2 to 4 cm beyond the face of the wall is usual, plus side "ears" (horns) that run past the edge of the reveal, either into the wall or onto its surface.

Cuts are made with a sharp saw blade meant for laminate or plastic, ideally a plunge saw on a guide rail, so the edge does not chip. With chipboard, any cut edge that ends up hidden should be sealed with sealant or varnish so it does not soak up moisture.

Preparing the base

The surface under the sill must be firm, dry and roughly level. Loose plaster, old mortar and dust are removed. Larger dips are filled with mortar or packed out with shims. Using a spirit level, the fitter sets the height so the sill sits tight under the window frame, or slides into the groove or under the window's sill adaptor profile if the window has one.

Bedding and fixing

The board is first laid dry on plastic packers or wedges and checked with a level. An internal sill is laid level or with a very slight fall into the room, never towards the window. Then it is fixed in place. The most common choices are low-expansion PU foam or a construction adhesive, for example one based on MS polymer.

Foam goes on in strips, not over the whole surface. While it cures, the sill is weighted down, for instance with sandbags, buckets of water or a prop wedged against the window head. Without weight, the foam lifts the board and the sill ends up rocking or sitting crooked. The weight usually stays on for several hours, according to the foam manufacturer, and ideally overnight.

Finishing

Finally, end caps are fitted if the system uses them, and the joints are sealed. The joint between sill and window frame is filled with a flexible sealant, usually silicone or acrylic in the frame colour. The joint to the plaster is normally done with paintable acrylic, so it can be painted along with the wall. The protective film on top is only peeled off once all other work in the room is finished.

The biggest decision you make is the material. It affects the look, how well the sill copes with daily use and how it is fitted.

Laminated chipboard or MDF

This is the most common type. A chipboard or MDF core is covered with a hard-wearing laminate, and the front edge is usually rounded by a process called postforming. These boards come in a wide range of finishes (white, wood effects and stone effects) and cope well with normal use. Their weak point is water: if it gets into a cut edge or through a damaged surface, the core swells, and that cannot be repaired. This is why end caps and carefully sealed joints matter so much.

Hollow PVC window board

A hollow PVC profile with internal ribs. It is light and not bothered by moisture, so it suits bathrooms, kitchens and windows where plant pots stand. It feels less solid to the touch, darker colours can expand more in direct sun, and sharp objects scratch it more easily. The end caps must belong to the same system, otherwise the hollow chambers stay visible.

Solid timber

Oak, beech or softwood sills made from glued solid timber suit timber-frame houses, cottages and interiors with wooden windows. They look good and can be sanded and refinished. Timber moves, though: it changes size as humidity changes, so it needs an expansion gap at the reveals, a good finish (oil or lacquer) on all sides and regular care.

Stone and engineered stone

Marble, granite or engineered quartz are very durable, but heavy, and they are usually fitted by stonemasons rather than joiners. This article focuses on the sills a joiner fits: laminated board, PVC and solid timber.

Depth and thickness

Laminated and PVC sills are usually around 2 cm thick and come in depths from narrow to very deep. A deep sill that projects well beyond the wall needs firmer support, otherwise it will sag over time. If you need a sill deeper than standard sizes, ask the fitter about support brackets underneath.

Why is the radiator almost always under the window? Warm air rises along the glass and forms a curtain that stops cold air dropping off the window and reduces condensation on the glazing. The sill must not block this curtain.

• The sill should not cover the whole radiator. A gap between the front edge of the sill and the radiator is usually recommended so air can flow.
• If you want a deep sill above a radiator, there are versions with routed ventilation slots or a grille.
• A sill that is too deep over a radiator leads to condensation at the bottom of the window, mould in the corners and a room that heats less well.

The second issue is moisture from building work. Fresh plaster or levelling mortar under the sill must dry out first. If a chipboard sill is fitted onto a damp base, moisture is drawn into the core from below.

The sill is one of the last jobs around a window. Anything missed beforehand is hard to put right afterwards.

• Window installed and fixed. The frame must be firmly anchored, and the gap around it filled with foam and covered.
• Vapour control tape on the inside. With a well-fitted window, the gap around the frame is taped on the room side with an airtight, vapour-tight tape. This tape must run continuously under the sill as well. If the sill fitter cuts or tears it, moist air from the room gets into the foam and damp and mould can appear around the window over time.
• Reveals at the right stage. Most often the sill goes in after the reveals are roughly plastered but before the final skim and paint. That way the joint to the wall can be finished neatly. Fitting into already painted reveals is possible, but expect a little touching up.
• Radiator and electrics sorted. If the radiator is being replaced or a socket under the window is moving, the heating engineer and electrician should finish first. Sockets and wiring are always work for a qualified electrician.
• Clear access to the window. Curtains, plant pots and furniture under the window removed, and space to cut.

In a block of flats, replacing an internal sill on its own is internal work within your flat. If it goes together with new windows, it makes sense to speak to the building management or owners' association beforehand, because windows affect the look of the façade and in many buildings the lease or house rules set conditions. Whether any related building work needs approval under local building regulations is something to check directly with your local building authority rather than relying on general advice.

Even though the price list works per linear metre, the effort involved in a particular installation can vary a lot.

Easier situations

• New build or new plaster with square, straight reveals
• Standard sill depths with no adjustments
• A window with a sill adaptor profile that the board slots under neatly
• Several identical windows in a row, where boards can be cut in one go

Harder situations

• Removing an old sill, especially a concrete or terrazzo one in older houses
• Crooked, sloping or uneven reveals that need precise scribing to a template
• Thick walls and very deep sills that have to be joined or supported
• Corner windows, bay windows and curves, where sill pieces meet at an angle
• Cut-outs around heating pipes or other obstacles
• An uneven or crumbling base that has to be repaired first

In older buildings, old sealants or boards may contain hazardous materials, including asbestos. If you suspect this, have the material tested. Asbestos removal must only be carried out by a licensed contractor.

Most sill problems do not appear on the day of fitting, but after a few weeks or after the first winter.

• Foam sprayed everywhere with no weight on top. The foam lifts the board, so the sill bows upwards or pulls away from the frame.
• No packers under the sill. The board rests only on foam and flexes when you lean on it. Later the sealant at the frame cracks or the board snaps.
• Fall towards the window. Water from a plant pot or condensation runs to the frame, gets under the sill and into the wall.
• Unsealed cut edges on chipboard. In the corners by the window or the wall the sill eventually swells and the laminate peels.
• No expansion gap with solid timber. The wood expands in summer, pushes into the reveal and can bow or crack the plaster.
• Rigid filler instead of flexible sealant. The joint between frame and sill cracks with the first temperature swings.
• Damaged vapour control tape. Damp and mould appear around the bottom of the window, often only after a few years.
• Radiator fully covered. The window steams up in winter and the room is harder to heat.
• Projection or ears too short. A visible gap is left at the wall that is hard to hide later.

Fitting one sill usually takes from a few tens of minutes to about an hour. A whole flat with several windows is typically a one-day job. It takes longer when old sills have to come out or the base needs levelling, because the mortar has to dry before the new board goes on.

Fixing foam or adhesive usually cures within a few hours, according to the manufacturer, and normally reaches full strength by the next day. Silicone or acrylic in the joints needs about 24 hours before you touch it or paint over it. Until then, keep plant pots and heavy objects off the sill and do not lean on it.

If you have a saw with a guide rail, a spirit level and some patience, replacing an internal window sill is a job you can do yourself. Most of the time goes into accurate measuring and cutting, not the fixing itself. Do not take it on if it means touching the electrics or the heating pipework.

1. Measure the width of the opening at the front and the back, the depth from the frame to the face of the wall, and check that the reveals are square.
2. Add the projection beyond the wall (roughly 2 to 4 cm) and the side ears, and write the dimensions down.
3. For an uneven reveal, make a cardboard template and transfer the outline to the board.
4. Cut the board with a sharp blade, cutting the laminate from the side the saw manufacturer recommends so the edge does not chip.
5. Seal any hidden cut edges of chipboard with sealant or varnish.
6. Clean dust and loose plaster off the base, fill larger dips and let them dry.
7. Check that the vapour control tape under the window is intact, and do not cut into it.
8. Lay the board dry on plastic packers and set it level with the spirit level.
9. Lift the board out, leave the packers in place, dampen the base if the foam instructions say so, and apply low-expansion foam in strips.
10. Put the sill back, press it against the frame and check it again with the level.
11. Weight the sill evenly and leave it to cure as the foam manufacturer specifies, ideally overnight.
12. Fit the end caps if the system has them.
13. Seal the joint to the frame with flexible sealant and the joint to the wall with paintable acrylic.
14. Once the sealant has cured, peel off the protective film and make good the plaster around the sill.

The handover does not take long. A spirit level, your eyes and your hand are enough. Check the sill only once the foam and sealant have cured, preferably in daylight.

• The sill is level or has a slight fall into the room, never towards the window
• Pressing on the front edge does not make it flex or rock
• The board sits tight against the window frame along its full length, with no gap
• The joint to the frame is filled with flexible sealant, straight and unbroken
• The joint to the wall is sealed and can be painted
• The projection beyond the wall is the same along the whole length
• The ears and end caps are fitted neatly, with no open chambers or raw cuts
• The surface is free of scratches, peeling laminate and foam residue
• No foam sticks out under the sill and the vapour control tape is not visibly damaged
• There is space above the radiator for warm air to rise
• The area is cleaned up and you know when the sill can take weight