How to repair and maintain window sills

Window sills are constantly at the mercy of the elements and so need occasional maintenance and repair, not only to look good but also to perform their job properly. Repairing most types of sill is not as difficult as it seems

Damage to sills is often the result of faults in the basic design or construction of the window and sill and reaction to former repairs.

Some sills project well beyond the frame of the window itself so that rainwater is directed well clear of the supporting wall. Note that any projection beyond the width of the window opening is for the sake of appearance only—and does nothing to improve the function of the window sill itself.

The purpose of a window sill is to protect the wall underneath the window from rain. Although this may not seem necessary—after all, the rest of the wall is not protected in this way— rainwater striking the glass panes forms a concentrated, downward-running cascade which could penetrate a masonry wall or start rot in wooden siding.

Putting a projecting ledge—a sill— of some material between the window and the wall below stops this happening and prevents penetrating damp. But this means that the sill itself has to cope with the water flow and so it is constantly exposed to wear.

In older houses, particularly, the window frame is often deeply recessed —perhaps flush with the inside surface of the wall. In this case, and where the window frame itself has no sill, a separate sill has to be provided and these are prone to trouble.

It is important that the top surface is sloped away from the window so that rainwater does not settle. A rounded sill edge also encourages water run-off. For wooden and smooth stone or concrete sills, the slope need not be very great; for the rougher clay tile or brick sills, a steeper slope is necessary.

Unless rainwater runs off quickly, wooden sills may rot and stone or cement sills crack through frost action. Paint finds it difficult to adhere to damp surfaces and combined with the action of sun it can easily blister and peel, leaving the sill even more vulnerable to attack from the rain.

A drip groove on the underside of the sill is a small but important feature. Whatever the angle of slope on the top surface, some rain inevitably creeps around the front face of the sill; accummulated drips then work along the bottom edge and onwards to the wall—where they can attack the joint with the sill. The drip groove prevents this by halting the water’s progress, causing it to form into larger droplets that then fall harmlessly to the ground.

Some sills are formed in one piece with the rest of the window frame. Others—either by design or following a repair—are installed separately and if any joint between the two is left unprotected or badly sealed, you can expect trouble.

In other designs, a galvanized steel water bar may be used to prevent water from tracking along constructional joints to the inside. This will be bedded on edge in a lead-packed groove in the main section of a stone or concrete sill. The bar fits a corresponding groove in the underside of a fixed wooden frame or top sill to give complete protection from draughts and wind-driven water.

Irregularities in the design of the window frame may also spell trouble for the sill beneath it. The basic rule is that each part of the frame ought to overhang the part beneath it, and that all ‘horizontal’ surfaces should slope slightly towards the outside.

Other sill types

Another type of sill makes use of plain clay roofing tiles or quarry tiles.

Hand-made tiles are rough-surfaced and fairly porous, and so should be laid at a comparatively steep slope of about 30° to the horizontal; machine-made and quarry tiles need less. Two staggered layers of tiles are laid to prevent rainwater seeping through to the brickwork beneath—the bottom course is laid with the tile nibs along the outer edge, hanging down to form a crude drip groove.

Dealing with design faults

If you want to avoid recurring problems, it is important to correct any design faults during the course of routine maintenance or repair work. Start by checking the drip groove. Appearance can be deceptive: there may be a drip groove, but well hidden under layers of paint. Probe around to see if you can find one and, if you

There is little you can do to correct poorly made tiled sills except remove and replace them. But only do this once you are sure that it is the slope which is causing the trouble.

Similarly, there is little you can do about design faults in a window frame without replacing the frame completely. Where the upper frame sections do not overhang lower ones properly, it may be possible to fix additional thicknesses of wood to the frame. But unless this is done carefully and the joints between the timber very well sealed, it may prove more of a problem than the original fault.

However, if the joints between a wooden sill and the frame are exposed and letting in water, simply rake out the old jointing material and pack the cleaned gap with suitable mastic or filler before repainting. do, scrape the recess completely clear then prime and paint it, taking care not to repeat the fault.

Where there is no groove, you might be able to rout or chisel one. But a stop of some form is a much easier solution which should work just as well. You can make the stop from a length of quadrant beading and fix it to the underside of the sill with impact adhesive, about 35mm back from the front edge. Make sure that the sill is sound and properly painted around the site of the stop and repaint the entire underside once the stop is in place.

Next check that the sill slope is steep enough. Puddles of water on the upper surface indicate real trouble and will quickly cause rapid decay. Prod a wood sill with a pointed blade or bradawl to check for wet rot, and look for fine cracks and flaking on stone or concrete sills—typical signs of an inadequate slope.

A wooden sill is easily corrected by planing it to a suitable slope, though you can do this only if the timber is sound and deep enough.

A stone sill can be repaired and sloped by forming a mortar fillet on top. But this usually means that the back edge of the sill would come close to wooden parts of the window frame; you should first carefully chip away part of the sill in order to lower it.

Remove all dust and loose particles, then paint the surface with PVA adhesive so that the new mortar mix of one part cement to three of sand adheres properly. Run over this with an offcut of timber to give the slope you want, then remove the former and shape a rounded front edge.


Rotten window sills

If you have caught the rot in good time you may need to do nothing more than burn off all the paint on the sill, scrape the surface back to sound timber, and flood the area with wood preservative before repainting.

But if the damage is at all extensive, it is sensible to cut out the affected part and replace it. Some joinery stockists sell ready-shaped sills and if you can find one which has the same profile as your sill , it can be used for patching gaps. Otherwise you have to trim a piece of suitable timber to the correct shape.

As an alternative, replacing the whole sill with new material often makes better sense; patching involves making joints that may themselves let water in, thus creating further rot problems to repair.

Most modern wooden sills are made of softwood—usually redwood—which is perfectly suitable providing it has been properly treated with pressure-impregnated preservative.

Hardwood is a better material for sills because of its resistance to decay, but there are problems. Susceptibility to surface ‘checking’ may make traditional materials difficult to paint while some hardwoods such as Ramin have poor resistance to decay.

You may be able to find a suitable all-plastics sill. Obviously, this cannot rot but the wood at the joint between it and the existing frame will still be susceptible to damage. Do not cover wooden sills with a plastics covering such as sheet laminate. Almost certainly you will not be able to make the edges and joints completely watertight and any water entering these could cause rot very quickly.

Replacing a sill

If the sill is formed in one piece with the window frame, you must cut it free at a suitable point in order to replace the whole sill. If possible, arrange the cut so that the joint with the new sill will be covered by the next window frame member up. Otherwise simply cut off a generous width of sill, well beyond the depth of rot. Power sawing tools are useful for this operation—perhaps an electric drill with circular saw attachment.

Clean up the cut face of the remaining part of the sill by chiselling and planing until it is smooth and flat. Treat the face generously with preservative.

The new sill is bonded firmly to this surface. Although galvanized screws driven in at angles from below through the two pieces might do in some cases, a better solution is to make dowel joints between the two.

Whatever main method of fixing you choose the two surfaces must also be glued together, using a urea or resorcinol formaldehyde adhesive. When the wood is well bonded, rake out all the gaps between the new sill and the wall or the old sill and flood them with preservative. Finally, pack the gaps with a suitable waterproof mastic.

To repair just a small section of sill, cut out the affected part with a saw and chisel then use this as a template to cut the patching timber to size. Thoroughly treat the new wood— especially the cut ends—with preservative, then glue it in place with a urea or resorcinol formaldehyde adhesive—not ordinary PVA woodworking adhesive. Finally, screw the patch to the existing sill using galvanized screws countersunk well below the surface. Cover the heads with mastic or filler and if even greater strength is needed, fix a metal strap to the underside of the sill.

Stone or concrete sills

Non-timber sills rarely need much more than patching. Stone and concrete sills are easily patched with a 1:3 mortar mix, using a timber form-work arrangement similar to that used for increasing the surface slope. Tiled sills are best patched by replacing the damaged tiles themselves, on an individual basis.

However, badly cracked or decayed sills may need replacing altogether and the best way of doing it is to cast a new concrete sill in situ, using shuttering.

Although the job can be very tricky with the existing window frame in place, the chances are that it too will need replacing if the sill is beyond reasonable repair.


Prevention is much better than cure, so making sure that your window sills are properly painted is a good investment and an important household maintenance job.

Perhaps the most important point is to ensure that the woodwork is not damp when it is painted, as this quickly leads to blistering and peeling.

The old paint will probably have to be removed completely if it is in poor condition and in this case burning off is better than using stripper because it keeps the wood dry. Sand the surface smooth afterwards and treat any knots with knotting compound to seal them. Rub a fine wood filler into the surface to seal the grain—do not forget the ends of the sill. Finally paint with primer, undercoat and two top coats, making sure that no bare timber is left overnight.

Hardwood sills often develop small cracks, causing subsequent fracture and flaking of paint. The answer is good preparation; one treatment is to scrub thinned wood primer into the grain and then seal it with filler before applying another coat of primer.

Painting stone or concrete can also cause problems. Ordinary oil paint will mix with the alkali salts in the stone and soften or flake, unless you use an alkali-resistant for the first coat. It is far better to use emulsion or masonry paints, both of which resist alkalis fairly well.

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