Timber sills outside your windows are always vulnerable to decay as  they are exposed to all weathers. Long-lasting repairs are easy to make and you’ll be surprised what a difference the work will have on the appearance of your home.

Timber window sills
Flaking paintwork is one of the first signs of rotting wood, but even an outwardly sound paint surface can, on closer inspection, but hiding a multitude of troubles. If you suspect rot in a sill strip off the paint and cut back any rotting areas until you reach sound timber. If  the exposed timber looks grey it is suffering from surface decay, which you must skim away before repainting. When removing affected areas take out any nails or screws in the surface and plane down until you reach clean, healthy timber.

Don’t forget to check the underside of the sill as well. Dig a sharp penknife into the wood; if it goes in easily these soft parts will have to be dug out and stripped back as before.

An important part of the sill is the drip groove, a U-shaped channel running the length of the underside. This ensures a free passage for rainwater and must therefore be kept free dirt.

If you have discovered rot in its early stages you will have only small cavities to fill. This operation is carried out using a hard stopping such as an exterior wood filled or waterproof stopping.

Warning

If you use putty instead of a hard stopping, when the sill has been repainted the oil content trapped beneath the surface of paint film could cause blistering when the paint is subjected to excessive heat.

Let the stopping set according to the manufacturer’s instructions and sand smooth. Where resinous (sticky) knots are revealed apply a coat of shellac knotting and leave to dry before painting. Repaint the entire surface with primer, undercoat and top coat.

Where it has been necessary to remove a large chunk of rotten timber, to repair by filling would  not only be impractical but unsound. Here you will need to make good by cutting a new piece of timber with a panel saw to the same size as that removed. Position the new piece of timber and mark a drip groove in pencil, following the line of the existing groove. Remove and tenon saw the groove to the require depth and width, gouging out the waste timber with a narrow chisel. Glasspaper smooth for a clean finish.

Fix the new piece of timber in position with exterior adhesive and galvanized nails. Sink the nail heads below the surface with a nail punch and fill the cavities with exterior filler or waterproof stopping. Repaint as before.

The most troublesome timber is oak which, because of its open grain, tends to encourage breakdown of the paint. Here strip back to bare wood and rub fine surface filler well down into the grain with a piece of clean rag. Let it set and then smooth with glasspaper, working only with grain. Finally apply a coat of aluminium primer, then undercoat and top coats of paint.

Ideally oak is the best left in its natural state and protected by coating with boiled linseed oil or a timber preservative. So when you strip the sill decide which finish you want – natural or painted.

Making a concrete sill
In extreme cases of rotting, where the sill has to be removed completely, it is worth casting a new one of your own in concrete. This is not difficult or expensive and removes forever the possibility of rot.

Chisel out all remaining pieces of the old sill. Apply one coat of acrylic primer to the bare wood of the frame for protection and apply a second coat immediately prior to casting the sill. To reinforce the concrete, drive a row of 150mm (6in) nails, 150mm apart and a third of their length, into the timber along the bottom of the window frame.

You must then construct a shuttering box from 25mm (1in) thick timber, screwed together, into which you pour the concrete. The tops of the sides of the box should slope slightly downwards away from the wall to prevent rainwater from collecting on the sill and causing possible rotting of the window frame. To support the shuttering box crew a batten to the wall below the window frame, ensuring the screws are well anchored. Remember to make the top of the box the level you intend as the top surface of the sill, and that the inside measurements of the box will be the outside measurements of the sill.

For your drip groove, stretch a length of stout cord – sashcord is ideal – along the base of the box and through specially drilled holes in the side pieces, knotting at both ends to keep it taut.

Mixing the concrete
Using one part fine shingle, two parts clean and sharp sand and one part cement, add water gradually untill you have a buttery, rather than sloppy, consistency. Shovel the mix into the shuttering box and smooth level with the top edges of the timbers with a steel float. Allow a few days for the concrete to set thoroughly, remove the box and cord, then paint with a proprietary concrete paint.

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