Weatherproofing Timber Windows and Doors for Winter

Why Timber Needs Attention Before the Cold Arrives

Timber windows and doors are one of the best things about a British home, but they take a serious beating between October and March. Driving rain on a south-westerly gale, freeze-thaw cycles and warm, dry air indoors all work against them. Water is the enemy; paint, oil and sealant are the armour. Once the armour fails, water gets in, timber swells, joints open, draughts appear and rot takes hold.

The right time to work is a dry, mild autumn day — ideally above 5°C and with no rain forecast for 24 hours, so sealants and finishes can cure. Tackle one elevation at a time rather than trying to do the whole house in a weekend.

Start With a Careful Inspection

Walk round with a torch, a flat-bladed screwdriver and a notepad, and look closely at:

• Soft or spongy timber at the bottoms of frames, on sills and along door bottom rails. Probe gently — if the screwdriver sinks in easily, rot is present.
• Flaking, blistering or cracked paint, particularly on horizontal surfaces where water sits.
• Failed glazing putty that is crumbling, missing, or has parted from the glass.
• Open joints at frame corners and where the frame meets the surrounding wall.
• Flattened, missing or perished draught seals, worn door bottoms and slack hinges.
• Blocked drainage slots, weepholes and sill grooves.

Deal with rot before anything else. Rake out soft timber, treat with a wood hardener and preservative, then rebuild with a two-part epoxy filler rated for exterior use. If the rot runs deep or affects the structure, call in a joiner.

Draught-Proofing That Still Lets the Frame Move

Timber expands and contracts with the seasons, so seals must accommodate movement rather than fight it. You want draught reduction, not an airtight seal.

• Brush seals — 5 to 10 mm nylon or polypropylene brushes in a carrier strip, fitted into the frame rebate or the edge of the sash. Ideal for sliding sashes and casements.
• EPDM or silicone bubble seals — self-adhesive or slot-in, suited to hinged casements and doors. Choose a hollow profile so it compresses rather than splits.
• Door bottoms — a brush or wiper seal in an aluminium carrier, or a hinged flap seal. An aluminium or hardwood threshold plate gives the seal something solid to close against.
• Letterplates, cylinders and keyholes — easily overlooked, and a genuine source of cold air. Fit a draught-excluding letterplate and an escutcheon with a brush.

Never block trickle vents or weepholes. Ventilation manages moisture; sealing it shut simply moves the damp problem indoors.

Treating and Protecting the Timber

Bare or weathered timber needs feeding before it needs covering. Sand back to sound timber, dust off, then treat with a solvent-based wood preservative, paying particular attention to end grain and joints — end grain drinks water far faster than a face.

For bare timber, choose a microporous finish: a breathable wood stain or microporous exterior paint. Moisture vapour can escape, but liquid water cannot get in. That balance matters, because sealing timber under a non-breathable film traps moisture that will blister the finish from behind.

When overcoating existing paint, feather the edges, spot-prime bare patches and apply two thin coats rather than one thick one. Thin coats flex; thick coats crack. Focus on the bottom edges of doors and the undersides of sills, where rainwater actually runs. Oiled oak and hardwood need re-oiling every 12 to 18 months with a UV-resistant exterior oil — apply until the timber stops absorbing, then wipe off the excess.

Renewing Glazing Seals, Mastic and Joints

Old linseed putty shrinks and cracks over time. Where it has failed, rake it out fully with a chisel or putty knife and clean the rebate back to bare timber. Prime the rebate, then re-glaze with linseed oil putty on traditional frames or a suitable glazing silicone on modern ones.

For joints between frame and wall, use a low-modulus, exterior-grade silicone or acrylic sealant that stays flexible. If the gap is deeper than about 6 mm, fit a backing rod first so the sealant forms a proper hourglass shape rather than a thin skin — a sealant touching only two sides will tear the moment the frame moves.

Never seal a drainage slot, weephole or sill groove. Those exist to let water out. If water is sitting in a groove, clear it with a thin wire rather than plugging it. On doors, check the weatherbar and bottom rail; refit the weatherbar with a 5 to 10 mm drip overhang so water runs clear of the threshold. A dark, wet bottom rail is the first sign it is not doing its job.

A Simple Winter Routine

Once the work is done, keep half an eye on it. After a storm, check sills, thresholds and glazing for standing water. Every few months, wipe seals with a silicone-friendly cloth and check that hinges and locks are snug — a door that has dropped 2 mm will leak air even with perfect seals.

Keep gutters and downpipes clear, too. Plenty of "window leaks" on upper floors are actually overflowing gutters sending water down the wall and into the frame. It is free to fix and can save a fortune.

Done properly, an afternoon's work in the autumn keeps the heat in, keeps the rain out, and adds years to the timber you have already got.