Sash Window Maintenance: A Seasonal Checklist

Nothing on your windows has actually broken yet. They open, they shut, they rattle in a gale, and you cannot tell whether that means they are healthy or whether you are two winters from a four-figure bill. A box sash answers that in about ten minutes a window, once you know what to press and what to listen for.

Every part of one was meant to be replaced with the frame still in the wall: the cord, the beads, the putty, the bottom rail, even the pulley. Sash windows in London rarely die of old age. They die of twenty years of nobody looking.

Grade the windows before you plan any work

Find out which windows deserve your time. Four tests, no tools beyond a bradawl and your hands.

1. The drop test. Raise the bottom sash to about a metre and let go. A balanced sash stays exactly where you put it. If it sinks more than an inch, the weights are light for the glass or a cord has stretched; if it creeps upwards, there is too much weight in the box.
2. The bradawl test. Light pressure into the bottom rail, in from each corner, then into both ends of the cill. Sound timber pushes back. Timber that takes the point like cork has gone soft under paint that still looks perfect.
3. The putty press. Run a thumb along the fillets at the bottom of each pane, where rainwater sits longest, feeling for putty crazed into segments, hollow under pressure, or lifting off the glass.
4. The pulley listen. Run each sash slowly with your ear near the stile. A squeal is a dry axle, a grating note a worn or seized wheel.

A window that passes all four wants nothing beyond the seasonal lists below. Fail two or more and it has stopped being a maintenance job: cords, splicing in timber and anything needing the sashes out belong on a bench, as does all of this above ground-floor level.

Why twice a year, and why those two moments

Monthly checks tell you very little. Joinery moves with the seasons, and it is the two extremes that are worth catching.

A softwood sash takes up moisture through a wet London winter and swells across the grain, which means across the width of the stiles and rails. It runs in a couple of millimetres of clearance to begin with, so a window that binds in January often slides perfectly well in May with nobody touching it. Plane it in February and you will own a rattling window by August. Autumn is the same logic at the other end: September and October work is aimed at the six months that follow, and needs timber dry enough to take filler or paint.

Autumn: everything that keeps the water out

Work round the outside first, then the inside. Reckon on fifteen minutes a window.

1. Deal with every putty fillet that failed the thumb test. Small gaps can be sealed to get through the winter and done properly in early summer. Missing sections cannot wait: the rebate behind them is bare timber holding water against glass. The difference between linseed and acrylic putty decides how long the repair lasts.
2. Clear the drip groove. Run a finger along the underside of the cill and find the channel set in a short way from the front edge. Painted solid, it stops working, and water tracks back along the underside to the wall instead of dropping clear. Rake it out with a slim screwdriver or a groove scraper. This one job prevents more decay than anything else, which is why the cill has a guide to itself.
3. Check the joints at the bottom corners of each sash. A hairline is normal. A line you can get a fingernail into runs into end grain, and wants a flexible exterior filler before the rain sets in.
4. Look up. An overflowing gutter, a cracked hopper head or a downpipe dribbling onto a reveal will rot a perfectly sound window. A good share of reported window problems are drainage problems.
5. Test the fasteners. A fitch or Brighton fastener should pull the meeting rails tight together, not merely hold them close. If the sashes still shift with the fastener thrown, the keep needs packing or repositioning: a loose meeting rail rattles all winter and leaks the full width of the window.
6. Check the draught seals. Brush pile a decorator has painted over is finished. Pile that is flattened or missing in patches is doing about a third of its job.

Spring: the parts that move

By late April the timber is at its driest and the windows behave differently. This is the moment for anything mechanical.

1. Run every sash its full travel, top and bottom. Most London houses have an upper sash that has not moved in a decade, and that is where trouble hides.
2. Inspect the cords where they cross the pulley. Look at the section sitting on the wheel when the sash is down: that is the point that flexes every time the window moves, and where cord fails first. Fuzzy, flattened or showing its core means it is going. The usual London cord is No. 6 waxed cotton, 8mm and sixteen-plait; suppliers put a maximum sash weight of around 50kg on 8mm cotton, though figures vary by maker and construction, so read the data sheet. If one cord has gone, replace all four while the sashes are out, because they are the same age. Our note on choosing the right rope for a box sash sets cotton against nylon.
3. Service the pulleys. One drop of light machine oil on the axle at each end of the wheel, wiped off immediately, cures most squeals. If the wheel still will not spin freely under a finger, lubrication is not the problem: worn pulleys are a replaceable part rather than a reason to give up on the window.
4. Find whatever is binding. Where a sash rubs, the paint goes polished and shiny, and that shine tells you where to ease. A shaving off the right 100mm beats planing a stile that only needed its staff bead moved over.
5. Wash the glass and frames with warm water and a little detergent. Not a pressure washer: high-pressure water drives straight through a tired putty line into the rebate, and strips paint off a soft cill in seconds.

Wax on the running faces, oil only in the bearing

The faces a sash slides on want wax. Rub an ordinary white candle down the pulley stiles and the inner face of the staff bead, then run the sash a few times to spread it. It is the difference between a window you lift with two fingers and one you shove with a shoulder.

Keep WD-40 and similar aerosols off those faces. They are water-displacing blends that do contain a light lubricant, so they free something up for a fortnight, but the oily residue on a sliding timber face collects dust and grit into a fine grinding paste, and it soaks into bare timber where paint is meant to go later. Once a stile is oil-contaminated, primer will not hold on it. The axle of a pulley wheel is a bearing and takes a drop of oil. The stile is timber, and it takes wax.

Reading the paint before it fails

Three places tell you the truth about a paint system: the top of the cill, the bottom edge of the bottom rail, and the bottom 150mm of each stile. Water sits in all three, and the film opens there years before it goes anywhere else. Cracked, chalky or splitting along the grain there means the window is due. In London a south or west-facing sash generally wants repainting every five to eight years, and a sheltered north elevation goes longer. If the film has gone on a whole elevation at once, you are past maintenance: rebuilding a paint system across a full face of a house is restoration work rather than a Saturday.

Conditions decide the result more than the tin does, and data sheets differ, so check the one you are using. Dulux Trade's guidance for exterior work is to stop if the air temperature is below 10°C or relative humidity is above 80%, while some exterior wood finishes, Sikkens Cetol HLS Plus among them, are rated down to 5°C and up to 85%. Surfaces sit several degrees colder than the air early in the day, so an 11°C morning is not an 11°C window. That rules out most of November to February in London, and late afternoons from October, because the film wants to be touch-dry before the dew falls. Direct hot sun is the other trap: the surface skins before the film flows out, and the coating fails in three years.

Two details are specific to sashes. Lap the paint a couple of millimetres onto the glass to seal the putty-to-glass joint, and give the running faces primer and undercoat but never a heavy build of gloss, or the sash binds in its box. The order of work is in our guide to painting sash windows properly.

What winter is telling you

Watch, note, act in spring. The only winter job is the one that keeps water out tonight.

• Streaming condensation on the glass every morning is warm moist air meeting single glazing. A ventilation problem, not a leak: improve extraction now, talk about glazing in spring.
• Water pooling on the bottom rail is that condensation arriving where it does damage. Wipe it daily and treat the timber as suspect in April.
• A whistle in a strong wind means the meeting rails are not pulling tight, or there is a gap at the parting bead.
• A sash that suddenly will not move is swelling, a snapped cord, or paint bonding the sash to the bead. Diagnose before forcing: forcing turns a cheap fault into an open joint.
• A bang, then a sash that will not stay up, is a cord gone on one side. Not a spring job.

The habits that shorten a sash window's life

• Painting the window shut, then never opening it again. Ventilation is half of what keeps joinery dry.
• Running silicone along the meeting rails or beside the sash. It is not a draught seal on a moving surface, it stops the window working, and taking it off brings paint with it.
• Nailing the staff bead back with the same six nails every time. Old beads split. Buy new stock and fix with lost-head nails or brass screws through cups.
• Filling a decayed area with hard filler. Ordinary fillers shrink and set rigid; timber moves. The joint opens, water gets behind it, and the decay carries on out of sight. Use a two-part flexible repair system, and only where the timber underneath is sound. Anything deeper wants cutting out and splicing, as our guide to wet rot in sash windows explains.
• Propping a sash with a book instead of dealing with the cord that has already gone.

If the house is listed or in a conservation area

Routine maintenance is normally just that. Genuine like-for-like repair and repainting in the same colour are generally accepted as not needing listed building consent, and nobody expects an application to clear a drip groove. But "generally" is doing real work there: a repair in matching materials can still require consent where it affects the special interest of the building, and councils draw that line in different places. Changing the appearance is a separate question again, whether that is a new colour, a different finish, a change of glazing, or replacing rather than repairing.

In a conservation area, an Article 4 direction can remove the permitted development rights that would otherwise cover replacing windows, and those rights never applied to flats and maisonettes anyway. Ring your local authority's conservation officer before you change anything visible, photograph what was there beforehand, and take your council's answer over a neighbour's.

Keeping a record, and knowing when the list stops

The first pass round the house is the slow one, about an hour and a half in a three-bedroom terrace, because you are writing things down as well as checking them. Keep a page per window: when the cords went in, when it was last painted, which panes have been re-puttied, where the bradawl found something soft. From the second year it takes forty minutes, because the notes say which windows to look at hard.

Four findings turn a maintenance list into a priced job: a cord gone, a pulley wheel that will not turn, soft timber behind a bead, and a bottom corner joint that has opened rather than hairlined. None improve over a summer, and all are cheaper bought together, because most of the cost is the taking apart rather than the mending. That is how our sash cord replacement visits are put together. If your notes have turned up something you cannot place, we work across every London borough and can usually say on the phone whether it is a job for this spring or one that will keep another year.