Sash Cord: Waxed Cotton, Nylon or Jute? How to Choose

The answer is dull, and on nine London sashes out of ten it is the same answer: braided waxed cotton over a synthetic core, 6mm or 7mm, pre-stretched, bought as one coil. Everything below is why that beats the nylon sitting next to it on the shelf and the jute in the bag marked sash cord, and which small decisions turn a fifty-year rope into a five-year one.

Start with the diameter, not the material

Cord is sold from about 4mm up to 8mm. Domestic London box sashes take 6mm or 7mm almost every time. Eight millimetre belongs on ground-floor bays, on tall plate-glass sashes, and on original sashes that have been reglazed with sealed units and are now carrying weight they were never made for. The old numbering still gets said on site, No. 6 and No. 8, and it lands on the 6mm and 8mm coils you buy today.

Makers rate cord by the weight of the whole sash it will carry. One of the larger UK suppliers publishes these figures for its waxed cotton:

Coils of 4mm and 5mm are sold as well, generally for small and light sashes, but the suppliers who publish weight ratings do not publish them for those sizes. Take that as a hint about what they expect them to be asked to do.

The right-hand column is the one people misread. Each sash hangs on two cords, and each cord carries one weight, not the whole sash. On single-glazed London sashes those weights usually sit somewhere between 4kg and 9kg a side, the larger figure going with big panes and thick glass. So a correctly chosen cord spends its entire working life a long way inside its own rating. The number on the coil is a size band, not a limit you are creeping up on. If the mechanism is new to you, our guide to how a sash window works sets out the weights, the pulleys and the balance between them.

Ratings are not consistent between suppliers, either, and that is worth knowing before you trust a rule of thumb. One merchant's page for a 100m coil of 8mm quotes 50kg as the maximum sash weight in one line and 75kg in another, for the same product. Where a window is anywhere near a stated limit, go up a size. The cord for a whole window is a few pounds and a snapped one is a visit.

What the standard cord is actually made of

Good sash cord is a braided cotton jacket over a synthetic core, impregnated with wax. Three materials, three separate jobs.

The cotton jacket takes the abrasion at the pulley and stays flexible for decades, which is what a fibre bending over a wheel a few thousand times a year has to do. The core carries the load without lengthening. It is worth knowing that the core is not the same everywhere: some ranges are built on polyester, others on polypropylene, and the difference is rarely on the label. The wax repels moisture inside a box that is open to the outside air through the pulley slots, and it cuts friction where the rope rides the wheel. Waxed cotton is normally supplied pre-stretched, which matters, because rope that has not had that done settles in its first months and drops the top sash out of line at the meeting rails.

None of this is a modern corner cut on tradition. It is a better rope than the pure cotton that came out, because it puts the non-stretching fibre where the load is and the flexible, grippable fibre where the wear is.

Nylon is stronger on paper. The objections are about everything else

Take the case at its strongest. The same supplier that rates its 6mm waxed cotton at 34.9kg rates its 6mm pre-stretched nylon at 54.4kg, and sells it as the longer-lived product. Nylon does not rot. It is sold pre-stretched precisely because bedding-in was the known weakness. On the numbers, it wins.

Then you hang it in a weight box, and the numbers stop being the argument.

Knots are the first problem. The standard fixing at the weight end is a knot, and a slick braid works loose where waxed cotton holds. The second is at the other end: where the cord is nailed into the groove in the sash stile, a hard synthetic can work past clout heads instead of gripping around them. The third is slower. Pre-stretching removes the stretch that comes from how the rope was built; it does not change how a polymer behaves with a cast weight hanging on it for twenty years. Where a synthetic does drift, the symptom is not a break at all. The upper sash gradually sits low, the meeting rails come out of line, and the window starts knocking about in a wind, which is one of the less obvious causes covered in how to stop sash windows rattling for good. Cheap polypropylene has a fourth problem, which is daylight through a gap in a pocket piece.

Where nylon is a reasonable choice: a sash so heavy that cotton would be at the top of its band, a back-extension window nobody sees, or an outbuilding where rot is the likelier enemy than wear.

Jute, and the bag marked sash cord

What a general hardware shop sells under that label is often jute, or a coarse unwaxed cotton. It is cheap, it looks the part for about a week, and it belongs on a garden gate.

Jute is a bast fibre with very little abrasion resistance and a strong appetite for moisture. In a weight box open to the outside through the pulley slots it takes up damp, loses a large share of its strength while wet, and rots from the inside where nothing shows. It also sheds fibre continuously, so it thins at the wheel faster than anything else you could have chosen. The cords that come out of London windows still sound at the pulley after decades are, with very few exceptions, waxed cotton. Jute rarely gets that far.

Flax and hemp cords were genuinely used historically and some survive, but they were spun and laid to a standard with no relationship to the jute on sale now. Unwaxed cotton sits between the two and has no reason to exist with waxed on the shelf beside it. The saving across a whole house is a few pounds, and the price of it is taking every sash out again inside a decade.

Chain, and the windows that genuinely want it

Solid link or knuckle chain has one real application: sashes heavy enough that any rope becomes a consumable measured in years rather than decades. Shopfronts, tall plate-glass Victorian sashes, and original sashes retrofitted with sealed units all qualify. Chain does not rot, does not lengthen, and does not abrade the way plaited fibre does. Brass and bronze chain outlast plated steel, which will rust in a box that breathes damp air through the slots.

It carries conditions, and they are not optional. It needs its own pulley: makers sell wheels specifically for chain, in 2in and 2¼in, cut differently from a wheel meant for rope, and running chain over a cord wheel wears both. It cannot be knotted, so it wants a proper fixing at the weight and a plate or pin at the sash. And it is audible, both over the wheel and moving inside a hollow timber box, which in a bedroom is a reasonable objection to raise before the work rather than after.

Before reaching for chain on a sash that has only recently become heavy, check the balance first. A sash that has gained several kilos from new glass usually needs different weights rather than a different suspension, and working out the right sash weight sets out the arithmetic.

Reading the end that broke

Whatever you buy next, spend a minute on the frayed end hanging below the pulley, because it tells you what went wrong and whether the new rope will meet the same fate.

Nine times in ten the failure is up at the wheel, because that is the one spot on the rope that gets worked. Everywhere else the cord simply hangs, while the few inches riding the pulley are bent round the wheel and pulled straight again on every opening. Do that for a century and the wax migrates out of the braid, the outer fibres saw against the wheel and against each other, and the rope thins from the outside in.

What varies is how the end looks. Furred, grey and visibly thinned over 30mm or 40mm is honest fatigue: the cord did the job it was made for. Stiff and slightly shiny, with the braid sealed together into a board, is paint. Gloss run down the pulley stile fills the braid and seals the wax under a film, and a rope that used to roll over the wheel starts sawing across it instead. Dark, damp fibre that crumbles between finger and thumb, usually with a musty smell out of the box behind it, is rot, and the cord is not really at fault. Water is getting in through a pulley slot or a tired cill, and the timber wants attention before anything new is threaded through.

While the staff bead is off, put a finger on each wheel and spin it. A wheel that has seized, or one turning dry in a paint-choked housing, stops rolling and starts dragging, so the new cord will wear in one fixed spot instead of over a moving surface. Squealing is the audible version of the same fault, and what wears out inside an axle pulley covers when the wheel itself is past saving.

Why all four cords go in together

The rope is the cheapest thing in the whole job by a long way. The cost sits in getting to it: lifting the staff bead and the parting bead without splitting them, opening the pockets, and setting the sashes back so each one stays exactly where you leave it. Do that once and four cords come out of it. Do it four times across a decade, as one snapped cord at a time forces you to, and the beads are a little more chewed on each visit and the bill has been paid four times over.

The other three cords in that window are the same age, have run over the same wheels under the same weights, and are somewhere within the same season of their working lives. Buy one coil rather than two while you are at it, because rope from different batches or different makers will not wear at the same rate on the same window, which puts you straight back to replacing them one at a time. The single-cord repair is the most reliably expensive habit we come across in the London boroughs our box sash work covers.

None of this is notifiable work, which occasionally surprises people. An external window is a controlled fitting under the Building Regulations, so replacing one, or replacing its glazing, brings those rules into play. Re-cording an existing sash replaces neither, and what the regulations actually cover on a repair is worth a read if you are being told otherwise. On a listed building, routine repair is normally accepted without listed building consent, though "like for like" is not a phrase that appears in the legislation and councils draw the line in different places, so anything beyond a cord is worth a call to the conservation officer first.

Measure, weigh, then buy one coil

Five minutes with a tape and a set of bathroom scales settles most of what is left.

Reckon on roughly 4m of cord per sash as a starting figure, then measure properly rather than trusting that. Weigh the weights: the pair for one sash should total slightly more than an upper sash and slightly less than a lower one, and if they do not, new cord simply restarts the same clock. Find out whether the sash has ever been reglazed, since heavier glass hung on original weights is why so many cords spend their lives working outside the band they were sold for. Check that the cord seats down into the pulley groove without rubbing the sides; an 8mm cord binding in the wheel means either the wheel was made for something smaller or it is choked with old paint. And keep the decorator off it: fit cords once the last coat of gloss has hardened, or mask them, because paint in the braid finishes a good rope in a handful of winters.

Then buy one coil in the right diameter. A 100m coil runs from a little under £30 to about £75 including VAT depending on size and supplier, and it will re-cord a house with plenty left over.

If you are doing the work yourself, the full sequence from pocket to parting bead is the order to take the window apart in. If the sashes are painted in, or the window is a bay above head height, our sash cord replacement service does all four cords, the pulleys and the balance in one visit. Either way the specification fits on the back of a hand: waxed cotton over a synthetic core, pre-stretched, in the diameter the weights ask for, one coil, kept out of the paint. Nothing on that list costs more than a few pounds. All of it decides whether the next person to open that pocket is you in five years or nobody at all in fifty.