Entry 19 · Windows & Glazing
Stone sills, thresholds and window surrounds
How natural stone sills, thresholds and surrounds behave in English rain, frost and salt air, and what to check before specifying or replacing them.
Natural stone at a window or door is a weathering detail before it is a decorative one. In England, a sill or threshold has to shed rain driven sideways, survive freeze and thaw cycles, and stay walkable when wet. The stone type, the finish and the fall all matter more than the colour.
A sill that works is one where water leaves the building faster than it arrives, and where the stone is dense enough not to hold that water through winter. A natural stone guide covering marble, granite and travertine from quarry to surface is a useful reference when comparing absorption and hardness figures before choosing a sill or threshold.
01 What does a stone sill actually have to do?
A window sill has three jobs: throw water clear of the wall below, close the gap between frame and masonry, and tolerate the movement between the two. In English weather the first job is the one that fails most often.
Water arrives not only from above but from the side, driven by wind. A sill with too little projection, or with a fall of less than about 1 in 40, will let water track back to the reveal. Once it reaches the frame, the damp shows up as staining on the internal plaster rather than at the sill itself, which is why the cause is often misread.
The second job is the joint. A sill bedded on a weak mortar or on a bead of sealant alone will move. Traditional practice beds external stone on a mortar that is softer than the stone, so any movement cracks the mortar rather than the sill. Modern work often uses a polymer-modified adhesive, and the classification matters: a C2 adhesive to BS EN 12004 is the grade normally specified for external stone where frost and movement are expected.
The third job is drainage. A drip groove, sometimes called a throat, cut into the underside near the outer edge stops water running back along the soffit. Without it, even a well-projected sill can wet the wall below.
02 Which stones cope with frost and rain?
Frost resistance is largely a function of how much water a stone absorbs and how it behaves when that water freezes. The relevant measure is water absorption by mass, tested to BS EN 13755, and the lower the figure the better for external use in a wet, freezing climate.
Granite is generally the safest choice for sills and thresholds. It is dense, absorbs very little water, and its hardness on the Mohs scale means it resists the abrasion a threshold takes from foot traffic and grit. Dark granites show salt bloom and dried rain less than pale ones, though they show dust more.
Travertine sits in the middle. Its absorption depends heavily on how open the natural voids are, and unfilled travertine is a poor choice for an external sill in England. Filled and honed travertine can perform well if it is bedded correctly and kept clear of standing water, but it is not the material to choose for a north-facing threshold that stays damp.
Marble is the most difficult of the three externally. Carrara and similar marbles are softer, absorb more water, and are sensitive to acid. In a city or near the coast, rain carrying pollutants will etch a polished marble surface over a few winters. Where marble is wanted at a doorway for appearance, a honed or brushed finish hides the etching far better than a polish.
03 Which finish should be used outside?
Finish is not only about appearance. A polished surface is the least absorbent in the short term because the pores are closed by the polishing process, but it is also the most slippery when wet and the most likely to show acid etching. For an external threshold, a polished finish is usually the wrong answer on both counts.
Honed, brushed and sandblasted finishes open the surface slightly, which raises absorption but gives grip. A sandblasted or brushed threshold is the standard answer for a doorway that faces the weather. Flamed finishes, produced by playing a high-temperature flame across the surface, are common on granite and give a rough, slip-resistant texture without sealing the stone.
Bush-hammered stone, worked with a serrated hammer, gives the roughest texture of the common finishes and is often used on steps and copings rather than sills, where it would trap dirt against the frame.
Whatever the finish, an external sill should be treated with a breathable water repellent rather than a film-forming sealer. A film traps moisture beneath it and can lift in frost. A breathable impregnator reduces water uptake while letting the stone dry.
04 How are sills, thresholds and surrounds fixed?
The detail that decides whether a stone sill lasts is the bed and the joint, not the stone.
Sills are normally bedded on a mortar or adhesive that fully supports the underside, with no voids. Voids collect water and are where frost damage starts. The joint between sill and frame should be a backer rod with a low-modulus sealant, not a rigid pointing, so the frame can move without cracking the stone.
Thresholds take traffic as well as water. They need full bedding, a slight fall outward, and a joint at the door line that can be compressed. Where a threshold meets a timber floor inside, a movement joint is normal, because stone and timber move at different rates with changes in humidity.
Surrounds, the stone jambs and heads around a window or door, are usually the most exposed part of the detail. They should be set with a slight forward fall on any horizontal surface, and any coping above should overhang the surround below it. A surround that sits flush with the wall above will stain within a season.
05 Can a stone sill be replaced without removing the window?
Often, yes, but the sequence matters. The internal reveal is usually the first thing to be cut back, because the sill is bedded under the frame and the frame has to be supported while the old sill is removed.
A temporary support under the frame, a cut line through the old bed, and a new sill slid into place on a fresh bed is the usual method. The frame is then re-fixed and the internal reveal made good. The work is weather-dependent: bedding mortar and sealant both need dry conditions, and a sill replaced in November may not cure properly before the first frost.
Where the frame itself is being replaced, the sill is normally renewed at the same time, since the cost of the stone is small compared with the cost of a second visit.
06 What should be checked before signing for stone work?
Ask for the stone's water absorption figure and its frost resistance, not just its name. Ask which adhesive or mortar is specified, and whether it is a C2 grade to BS EN 12004 for external work. Ask for the fall on every horizontal stone surface, and where the drip groove is.
Check the finish against the location. A polished threshold on a north-facing door is a maintenance commitment, not a detail. A honed or brushed finish on the same door is not.
Check the joint design. A rigid pointing between stone and frame will crack; a low-modulus sealant over a backer rod will not.
Finally, check the maintenance instruction. External stone in England should be cleaned with a pH-neutral cleaner, not an acid, and re-treated with a breathable repellent every few years depending on exposure. Oil and wine stains on a threshold are treated by drawing the substance out, not by scrubbing it in, and a poultice is the usual method.
Get those five things in writing, and the stone at the window or door will outlast the frame it sits against.