A cracked window pane is one of those faults that looks small until the weather turns, the crack spreads, or a child brushes a hand across it. We see it in London flats, shopfronts, period terraces and rented properties after knocks, thermal stress, failed putty, stiff sashes and the occasional football.
The main mistake is assuming every crack needs a whole new window, or the opposite, taping over it and hoping it will last until next winter. Both can cost you. A needless full replacement can run into hundreds or thousands of pounds, while leaving unsafe glass in place can mean water damage, draughts, security problems and a failed landlord inspection.
The right answer depends on the glass, the frame, where the pane is fitted, and whether the crack is a surface crack, a failed sealed unit, or a sign that the frame itself has moved. Here is how we judge it on site before recommending repair or full replacement.
Understand Why the Pane Has Cracked Before Choosing the Fix
Glass rarely cracks for no reason. A clear impact point usually means something has hit it, but a long clean crack starting at the edge can come from movement in the frame, a tight glazing bead, thermal stress, or old putty holding the pane too rigidly. In Victorian and Edwardian timber sashes around Kensington and Chelsea, we often find cracked panes where old linseed putty has gone hard, the sash has swollen, and the glass has been squeezed as the window is forced open.
Modern double glazed units fail in a different way. You might see a visible crack in one leaf of glass, or you might see misting between the panes because the perimeter seal has failed. Once the seal has gone, the sealed unit cannot be properly repaired with a tube of sealant from a DIY shop. The glass unit usually needs replacing, even if the frame can stay.
Location matters as well. Glass in doors, beside doors, low-level glazing and some bathroom or stair areas may need to be safety glass to meet Building Regulations Part K and relevant British Standards such as BS EN 12600. If safety is involved, get the glass inspected professionally rather than guessing from appearance.
Make the Area Safe Immediately
Keep people away from the pane until you know whether the glass is stable. A small crack can run suddenly if the pane is under tension, especially when a window is opened, slammed, or heated by direct sun after a cold night. Wear gloves and shoes if you are clearing loose fragments, and do not press on the glass to test it.
For a short-term measure, apply clear packing tape across both sides of the crack if you can reach safely. This is not a repair, it only reduces the chance of loose shards falling while you arrange the proper work. If the pane is badly broken, at ground level, in a commercial frontage, or exposes the property to entry, arrange emergency boarding or same-day make-safe work. In London, a basic emergency boarding visit is commonly around £120 to £220, depending on size, access and time of day.
Identify the Type of Glass and Window
Before anyone can price the job properly, they need to know what type of glass and frame they are dealing with. A single glazed pane in an old timber sash is a different job from a toughened double glazed unit in a uPVC casement, and both differ from laminated shopfront glass. The thickness also matters, with older single glazing often 3mm or 4mm, while replacement units and safety glass vary according to size and use.
Look at the frame material, the opening style, and the condition of the beads or putty. Timber sashes may need the old putty raked out and new glazing sprigs fitted. uPVC and aluminium windows usually have removable beads, but brittle older beads can crack during removal, adding to the job. If you are in a leasehold flat or a building with a communal freehold, check whether the window is your responsibility before ordering a full replacement.
Check Whether a Pane Repair Is Enough
A repair usually means replacing the damaged pane or sealed glass unit while keeping the existing frame. This is often the sensible choice if the frame is sound, the window opens and shuts properly, and there is no rot, distortion or repeated cracking. For single glazing in a timber sash, a like-for-like pane replacement in London will often sit around £120 to £250 for a modest pane, including fitting, though awkward access or sash removal can increase that.
For a blown or cracked double glazed unit, the frame can usually stay if it is square and serviceable. A typical replacement sealed unit might cost around £180 to £450 fitted for common domestic sizes, with higher prices for toughened, laminated, patterned, acoustic or specialist low-emissivity glass. If the frame is good, this avoids the cost and disruption of replacing the whole window.
Pane replacement is also useful in period properties where keeping original joinery matters. In conservation areas, changing the whole window can raise planning and appearance issues, while replacing a broken pane like-for-like is usually more straightforward. Listed buildings are different, so always check before altering historic glass or frames.
Decide When Full Window Replacement Is the Better Option
Full replacement makes sense when the crack is only one symptom of a failing window. Rotten timber rails, loose joints, distorted uPVC frames, perished seals, draughty units and repeated water ingress can make another pane replacement a false economy. If the sash rattles, the casement has dropped, or you can see daylight at the frame, the glass is not the only problem.
Costs vary widely. A straightforward uPVC casement replacement might be around £450 to £900 per window in many London properties, while a made-to-measure timber sash replacement can be around £900 to £1,800 or more depending on size, glazing specification, ironmongery and decoration. If scaffolding is needed for upper floors or rear elevations over extensions, that can add several hundred pounds.
Replacement windows also bring regulatory points. New windows in dwellings normally need to comply with Building Regulations, including energy performance requirements under Part L and safety requirements where applicable. Installers often self-certify through schemes such as FENSA or CERTASS, or the work can go through Building Control. If you are a landlord, keep the paperwork because buyers, managing agents and insurers may ask for it later.
Measure and Price the Work Properly
Do not rely on a rough width and height taken from the visible glass alone unless you know how the system is built. For single glazing, the rebate depth and required edge clearance matter. For double glazed units, the thickness of the sealed unit, spacer bar, coating, gas fill, safety rating and any pattern must match the window system.
On a proper visit, we check the pane size, frame condition, access, glass type, safety requirements and whether the opening still works correctly. In older London houses, we also look for sash cord failure, painted-shut windows and rotten cills, because disturbing one pane can expose a bigger maintenance issue. If timber repair is needed around the pane, allow extra. Small splice repairs and making good may add around £120 to £350 depending on how much timber has failed and whether decorating is included.
If a tenant reports the crack, take clear photos from both sides and note when it appeared. Landlords should avoid delaying, because a cracked pane can become a safety and security concern, especially in common parts, front doors, bathrooms, children’s rooms or shop premises.
Remove the Damaged Glass and Prepare the Frame
For single glazed timber windows, the old putty or glazing compound is carefully cut out, loose sprigs or pins are removed, and the cracked glass is lifted away in sections. The rebate should be cleaned back to sound timber, not just skimmed over, otherwise the new putty will fail quickly. If the timber is damp or rotten, it needs repair before the new pane goes in.
For uPVC or aluminium, the beads are removed in the right order and the sealed unit is supported as it comes out. Packers are important because they keep the unit sitting square and stop the opening sash dropping. A poor fit can create pressure points, which is one reason replacement panes crack again.
This is the point where DIY often goes wrong. Broken glass is sharp, larger panes are heavier than they look, and toughened or laminated glass behaves differently when damaged. If the pane is above ground floor, large, near electrical fittings, or in a public-facing business frontage, call a professional rather than taking chances.
Fit the Correct Replacement Glass
The replacement should match the use of the window, not just the hole it fills. Bathrooms may need obscure glass, street-facing bedrooms may benefit from acoustic laminated glass, and doors or low-level areas may require toughened or laminated safety glass. For double glazed units, matching the thickness and spacer colour also helps avoid one odd-looking pane in a run of windows.
In timber frames, the pane is bedded evenly, fixed with proper glazing pins or sprigs, and finished with suitable putty or glazing compound. The putty line should shed water cleanly and should not be left proud in a way that traps moisture. Once cured, it needs painting to seal it, otherwise water gets behind it and the same problem comes back.
In beaded systems, the unit should be packed correctly, beads refitted securely, and seals checked for gaps. A quick water check around the frame is worthwhile, especially on exposed elevations where driving rain hits hard. Many upper-floor rear windows in London take more weather than owners realise because extensions and lightwells channel wind and rain awkwardly.
Test the Window and Finish the Surrounding Area
Once the glass is in, open and close the window fully. A sash should slide without grinding, and a casement should not catch at the top or lock under strain. If the frame has shifted, forcing it shut will only put stress back into the new pane.
Check seals, draughts and any making good around the frame. If timber has been repaired, it should be primed and painted properly once materials are ready, not left bare. Our decorating team often sees window repairs fail early because the glazing was sound but the exposed timber was never finished after the glass work.
For businesses, also check alarms, contacts, blinds and display fittings after a pane repair. A cracked shop or office pane is often tied into security sensors, vinyl signage, or internal partitions, so a simple glass replacement can affect more than the window itself.
Plan the Timing, Access and Permissions
Small pane replacements can sometimes be completed in one visit if the glass is standard and available. Bespoke double glazed units are usually measured first and made to order, often taking several working days depending on specification. Safety glass, laminated glass, acoustic units and heritage-style slim double glazing can take longer.
Think about access before booking the work. A ground-floor sash facing a quiet garden is simple. A fourth-floor flat above a shop, a rear window over a conservatory, or a pane in a communal stairwell may need extra labour, tower access, or agreement with a managing agent. In mansion blocks and converted houses across Kensington, Chelsea, Notting Hill and Holland Park, access rules and working hours can be stricter than owners expect.
Season matters too. Winter cracks often show up after cold nights followed by direct sun, while summer heat can expose tight glazing and poor edge clearance. If the room is already draughty or damp, do not wait months, because moisture around timber frames quickly turns a glass job into a carpentry and decorating job.
Frequently Asked Questions
Q1: Can a cracked window pane be repaired without replacing the glass?
Usually no. Tape or resin may hold a small crack temporarily, but domestic window glass is normally replaced rather than invisibly repaired. The proper fix is to replace the pane or sealed unit and check why it cracked.
Q2: Is a cracked double glazed unit dangerous?
It can be, depending on which pane is cracked, where it is located and whether it is safety glass. It also loses performance if the seal is damaged, so arrange a professional inspection rather than leaving it.
Q3: How quickly should a landlord deal with a cracked pane?
As soon as reasonably possible, especially if it affects safety, security, weatherproofing or a tenant’s normal use of the room. If the glass is loose or accessible to children or the public, it should be made safe urgently.
Q4: Will replacing a window pane need planning permission?
A like-for-like pane replacement usually does not, but listed buildings, conservation areas and full window replacements can be different. If the external appearance changes, check before ordering the work.
Q5: Should I replace all the windows if one pane has cracked?
Not unless the frames are generally failing or you are already planning an upgrade. A sound window usually only needs the damaged glass replaced, but repeated cracks, rot or poor seals may point towards full replacement.
A cracked pane needs a calm decision, not a guess: make it safe, identify the glass and frame, then choose pane replacement if the window is sound or full replacement if the whole unit is failing. Our team at Kensington Maintenance Services Ltd handles pane replacement, timber repairs, sash window making good and wider handyman services across local homes, rentals and business premises, and if you want us to assess the safest and most cost-effective route you can get a quote or contact us and we will talk you through the options plainly.