Concrete window sills: what defects mean when buying a property

Concrete window sills are found on a wide range of properties built from the mid-twentieth century onwards. Durable when properly maintained, they are nonetheless vulnerable to water ingress, frost damage and the gradual effects of weathering. On older properties especially, cracking, spalling or crumbling sills are frequently flagged in survey reports.

Knowing what to look out for at a viewing, and what a surveyor's findings actually mean, puts buyers in a stronger position when it comes to negotiating or deciding whether to proceed.

The most common forms of deterioration a surveyor will look for include:

Cracking: horizontal or vertical cracks are the most visible sign of damage. Surface cracks can allow water to penetrate, which accelerates further deterioration and can lead to damp problems internally.

Spalling: the flaking or breaking away of the concrete surface, often caused by water seeping in and corroding the steel reinforcement within the sill. Once the steel begins to rust and expand, it forces the surrounding concrete apart. The same defect is common on brickwork too, and the underlying cause is often the same.

Crumbling or soft concrete: concrete that feels soft to the touch or breaks away easily indicates significant degradation. This is more serious than surface cracking and usually requires full replacement rather than a repair.

Warping or bulging: a sill that appears to bow outward may indicate internal structural movement or expanding corroded reinforcement beneath the surface.

Discolouration: staining or colour changes can point to water damage, mould growth or mineral deposits leaching through the concrete.

The severity depends on the extent and nature of the damage. Minor surface cracking is relatively common on older properties and is usually straightforward to address. It becomes more significant when:

• Water has been entering the wall behind the sill for an extended period, potentially causing damp to spread internally
• The sill structure itself has failed, meaning water is no longer being directed away from the wall properly
• Corroded reinforcement is causing the concrete to break up from within, which requires more substantial remedial work

A surveyor will assess whether the damage is cosmetic or whether it has led to secondary problems such as damp penetration or damage to the window frame and surrounding masonry.

As part of a RICS Level 2 HomeBuyer Report or RICS Level 3 building survey, the surveyor will inspect all accessible window sills for signs of cracking, spalling, deterioration and any evidence of water ingress.

Where damage is found, it will be rated according to severity and recommendations made on the appropriate course of action. In a Level 3 survey, the surveyor will also look at the wider context: the condition of surrounding masonry, the state of window frames and any evidence of damp internally, to give a fuller picture of what the defect has led to.

If you are buying an older property and can already see signs of deterioration at the viewing, it is worth considering a Level 3 survey rather than a Level 2. Our house survey cost guide explains what each level covers and what to expect to pay.

If a survey flags damaged concrete window sills, that information has practical value in the buying process. Depending on the severity of the findings, you may be able to use the report to renegotiate the agreed price, ask the seller to carry out repairs before exchange, or simply factor the likely cost of remedial work into your decision.

For more information see our guide on buying a house and what to expect from the survey process.

A property survey will identify any issues with concrete window sills and give you a clear picture of their condition before you commit to a purchase.

At Surveyor Local, we work exclusively with RICS-accredited chartered surveyors across England and Wales. Our team can arrange a survey quickly and at a competitive price.

Call our friendly team on or get a survey quote online.