4th Floor
Silverstream House
45 Fitzroy Street
London
W1T 6EB
Silverstream House
45 Fitzroy Street
London
W1T 6EB
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4th Floor, Silverstream House, 45 Fitzroy Street, London, W1T 6EB
E: info@kelpstudio.co.uk
T: 020 3835 2633
M: 07561 417319
Is this crack serious? A structural engineer’s guide
19 October 2026 · Blog
Almost every house has cracks, and most are nothing to worry about. But some are an early sign of movement that’s worth catching. Here’s how we tell the difference when we inspect a property.
1. How wide is it?
Width is the first thing we measure. Engineers in the UK use the BRE damage scale, which groups cracks roughly like this:
• Category 0 to 1, hairline up to about 1mm: usually cosmetic. Fill and redecorate.
• Category 2, up to about 5mm: still mainly cosmetic, but doors and windows may start to stick. Worth getting looked at.
• Category 3, about 5 to 15mm: moderate damage. Repairs are likely and the cause needs investigating.
• Category 4 to 5, over about 15mm: severe damage that needs prompt professional advice.
A coin is a handy check: if you can slide a 1p coin into the crack, it’s over 1.5mm wide.
2. Which way does it run?
• Diagonal cracks running out from the corners of doors and windows, especially if they’re wider at one end, often point to foundation movement.
• Stepped cracks following the mortar joints in brickwork can mean the same.
• Straight vertical cracks where an extension or bay meets the house are common where the two parts sit on different foundations.
• Horizontal cracks in external walls can indicate problems with wall ties or a wall bowing outwards.
• Fine cracks where walls meet ceilings, or across new plaster, are usually just shrinkage.
3. Is it moving?
A crack that’s old and stable is very different from one that’s still opening. Signs of ongoing movement include cracks that reappear after decorating, doors and windows that start sticking, and cracks that open in dry summers and close in wet winters, which is typical of clay soils and nearby trees. Mark the ends of the crack in pencil with the date, and photograph it next to a ruler, so you can see whether it changes.
4. What’s nearby?
Large trees close to the house, leaking drains, recent building work next door and previous alterations such as removed walls or chimney breasts are all common causes. Tell the engineer about anything like this.
When to get help straight away
Call a structural engineer promptly if cracks are wider than about 5mm, are growing quickly, or come with bulging walls, sloping floors or doors that suddenly jam. If you suspect subsidence, check your home insurance policy, as most require you to report it.
What we do
We inspect the cracks, measure and categorise them, work out the likely cause and give you clear recommendations: no action, monitoring, repair or further investigation. Because we also design drainage, we always consider whether leaking drains are involved.
Guide prices: specific inspection of one crack or defect £950 + VAT, general inspection of the whole property approx. £1,700 + VAT. We can usually visit within 3 working days.
Our subsidence and crack reports →
Book an inspection or call 020 3835 2633
A crack that’s old and stable is very different from one that’s still opening. Signs of ongoing movement include cracks that reappear after decorating, doors and windows that start sticking, and cracks that open in dry summers and close in wet winters, which is typical of clay soils and nearby trees. Mark the ends of the crack in pencil with the date, and photograph it next to a ruler, so you can see whether it changes.
4. What’s nearby?
Large trees close to the house, leaking drains, recent building work next door and previous alterations such as removed walls or chimney breasts are all common causes. Tell the engineer about anything like this.
When to get help straight away
Call a structural engineer promptly if cracks are wider than about 5mm, are growing quickly, or come with bulging walls, sloping floors or doors that suddenly jam. If you suspect subsidence, check your home insurance policy, as most require you to report it.
What we do
We inspect the cracks, measure and categorise them, work out the likely cause and give you clear recommendations: no action, monitoring, repair or further investigation. Because we also design drainage, we always consider whether leaking drains are involved.
Guide prices: specific inspection of one crack or defect £950 + VAT, general inspection of the whole property approx. £1,700 + VAT. We can usually visit within 3 working days.
Our subsidence and crack reports →
Book an inspection or call 020 3835 2633