It’s a question surveyors hear all the time. You notice a crack, maybe above a door frame or trailing down the corner of a room, and the worry sets in: is this something minor, or something serious?
The truth is, many cracks are not structural. They’re often classed as ‘cosmetic’, caused by things like shrinkage or minor thermal movement, or the natural settling that happens in homes over time. But some cracks are different. Some hint at deeper problems, and the hard part is knowing which is which.
Professionals use BRE Digest 251 as the go-to guide for assessing cracks. It breaks them down into categories based on width and severity. Category 0 is hairline, barely 0.1mm wide, the kind you would struggle to see unless the light hits it just right. At the other end, Category 5 cracks are over 25mm wide and may suggest significant structural movement.
That said, even some cracks in the middle of that scale can look dramatic but mean very little, particularly if you already own the property and there are no other signs of movement. Cracks between 1mm and 5mm wide fall under Category 2 in BRE Digest 251. The Digest does not comment on location, but in practice, surveyors often see cracks of this size around window openings or where different building materials meet. In many cases, if stable and isolated, these cracks can be safely filled and redecorated. But if the same crack keeps reappearing, grows over time, or comes with other symptoms, that is when it needs more than just filler.
This is where the reality kicks in. Whether you are a homeowner or a buyer, if the crack raises real concern, a general building surveyor, unless they are also a structural engineer, is not qualified to give you a final answer. They might offer an opinion based on experience, but more often than not, they will recommend further investigation. That usually means a residential structural engineer.
When cracks extend all the way to the ground, the issue may lie in the substructure. Assessing that properly is beyond a general residential surveyor’s remit. One of the first things a structural engineer is likely to ask about is the condition of the below-ground drainage. Collapsed or leaking drains are a known cause of ground movement, particularly in older properties. The next question is often about trees or large hedges, whether they are current or historic, especially in areas with shrinkable clay. Not all cracks are caused by subsidence. Some are caused by heave, which is when the ground swells, often after trees have been removed. Either way, this points to ground-related movement, and that is structural territory.
There are a few classic warning signs surveyors look out for, and you can do the same. Cracks that are visible on both the internal and external faces of a wall. Cracks that reach all the way to ground level, especially in brickwork, or that follow a stepped or diagonal pattern through mortar joints, may point to foundation movement such as subsidence or heave. When these signs appear together, for example, a wide, stepped crack that runs inside and out and continues down to ground level, that is the point at which proper structural assessment is warranted – you can skip a general surveyor.
Context is everything. A crack in a plasterboard-lined modern extension might look alarming but mean very little. The same crack in a bay window of a 1920s house built on clay soil could tell a very different story. Seasonal movement is common, and dry summers, thirsty tree roots, and neglected drains all play a part. Sometimes the movement settles. Sometimes it does not.
The guidance from structural experts like Rupert Pool is consistent. Width, direction, pattern, and context matter more than how dramatic a crack looks. What is really needed is a clear explanation and a conclusion. Is it a concern, or is it not? That is what a structural engineer is qualified to deliver. A residential surveyor might offer a solid opinion, but if there is any doubt, they will recommend you speak to a structural engineer anyway. They will frame it as being in your best interest, and to be fair, it is. But you could have avoided the survey cost by going straight to the structural engineer. Most engineers will not say that out loud, and why would they? We often see structural engineers thanking surveyors for the referral. Same with damp specialists. Of course they would applaud the middleman. Why bite the hand that feeds you?
This is where we think the industry sometimes lets people down. Advice can swing wildly from dismissive comments like “continue to monitor” to alarming warnings like “structural engineer comment is required” when most people just want plain answers.
At Survey Shack, we are not here to treat defects or upsell repairs. We are here to help you understand what is relatively normal, what is not, and when it is time to bring in someone with the right qualifications. Most cracks are part of a property’s story. Some need minor repairs. Some need proper investigation by a structural engineer. Knowing the difference is what makes the rest manageable.
So if you have spotted a crack, do not jump to conclusions and do not get brushed off either. Measure it. If you are the owner, think about when it appeared. Buyers and homeowners alike should check what is nearby. And if it looks like more than surface-level movement, skip the loop. Call a structural engineer. That is where real clarity starts.
Survey Shack’s guided survey is a smart, low-cost way to gain clarity – without the upfront expense of a residential building surveyor. Whether you’re looking for a budget-friendly alternative or simply trying to understand which type of survey is right for you, Survey Shack provides an affordable first step. It’s the modern buyer and seller’s best ally.
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