What it looks like
The signs below are the ones homeowners most often notice first. Spotting one early makes the problem far easier and cheaper to put right.
Why it happens
Understanding the cause matters — the right fix depends on it. These are the usual culprits behind sagging roof.
Undersized or overloaded timbers
If rafters were too small for the span, or a heavier covering (for example concrete tiles fitted over an old slate structure) was added, the timbers gradually deflect under a load they were never designed for.
Water damage and rot in the structure
A long-running leak softens rafters, purlins or the wall plate. Once timber rots it loses strength and begins to give way, so an untreated leak can turn into a structural sag over time.
Failed or removed internal support
Purlins, struts and collars brace the roof. If one has failed, or was cut out during a loft conversion without a proper replacement, the roof loses part of its support and starts to spread.
Foundation or wall movement
Movement lower down the building — subsidence or a bowing wall — can pull the roof structure out of line, so a sag is sometimes a symptom of a problem elsewhere in the property.
What to do
- 1
Treat a new or worsening sag as urgent
A dip that has appeared recently, or is visibly getting worse, needs prompt professional assessment. Sudden movement can precede a partial collapse.
- 2
Inspect the loft for the cause
Look for wet or cracked timbers, cut or missing struts and any sign of past leaks. This helps a surveyor or roofer understand whether it is a load, a rot or a support problem.
- 3
Keep weight and traffic off the roof
Do not add load or allow anyone onto a roof that is already sagging. Its capacity is already compromised and further weight is dangerous.
- 4
Get a structural assessment
A roofer or structural engineer can determine whether the sag is historic and stable or active, and specify the right remedy — from renewing timbers to adding proper bracing.
When to call a professional
Some situations are worth getting a roofer to look at straight away:
- The sag is new, growing, or accompanied by fresh cracks
- You can see rotten, split or deflected timbers in the loft
- A strut, purlin or collar appears to have been cut or has failed
- Doors and windows below the roof have started to stick or crack
Safety first
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