What Does a Sagging Roof Mean?

What you may be noticing

Roof sag is usually spotted from across the road, or in a photograph, rather than up close. A straight edge helps: ridges and eaves lines were built straight, so a visible curve is a real change.

Signs worth recording

• A dip or curve along the ridge line
• A slope that looks hollow or wavy rather than flat
• The eaves or verge line no longer running straight
• Slates or tiles opening up, riding or sitting unevenly in one area
• Gaps appearing at a ridge, hip or against a party wall
• Doors or ceilings in the rooms below behaving differently than before
• Whether the shape has changed recently or has looked like that for years

Possible contributing factors

Possible causes include the following, singly or in combination. Older roofs in particular can show a gentle settled curve that has been stable for decades, while a new or worsening sag is a different matter.

What may be involved

• Long-term deflection of roof timbers under sustained load
• Additional weight the roof was not originally built for — for example a heavier covering laid over a lighter one
• Deterioration of timber where water has been getting in over a long period
• Weakened, corroded or failed fixings and connections
• Alterations within the roof space, such as timbers cut or removed during earlier work
• Movement or deterioration in the supporting structure below the roof
• Damage to the covering itself making an otherwise sound slope look uneven

Is it new, or has it always been like that?

This matters more than almost anything else. An old roof with a settled curve that appears in twenty-year-old photographs of the house is a different situation to a dip that has appeared or deepened in the last couple of winters.

Old photographs, a neighbour's memory of the house or an estate agent's listing image can all help establish whether the shape is changing. If it is, that should be assessed rather than monitored casually.

What a roofing inspection can tell you

A roof inspection looks at the condition of the covering, the details and the visible roof structure where there is safe access to the roof space. It can identify roofing defects, water ingress, deteriorated timber that is visible, and whether the covering appears to have been changed.

That is often enough to establish whether the problem is roofing-related and repairable, or whether it needs a structural professional's involvement before anyone considers roofing work. We would rather tell you that than quote for a re-roof over a structure nobody has assessed.

Where roofing work does come into it

Where the sag is limited, the structure is confirmed to be sound and the problem is with the covering or localised timber, roof repairs may be the right answer.

Where the covering has reached the end of its life and the roof needs stripping in any case, a new roof or slate roof replacement is the point at which underlying timber can be seen properly and put right. Whether that is appropriate depends entirely on what the assessment finds.