Not every dip in a roofline is a problem. Old roofs settle. What matters is whether the movement stopped fifty years ago or is still going.
Old settlement versus active movement
A gentle even dip along the ridge of a period cottage is often historic. Hand cut timbers settled into their final shape a century ago and have not moved since. It is part of the character and, on many Cotswold properties, entirely expected.
Active movement looks different. It is usually localised, the line is irregular rather than smooth, tiles around it are lifting or opening at the joints, and there is often a matching crack in a wall or ceiling below. If a dip has visibly worsened in the last few years, treat it as active.
Undersized or overloaded rafters are the most common structural cause, often after a heavier covering was fitted than the roof was designed for. Slate replaced with concrete tile is the classic example, because concrete can be nearly twice the weight.
Other causes are rot at the rafter feet where a gutter has overflowed for years, purlins that have deflected or lost their support, removal of internal walls or bracing during a loft conversion, and death watch or wood boring insect damage in older oak.
From outside, we sight along the ridge and the eaves line and compare against the neighbours where the houses are of the same age. From inside the loft, we check purlin deflection, look at the rafter feet at the wall plate, probe suspect timber and look for old repairs.
Where movement looks structural rather than cosmetic, a structural engineer's report is the right next step. It is a few hundred pounds and it prevents a great deal of guesswork on both sides.
The repairs, and what they cost
Sistering rafters, which means bolting a new timber alongside the tired one, runs a few hundred pounds for a small area and is common where a handful of rafters have sagged. Replacing rotten rafter feet and wall plate is more involved because the covering has to come off at the eaves, typically 1,200 to 3,000 pounds for a run.
Adding or repairing purlin support, or fitting a steel where a wall has been removed, is engineer led and costs from around 2,000 pounds upwards. Where the covering is also at the end of its life, doing the structural repair as part of a re roof is far more efficient than doing it separately.
Is a sagging roof dangerous?
Rarely an immediate collapse risk, but active movement gets worse and lets water in as the covering opens up. Historic settlement that has stabilised is usually not dangerous at all.
Will a surveyor fail a house for a sagging roof?
A survey will flag it and normally recommend a specialist inspection, which can affect a sale. A clear roofer or engineer report showing the movement is historic often resolves it.
Can a sagging roof be jacked back straight?
Sometimes, gradually, but on an old roof it is often the wrong aim. Stabilising the structure and re covering to the existing line is usually safer and cheaper than forcing century old timber back.
Did my heavier tiles cause this?
It is a very common cause. A slate roof re covered in concrete tile without strengthening puts far more load on the same rafters, and the sag shows up years later.
Does insurance cover a sagging roof?
Generally no, because it is gradual deterioration or a design and loading issue rather than sudden accidental damage.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.