Roof spread is when the rafters of a pitched roof push the tops of the walls outwards, usually because the timbers that should tie the roof together are missing, cut, or not doing their job. A sagging roof — a dipped ridge or a roof slope that dishes between supports — is often linked to it. Many older roofs have a gentle, long-settled sag that is nothing to worry about; others are still moving and need strengthening.
Here’s what causes both, how to spot them, and how to judge whether a sagging roofline is a problem.
How a traditional pitched roof holds itself up
A pitched roof is basically a series of triangles. Pairs of rafters lean against each other at the ridge. Their weight wants to push their feet outwards, and something has to stop that:
- Ceiling joists (or tie beams), fixed to the rafter feet at eaves level, act as ties holding the triangle together
- Collars — horizontal timbers higher up — help resist spread and stiffen the rafters
- Purlins — beams running along the length of the roof, part way up the slope — support the rafters mid-span so they don’t bend
- Struts and binders carry purlin loads down to internal walls
Most houses built before the 1960s have this kind of “cut” roof, built from loose timbers on site. Newer houses typically have trussed rafters — factory-made triangulated frames — which work on the same principle but rely on every member being intact.
Take out or weaken one part of the system and the forces go somewhere else, usually into the walls.
What causes roof spread and sagging?
Heavier roof coverings
This is one of the most common causes in older Northern housing. Many Victorian and Edwardian terraces were originally roofed in Welsh slate, a light covering. When those roofs were re-covered in the later 20th century, concrete interlocking tiles were often used instead. Concrete tiles are usually heavier than slate, and conservation guidance warns that using them without strengthening the roof structure can cause damage to the roof and supporting walls.
The extra weight bends the rafters and purlins, and the added thrust at the eaves pushes the walls out. Planning Portal advises that if a new covering is significantly heavier than the old one, the roof structure may need strengthening, and that most re-roofing work needs Building Regulations approval.
Pennine stone slate roofs are heavier still. Those roofs were built with heavy timbers to carry them, but where timbers have decayed or been altered, the weight shows quickly.
Loft conversions and alterations
Converting a loft often means cutting through ceiling joists, collars, struts or purlins to create headroom or fit dormers and roof windows. Building control guidance is clear that if these members are removed, alternative support — such as a structural ridge beam or new steel beams — needs to be designed by an engineer. Where that wasn’t done, or was done poorly, the roof can spread or sag.
Cutting trussed rafters to make loft storage space is a particular problem: each member in a truss is doing a job, and LABC has warned homeowners specifically against altering trusses without engineering advice.
Undersized or poorly supported timbers
Some roofs simply had undersized purlins or rafters, or purlins that were never propped down to an internal wall. Over decades they’ve crept and sagged under load.
Decay
Wet rot in rafter feet, wall plates or purlin ends — usually from a long-term leak at a gutter, valley or chimney — weakens the joints that hold the roof together. Woodworm can do the same, more slowly. Rot at eaves level is a classic cause of spread because it’s exactly where the tie and rafter meet. See our guide to dry rot vs wet rot and woodworm signs and treatment.
Removed chimney breasts and internal walls
Internal walls and chimney breasts sometimes carry struts or purlin loads. Remove them without providing alternative support and the roof can drop. See our guide to chimney breast removal.
Signs of roof spread and a sagging roof
From outside:
- A dipped or “hogged” ridge line — sagging in the middle, or humped over a party wall
- Roof slopes dishing between purlins or party walls, or a wavy line of tiles
- Walls leaning outwards at the top, often visible by sighting along the wall or comparing against a downpipe
- Horizontal cracking in the wall just below the eaves
- Gutters out of line or brackets pulling away
- Disturbed slates or tiles at the ridge or eaves
From inside:
- Gaps between the ceiling and the top of the wall in upstairs rooms, often along the front and back walls
- Cracks in upstairs ceilings running parallel to the external walls
- Doors upstairs out of square
In the loft:
- Rafters bowing, or rafter feet pulling away from ceiling joists or the wall plate
- Purlins sagging or not supported at their ends
- Cut or missing collars, struts or truss members
- Rot or staining at wall plates and rafter feet
Some of this is visible from the street or garden. A drone can help get a closer look at roof slopes and ridges that aren’t visible from the ground — our guide on what a drone roof survey can and can’t tell you explains its limits — and our drone surveys can be added where roof condition is a key concern.
How serious is a sagging roof?
It depends mostly on whether it’s still moving.
| Situation | Usually means |
|---|---|
| Gentle, even sag in a Victorian roof, no cracking, sound timbers | Long-term creep that has stabilised; often just monitored |
| Sag following heavier re-roofing, with spread at the eaves | Roof overloaded; may need strengthening or a lighter covering |
| Cut timbers after a loft conversion or alteration, with new cracking | Needs structural advice and evidence the work was approved |
| Rot at rafter feet or wall plate, with walls leaning | Needs repair; walls may need restraint |
| Fresh cracks, gaps widening, slates slipping | Active movement — get it looked at promptly |
Many older roofs in the region have an obvious, characterful dip that’s been there for a century and has long since stopped. The concern is a roof that is actively spreading — new cracking, gaps getting wider, slipping slates — or one where timbers are rotten or have been cut without replacement support.
Roof spread and buying a house: Level 2 or Level 3?
Roof structure is one of the areas where survey level makes a real difference.
A Level 2 survey looks at the roof from the ground and the loft hatch, and the surveyor will look into the loft where it’s safely accessible. It will flag visible signs of spread or sagging and recommend further investigation if needed.
A Level 3 survey is better suited to older houses, altered roofs, heavier re-covering, loft conversions, or any property where the roofline already looks out of true. It’s a more thorough inspection, with more detail on the likely cause and what to do about it. Our guide to which survey level you need explains the differences.
If the house has a loft conversion or a recent re-roof, ask the seller for:
- Building Regulations completion certificates for the loft conversion or re-roofing
- Structural calculations and drawings for any beams or altered timbers
- Guarantees from the roofing contractor
What does fixing roof spread involve?
Fixes depend on the cause, and should be designed by a structural engineer where structure is involved. Typical approaches include:
- Adding ties or collars to stop the rafters spreading
- Strengthening or propping purlins back to load-bearing walls
- Repairing rotten rafter feet and wall plates
- Restraining leaning walls with straps tied into the roof or floor structure
- Replacing a heavy covering with a lighter one, sometimes the simplest fix where concrete tiles overloaded a roof designed for slate
- Installing a structural ridge beam where a loft conversion removed ties
Costs depend on access, the extent of timber repairs, whether the covering has to come off, and whether walls need rebuilding or restraint. Anything involving the roof structure is likely to need Building Regulations approval.
Need an independent view?
If you’re buying a house with a sagging roofline, or you’ve noticed gaps opening upstairs and want to know if the roof is moving, we can help. A Level 3 survey is the right starting point for most purchases, and a building defects investigation suits owners with a specific concern. Contact us with the address and a photo of the roofline.
Sources and further reading
- Building regulations: re-roofing — Planning Portal
- Building regulations: work to an existing roof — Planning Portal
- Roof truss “alterations” (or why not to cut your roof trusses) — LABC
- Structural timber repairs — The Building Conservation Directory
- Repairing clay-tiled and slated roofs — The Building Conservation Directory