A traditional pitched roof works as a series of triangles. The rafters lean against each other at the ridge, and their feet would slide outwards if nothing stopped them. What stops them is a tie across the bottom of the triangle - usually the ceiling joists, nailed to the rafter feet - and sometimes collars higher up. When that tie is missing, weak or disconnected, the rafters push the tops of the walls outwards. That is roof spread.
It is common in older Yorkshire housing, particularly stone terraces and cottages with heavy stone slate roofs, and in houses where the roof structure has been altered.
What causes it
- Ceiling joists that were never properly fixed to the rafter feet, or that run the other way and so tie nothing
- Collars or ties removed to make space in the loft - in a loft conversion done without design, or just for storage
- A heavier roof covering than the structure was built for - concrete tiles in place of slate are a common example
- Purlins that are undersized, overspanned or poorly supported, so the rafters sag and push
- Decay at the wall plate or rafter feet, often from a long-standing leak at the eaves or a blocked gutter
- In stone buildings, walls that were never stiff enough at the top to resist the thrust
What it looks like
- The top of a wall bowing outwards at the eaves, most visible when you sight along the elevation from a corner
- Horizontal cracking near the top of the front or rear wall, a course or two below the eaves
- A ridge line that dips in the middle, or roof slopes that look dished rather than flat
- Gaps opening between the ceiling and the top of the wall inside upstairs rooms
- In the loft, rafter feet that have moved off the wall plate, or ceiling joists pulled away from rafters
A dipping ridge on an old roof is not always active spread. Many roofs settled into a shape decades ago and have not moved since. The question, as with most structural movement, is whether it is historic and stable or still going on.
Re-roofing and Building Regulations
Replacing a roof covering like for like is generally not notifiable. Changing to a significantly heavier covering is different: the roof structure has to be shown to be capable of carrying it, and Building Control will expect that to be checked. Re-covering is also the moment when spread caused by a previous heavier covering often shows itself, so it is worth having the structure looked at before the new covering goes on.
What you can check yourself
- Sight along the eaves and the ridge from each end. Note any bow or dip and photograph it from a fixed point
- Look in the loft for ties and collars that have been cut, removed or never fixed, and for rafter feet that have moved
- Check whether the ceiling joists are nailed to the rafters at each pair, or run parallel to the ridge
- Look for damp and decay at the eaves, and keep gutters clear
- Note whether any cracks under the eaves are the same width at each end and whether they change over months
How it is repaired, when it needs repairing
If the spread is historic and stable, the usual answer is to leave the structure alone and repair finishes. If it is active, the triangle has to be closed again: new or reinstated ties and collars properly fixed to the rafters, steel ties or straps across the roof, better support for the purlins, and restraint straps tying the wall plate and the tops of the walls to the roof structure. Where a wall has been pushed well out of line, the wall itself may need attention.
In a loft conversion, ties and collars cannot simply be put back where they were, because that is the space being converted. The new floor, the purlins and often steel beams are designed to take over the job, which is why a loft conversion always needs structural design.
When to call an engineer
A threshold, not a plea: get it looked at if the top of a wall is visibly bowing, if cracks under the eaves are growing, if ties or collars have been removed, if you are buying a house with a dished roof and the surveyor has referred it, or before you re-roof with a heavier covering.
How we would investigate it
An inspection outside and in the loft, measurement of how far the walls are out of plumb at the eaves, a check of ties, collars, purlins and bearings, and a written opinion on whether the movement is historic or active and what, if anything, needs doing. Where repair is justified we design and specify it; we do not carry it out and take no commission from anyone who does.
A structural survey and report is a fixed £580 + VAT, with the report inside 10 working days of the inspection, guaranteed.
Frequently asked questions
Is a sagging roof dangerous?
Often not. Many old roofs settled into a dished shape long ago and have been stable since. A sagging roof needs looking at if it is getting worse, if the walls at the eaves are being pushed out, or if timbers are decayed or have been cut.
What is the difference between roof spread and a sagging ridge?
Roof spread is the rafters pushing the tops of the walls outwards because the bottom of the roof triangle is not tied. A sagging ridge is often the visible result, but it can also come from undersized or poorly supported purlins without much spread at the eaves.
Can I replace slates with concrete tiles?
Concrete tiles can be considerably heavier than the slates they replace. Changing to a significantly heavier covering means the roof structure has to be shown to carry it, and Building Control will expect that check. Ask for it before the old roof comes off.
Does a loft conversion cause roof spread?
It can, if ties or collars are removed without the new structure being designed to take over their job. A properly designed loft conversion replaces their function with the new floor, purlin support and often steel beams.
Do you carry out roof repairs?
No. We inspect, diagnose, and design and specify repairs where they are justified. We do not carry out repairs and take no commission from anyone who does.