Sagging roofs and roof spread
A pitched roof is a triangle, and a triangle only holds its shape if the bottom is tied. Most sagging roofs are the visible result of that tie being weakened, cut or never adequate.
What does a dipped ridge line mean?
Usually that the rafter feet have spread and the roof has flattened slightly, dropping the ridge. It is the visible signature of roof spread and normally means the tie across the bottom of the roof is not doing its job.
Why a roof spreads
Rafters lean against each other at the ridge and bear on the walls at the bottom. Under load they try to flatten, which pushes the feet outwards. Something has to resist that thrust, and in most traditional roofs it is the ceiling joists, acting as a tie across the bottom of the triangle.
When the tie is effective, the roof holds its shape and the walls take only vertical load. When it is not, the rafter feet move outwards, the ridge drops in the middle, and the tops of the walls are pushed out. The classic external sign is a dipped ridge line seen along the roof from the side.
The thrust involved is substantial and it is permanent. It does not settle down or find equilibrium on its own, which is why roof spread tends to keep progressing slowly until something is done about it.
What removes the tie
Loft conversions are the most frequent cause. Ceiling joists get cut for a stairwell, or the new floor is installed without maintaining the tie action, and the roof loses the restraint it depended on. The movement may not appear for some time, which obscures the connection.
Poor connections are as damaging as cutting. A ceiling joist that is not properly fixed to the rafter foot, or one lapped over a wall without a mechanical connection, cannot carry the tension it is being asked to carry. The tie exists on the drawing and not in reality.
Reroofing with a heavier covering increases the thrust. Replacing slate with concrete tiles, which are considerably heavier, raises the load on a structure designed for something lighter, and a roof that was marginal becomes inadequate.
Putting the thrust back under control
The direct remedy is to restore the tie. New ties at rafter foot level, properly connected at both ends, put the triangle back together. Where the loft is in use and a tie at floor level is not acceptable, collars higher up can be used, though they are less efficient the higher they sit.
Where a tie cannot be accommodated at all, the thrust has to be taken by something designed for it. A ridge beam carrying the rafters removes the spreading action entirely, because the rafters then hang from the beam rather than leaning on each other. That usually means structural steel design and support down to foundations.
Restraint straps tying the wall plate to the walls, and the walls to the floors, address the consequences at the wall rather than the cause at the roof. They have their place, particularly alongside a proper tie, but strapping alone does not stop a roof that is still spreading.
What we check.
The points our calculations resolve for a project like this.
- Whether ceiling joists are present, continuous and properly connected at rafter feet
- Whether a loft conversion cut or bypassed the tie
- Weight of the current roof covering against what the structure was built for
- Amount of ridge dip and outward movement at the wall tops
- Whether a tie can be accommodated at floor level or a ridge beam is needed
Issued drawings, redacted.
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What does a dipped ridge line mean?
Usually that the rafter feet have spread and the roof has flattened slightly, dropping the ridge. It is the visible signature of roof spread and normally means the tie across the bottom of the roof is not doing its job.
Can a loft conversion cause roof spread?
Yes, and it is one of the commonest causes. Ceiling joists frequently act as the tie holding the rafter feet together, and cutting them for a staircase or a new floor removes that restraint.
Do restraint straps fix roof spread?
They help hold walls to floors and roof, but they treat the consequence rather than the cause. If the roof is still spreading, the thrust needs to be tied or carried by a ridge beam.
Is a sagging roof dangerous?
It is rarely a sudden risk, but it does not stop by itself and the walls take the thrust in the meantime. It is worth assessing rather than watching, because remedies get more involved as movement continues.
Related in structural problems & symptoms.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project