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How much does it cost to repair a sagging roof?

If you have noticed that the roof on your house has started to sag, it could be the sign you need that repairs are necessary, you may even need a new roof. The cost will depend on the extent of the rot and how much of the roof timber needs to be replaced. A fair average to budget on would be £1,000-£2,000 to repair but this could easily go up if the underlying timber is rotten or waterlogged.

Last Updated: October 2022

Whether you can repair the joist will depend on the extent of the rot and sometimes this is not always completely visible and cannot be assessed properly until work starts. A new roof can cost between £2,000 and £6,000 so it wouldn’t take much repair work before a new roof is actually a more economical option.

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What happens when a roof starts to sag?

When a roof starts to sag or dip, it means that the supporting timber structure is no longer able to do its job of bearing the weight of the roof tiles. There can be a number of reasons for this including:-

  • the underlying roof trusses or joists have become rotten after years of water ingress, incurring roof leak repair cost
  • the roofing material has been changed and replaced with something heavier such as going from slate to concrete tiles and the joists cannot bear the additional weight
  • there is a long-running problem with snow and ice due to the weather conditions; snow and ice are very heavy and will considerably add to the load on the roof and can prove the final straw for a roof which is already in trouble

The most common cause of roof sag is penetrating water which has caused rot in the roof joists and this is likely to have been going on for a number of years before the problem becomes visible. Repairing a sagging roof is possible but if the rot is extensive and depending on the cost, a whole roof replacement can be the better option. If the repairs are expensive you could be better off putting that money towards the cost of a new roof.

Older buildings and period properties are more susceptible to a sagging roof problem not just due to the age of the roof timbers but because the end of the beams tend to sit on or in the walls and this can make them even more vulnerable to damp. Once damp is present in one or two timbers, it can easily spread rot to adjacent joists within the roof space.

Regularly inspecting, maintaining and cleaning your roof can help prevent these problems. Learn more about the cost of roof moss removal and roof cleaning and coating costs to help keep your roof in shape.

How is the wood repaired?

Each timber is repaired individually one at a time if there is more than one with rot. The timber is supported structurally before the work starts and loft boarding or floorboards may need to be removed depending on where the rot is in the timber. The damaged wood is cut out and removed and a new timber section is fitted to fill the gap. The new timber is drilled with holes and a recess is created to accommodate high tensile steel bars which are inserted and then fixed firmly in place with a bonding agent. The new timber is fitted into place, attached to the current timber via the metal rods and then the remaining gap is filled with a wood resin and left to harden. The props are removed and then work can begin on the next timber. As an alternative to steel rods, metal plates can also be used to connect fresh wood to the old timber joist; these are called joist end plates and are screwed into place.

It is very important as part of the repair work to fix the problem and prevent water from continuing to enter the roof in addition to repairing the rotten timbers otherwise it is only half a job.

How to avoid your roof sagging?

Most people forget about the roof on their house unless and until there is a problem. But there are things you can do to keep your roof in a state of good order and repair including:

  • attend to any repairs – missing, cracked or chipped tiles or slates – promptly to prevent water ingress and to keep the roof weatherproof. See our page on the cost of slate roofs for more information on slate roofing and repairs.
  • Have your guttering and rainwater goods checked and cleared twice a year if not more frequently during which time the eaves and lower end of the roof can be inspected for wear and tear and other maintenance problems
  • Ask a roofing contractor to check your roof annually – some problems are simply not visible from the ground

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