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What is the cost of a replacing roof rafters?

Roof rafters are sometimes also called roof joists and they are the timber frame on which the roofing material is laid. They are usually timber although in some cases, they can be made of steel. Roof rafters run horizontally along the top of the house and connect the two opposing walls providing the base to support the roof. We would expect roof rafter replacement to cost around £275-£550 to repair one beam for a standard three-bedroom property.

Last Updated: September 2022

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What causes rafters to deteriorate?

Timber rafters, particularly in older or period properties, may have been in situ for many decades and will fall victim to weathering and rot just like any other piece of wood and despite the fact that they are substantially covered by the roof. Water is the most common enemy of roof joists so it is important to keep the roof itself in good order to ensure the supporting timbers are not compromised. Water can cause rot even in treated timber. Older buildings are more susceptible as the rafters or tie beams tend to sit on or in the walls which can make them even more vulnerable to damp. Damp and rot can then spread from one or two affected joists to other adjacent timbers.

Excess weight is another source of problems with roof rafters, perhaps a roofing material that is too heavy for the structural support beneath it. And finally, there can be damage caused by a falling tree or other objects.

If repairs or replacement are not undertaken then the efficiency of the roof itself can be affected and it will no longer continue to be weatherproof. The roof can start to sag, water will pool and potentially penetrate the rooms below. Leaks will damage the ceilings and internal decoration.

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Repairing roof joists

A good average to work to cost-wise would around £250-£500 to repair one beam for a standard three-bedroom property. The feasibility of repair depends upon the extent of the rot otherwise replacement is the only option. This is the process for the repair of a timber joist:-

  • The rotten timber is temporarily supported before the work starts
  • Loft boarding and floorboards may need to be removed to facilitate access depending on where the timber is being repaired
  • The damaged timber is cut out and removed. Some of the rot will be visible but there is bound to be more hidden underneath which is not immediately apparent
  • A new timber section is prepared by cutting timber to the requisite length and then holes are drilled into the front face of the timber along its entire length. High tensile threaded steel bars are inserted into the holes and fixed in place with a bonding agent
  • The existing clean timber is prepared by cutting a slot into it in a ‘U’ shape to accommodate the steel bars and the new timber is then put in place
  • Wood resin is mixed and poured into the space in the existing timber until the whole area is full
  • The resin is left to harden before removal of the prop and replacing any floorboards or loft boards
  • The timbers need to be assessed and repaired one at a time
  • Repairs should be made to any areas where water is leaking into the roof and which caused the rot in the first place

There is a similar repair system using metal plates to connect new wood to the old timber joist – these are called joist end plates. The rotten rafter has to be prepared just as before and the new and old wood is connected by using srew fit metal plates.

Repair or replace?

The correct route is usually determined by the extent of the decay and whatever your roofing contractor advises is the better of the two options. Replacing an entire roof will cost in the region of £2,000-£4,000 so it wouldn’t take many rotten joists at £400 or £500 a shot for repair to actually make a whole new roof better value for money. It is usually cheaper to repair a beam with timber resin materials than with metal but this is to some degree dictated by the amount of rot present.

You should also factor into your costs the work needed to resolve the source of the problem which has caused the rotten joists in the first place. A new roof might solve that as well. There is no point incurring expense repairing roof joists if water continues to penetrate. It is also perfectly possible that other problems will be uncovered once work on the roof begins so there should always be scope within the budget for any unforeseen costs.

Finding a roofing contractor

Shop around for quotes and also opinions on the best form of remedial works for your roof – different contractors may have different suggestions as to what you can do and which materials they prefer to use. Any estimate should have a clear split between labour and materials and plenty of detail about the exact works proposed.

The job itself will take around three to five days so allow a week. Complicated repairs and restoration could take longer. There will be noise and disruption but you don’t have to be at home whilst the work is carried out.

Do you need planning permission to repair rafters or replace the entire roof?

You don’t usually require planning permission to repair or even replace a roof but if you are planning to take the opportunity to alter the design, shape or height of the roof then this will usually need planning permission. If your home is listed or in a conservation area then even a new roof which conforms to the same size and type of the old roof should be checked with the local planning officer as you may need to obtain consent.

See here for more information on government building regulations and guidance from the Welsh government on building regulations for roofing projects.

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