Article - Joist Repair

Joist and beam repair help

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Summary: Repair joists and other floor timbers - using our Joist End Repair Plates, Joist Splicing Plates or Timber-Resin Splice Kits.

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BRE Joist Spacing Table

BRE Notching and Drillinf Holes in Joists


There are three main types of damage, but they are all caused by one problem - DAMP. Timber will start to decay at above 18% moisture content (in average softwood species, like an ordinary Pine), so elimination of dampness is our main concern.

1. Wet rot - does not spread beyond the area of wetness: it causes localised damage to timber

2. Dry rot - can spread well beyond the area of wetness and cause serious damage to timber

3. Insect attack - mostly in damp areas, can spread beyond, but mostly slow acting

Click to see timber decay treatments for wet rot, dry rot and woodworm attack.


Joist ends become damp because they rest inside or close to the support walls and easily absorb water from damp soil, humid air or water leaks. Any repair or treatment must include attention to the sub floor Air Bricks.

The easiest way to repair joist ends is to fit Joist End Repair Plates. These come as a pair of L shaped, metal plates complete with screws and instructions. They have the advantage that joists are extended back into or onto the original bearing, whereas repairs with bolted-on pieces of wood mean that you need a new bearing area, or a more complex solution using three pieces of wood.

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The end grain 'cut' and the support area should be thoroughly treated with Boron Ultra Gel and a piece of damp proof material, like DPC plastic, placed underneath, to prevent damp from being transferred. Use pre-treated timber if possible. Clean out the Air Bricks and fit additional to allow about one brick per 2.25 metres of outside wall. This will only work effectively if the external ground level is well below the air bricks - built-up soil, tarmac or pathways can easily prevent air flow.

Click to see joist end treatments for wet rot, dry rot and woodworm attack.


These are usually larger section timbers than the joists and may not be suitable for the Joist Repair Plate system. If larger than 10 inch x 2 inch (250mm x 50mm) then you will need to use the Timber-Resin Splice system instead. This provides a new piece of timber, with bars pre-bonded into one end, ready for you to fit using the resins provided in the kit.

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Because these tend to be thicker than joists (more than 2 inch, 50mm thick) they need additional treatment. As well as being thoroughly treated with Boron Ultra Gel they need injection with Boron Ultra Paste, via 10mm diameter holes. If permanently damp, like some embedded wall plates, such timbers may justify the insertion of Boron Rods, as well as Paste, both placed in the same holes to work together. These Boron based materials dissolve inside the wood and create a reservoir of Borate, which kills and prevents rots and insect attack.

If you need any further help with any type of floor repair or timber treatment, Property Repair Systems expert staff will be pleased to give you completely free, no obligation advice on 01626 872886 .

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