How to Strengthen and Replace Floor Joists


The floor joists of older houses often sag under load and sometimes the floor becomes springy. These are symptoms of defects which are often caused by:

  • overloading the floor
  • joists which are too small for their span
  • failure of strutting in single floors
  • collapsed sleeper walls
  • weakening of the joists as a result of attack by woodworm or decay.

These problems can generally be put right by strengthening the floor joists but eradi­cate any woodworm or rot first.

How to Strengthen and Replace Floor Joists  Floor Joists

Strengthening floor joists

A joist which is too small to support the loads it has to bear, or one which has been weakened – by woodworm attack, say -can be strengthened by attaching another new strong piece of timber to it. Attach the new joist to the existing one by bolts or by bolts and timber connectors. When fastening the two together, make sure that the new joist does not project above the level of the old joist – if it docs, it will prevent you replacing the floor properly. In an unboarded loft it docs not matter if the new joist projects above the existing ones. In a single floor, if the new joist projects below the existing ones, it could damage the ceiling.

Replacing sections of joist

Where ground-floor joists are supported by sleeper walls, it is usually possible to replace just those sections of joist which are damaged – remembering to replace any rotten or damaged wallplates at the same time. Use preservative-treated lim­ber of the same size, and treat cut ends with preservative.

You will have to remove the flooring and floorboards above the affected joists: the job can be made easier by sawing across the floorboards along the edges of joists adjacent to the area you want to remove. Take out enough floorboards to expose the sleeper walls on cither side of the affected section of joist.

Remove the defective section by sawing across the joist between the two adjacent wallplates. Cut the replacement joist long enough to overlap the ends of the existing joist and to rest on top of the two wall­plates. Manoeuvre the joist into position and nail it down on to the wallplates and existing joist ends.

Where the replacement section is at the end of a joist and the joist goes into the wall, wrap DPC material around the ends of the joist and, if necessary, pack it up with pieces of slate to bring it up to the level of the other joists.

Before replacing the floorboards, screw lengths of batten to the side of the two adjacent joists – flush with the top edges. The floorboards can then be nailed or screwed to these battens.

Replacing whole floor joists

On upper floors and severely damaged ground floors, it may be necessary to replace the whole length of the joists.

Remove floorboards as necessary to expose the floor joists to be replaced. With upper floors, also remove an area of the ceiling below. If you take a lot of care over replacing the joist, it may be possible to do it without removing the ceiling. But if the ceiling is constructed from plasterboard, you should expect it to suffer damage.

If joists are built into the walls, remove them by sawing through the joist at an angle this will let you extract the ends of the joist from the walls.

Cut the new joist to length – the dis­tance between the two supporting walls plus the amount of timber can be inserted into one of the holes in the walls. (This should be at least 150mm roughly 115mm of brickwork and 50mm of cavity.) Next, push one end of the new joist into one of the holes. Make sure that you push it as far as it will go. It should then be possible to insert the other end of the joist into the hole in the opposite wall. Adjust the position of the joist so that its weight (and the loads the joist will have to lake) is supported equally between both walls. Failure to do this may result in one end of the joist coming adrift from the wall.

How to Strengthen and Replace Floor Joists  Floor Joists 5

Older house often have solid walls, and for these the sliding method of inserting the joist – described above is not practi­cal. With solid walls you will have to chase out the wall above the hole to allow the joist to drop down on to the brick­work. Alternatively, you could brick up the holes and fix two joist hangers into or on to the face of the wall – then simply cut the joist to length and drop it into position on the hangers.

Finally, replace the floorboards and, if necessary, the ceiling.

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Related posts:

  1. How to Install Floor Joists
  2. How to Remove and Replace Floorboards
  3. How to Align the Floors
  4. How to Diagnosis Floor Problems
  5. How to Repair Timber Floors

Filed Under: Home & Maintenance

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About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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