Repairs: floors and flooring

Problem 3: Over-cut joists

Notching of the joists to accommodate central heating pipes or electrical cable runs is a further cause of joist bending and deflection. Sometimes this can be repaired without resort to joist doubling. But often the joists have been butchered and the notches are too large or in the wrong place and will affect the load-bearing capacity of the joist or beam.

Guidance is set out in Span tables for solid timber floors, ceiling and roofs (especially trussed rafter roofs) for dwellings, (cited in Building Regulations Approved Document A) and Timber joist and deck floors – avoiding movement, Information Sheet 1/36, both published by TRADA.

  • For joists up to 250mm the maximum hole should be a quarter of the joist depth and holes should only be drilled in the joist between 0.25 and 0.4 times the span of the joist.
  • Where the top edge of the joist is notched, the notching should only be carried out in a zone starting at 0.07 times the span, up to 0.25 times the span and to a maximum depth of 1/8 of the joist. Holes and notches should not be overlapped and should be kept at least 100mm apart.

Solution

Assess the amount of any bending that has already occurred. Often it will not be possible to rectify any sagging to the floor – only to reinforce the structure by way of damage limitation.

This can be achieved by bolting a new joist of approximately similar depth alongside. It is 'approximately similar' because if there is a lath and plaster ceiling, for example, then the plaster that has oozed through the timber lathing will make it very hard to set a new full-depth timber snugly alongside unless the ceiling is completely taken down first – which is of course an option.

Hopefully the original joist end bearing is sound and it should be a simple matter to 'plate' the defective timber by bolting the new piece alongside (as shown in the sketch that accompanies Problem 1, Solution 2).

At least 3 or 4 bolts are needed in this situation, and to spread loadings adequately across the notched part a new piece of at least 1.5m length is preferable – more probably.

The new joist will need to be notched too but less drastically, or if you are careful the hole drilled in the null load section of the joist may be usable, depending on how badly the old timber has been cut away.

When plating up old joists or doubling them up it is sensible to insert nogging pieces to help stiffen the structure and provide adequate ‘blocking’ to help resist twisting as the timbers are put under load.

Flitch beams

An alternative to plating up the joist is to construct a flitch beam. This incorporates a steel plate sandwiched between 2 conventional timber joists, which are bolted through typically at 300–400mm centres or less.

Although constructing a flitch beam is a straightforward operation, tolerances can be fine and difficult to prove using conventional rules of thumb. So, in critical applications it is advisable to have the loading values and stiffness of the beam checked or specified by a structural engineer.

Repair to excessively notched joists