Repair to drilled joists

[EntropyUK]

Folks I need some advice. We have had some work done by a cowboy whilst I was out of the country and he will not be repairing the problem.

Anyhow he has put the shower waste outlet through the joists of the bathroom. The joists are 170mm x 55mm and he has cut 61 mm holes in the upper third of the joists. A telephone call to a building surveyor confirmed my suspicions that the strength had been compromised especially considering the fact that these joists are also carrying a bath and the holes are aprox 300mm from the edge of the bath.

I am happy to carry out the repair as I was planning on steel plates either side of each joist which is drilled and bolted. What I wanted to know was:
1. How thick should the plates be?
2. How far either side of the hole centre should the plate run?
3. I was planning on a plate with a 'U' cut out of it to allow for the pipe work. How should the bolt holes be located.

Thanks
Ent

 

[freddiemercurystwin]

Prolly 6mm thick, holes for 10mm bolts at 300 centres staggered top and bottom, 40mm from the top and bottom edges if you get my gist, plates say 1m long. Something like that will see you OK.

 

[tony1851]

How far from the ends of the joists are the holes, and what is the overall span of the joists?
May not be as big a problem as you imagine, regardless of the bath.

 

[AronSearle]

How far from the ends of the joists are the holes, and what is the overall span of the joists?
May not be as big a problem as you imagine, regardless of the bath.

If it is in the upper third, they can only make knotches 22mm deep.

If it is in the centre they can make 43mm holes.

61mm holes are outside of the rules whichever way you cut it.

 

[tony1851]

Those are the 'deemed-to-satisfy' provisions and are regarded as safe.

But by doing calculations, it is possible to prove adequacy of bigger notches, particularly near the end of the beam.

The only issue near the end of the joist is shear stress, and that is rarely a problem in domestic floors, even under a bath. Timber joists can sustain quite high stresses for short- and medium term loadings.