People Should Jump Up & Down a Bit More

I have been out at my daughter's house over the Christmas break, knocking down a few walls. Nothing like a bit of demolition to wake up the metabolism after sitting about eating and drinking too much. Her boyfriend enjoyed it as well. Taking a sledgehammer to a wall seems to appeal to boys of a certain age, usually between six and sixty.

Of course you have to take care, you shouldn't just start knocking down walls. He was interested to discover how you go about supporting the structure with Acrows when you take out a load bearing wall. Then you need to know what to put back in place of that wall. The structural engineer did his calculations based upon the information I gave him. I measured the depth of the joist and their distance apart and then the span.

By modern standards the joists were over-spanned even before we took out the wall. Even putting a steel beam back in place of the wall left the joist over-spanned because there was a breeze block wall on top which added considerably to the weight. I was about to email off the measurements when I took a floorboard up and what I then discovered made me rethink the whole thing.

Whoever put the central heating in had notched the joist to an excessive depth. The central heating was a good job but my guess is that they got the apprentice or labourer to go around lifting boards and notching joists. They gave him very little guidance.

The rule of thumb is to notch no more than one sixth of the depth and the notch should be in the first quarter of the span. This clown had taken out a 2x2inch section of every joist. It amounted to a quarter of the depth right in the middle of the span (the weakest point) very near where the block wall was. So instead of there being 8 inch joists there were effectively only 6 inch joists. Either the load had to be reduced or the span reduced by putting in more support. It was decided that the best bet would be to put in another steel beam to divide the span into 3 parts.

It was a good job I discovered it because details such as how deeply the joists have been notched will never appear on a survey but they can make quite a difference. You might think that I over-reacted to the problem, after all one sixth or one quarter is to be the difference between the floors collapsing. In all likelihood over-spanning will just make them a bit lively. You can usually tell whether joists are on their limit by things such as cracked ceilings. It is also likely to show up if you jump up and down. This is tricky to do if you are just going to view a house particularly because it might just cause a lump of plaster to become detached from the laths. This won't make you very popular but it'll tell you a lot about the house you are about to make an offer on.

You might of course be happy to have a little shake and rattle on your upstairs floors. A lot of people, even in flats can hear the light fittings rattle when people walk around upstairs but if you are putting down ceramic floor tiles for a bathroom or wet room then you cannot afford to have more than 1mm of deflection in the joists. In my daughter's case it was highly likely that with a bath full of water and somebody walking across the bathroom floor the floor tiles would have cracked due to this deflection. People are very quick to blame the tiles or even the adhesive in such cases, but the provision of a rock solid base is the first essential and it is worth spending a bit of time at this stage to avoid problems in the future.


Roger Bisby
Information for Building Projects, Renovation and DIY at SelfBuilder.net