Monday 4 July 2011

Skepticism in the construction industry

Misunderstandings abound in everyday life, at home, at work, in education, in government. This is an example from my own field — construction.

Herringbone strutting is an arrangement of short lengths of timber nailed between timber floor or roof joists to reduce their likelihood of twisting as they dry out. But very few builders actually use herringbone strutting, preferring what they call solid strutting, which is usually short lengths of the same joist timber, nailed at right-angles between the joists. Strutting isn't usually needed between joists less than 2.4 metres long, but timbers that span farther than this can warp as their moisture content decreases.

Solid strutting, however, is next to useless. The short lengths of timber can be jammed in between the joists as hard as you like, but as the joists dry out they shrink, and if the solid strutting wasn't nailed in only the ceiling (usually plasterboard) would prevent it falling out. Herringbone strutting on the other hand exploits the cross section of the joists, which are deeper than they are thick, so as they dry out, the vertical shrinkage is greater than the horizontal shrinkage, and the strutting (which is fixed diagonally — with the top of each joist being strutted against the bottom of its neighbour — and vice versa) actually increases its action, applying greater pressure between the joists as the moisture content reduces.

It's only in fairly old books about construction techniques that I've seen this explanation of how herringbone strutting works. Modern instructions seem oblivious to its unique cleverness, stressing its use in spreading point loads to adjacent joists, and suggesting that solid strutting is an adequate alternative. It isn't. The only effective way of using solid strutting is to drill a hole through each joist next to the strutting and fit a tension rod with nuts at each end. The joists can then be cramped up by force to ensure they won't twist. Needless to say I've never seen this done, and when I suggest it the response is derisive laughter or plain astonishment. But then I point out that herringbone strutting would be a cheaper, easier and effective alternative.

When timber arrives on site green there's a chance it will be used in the construction while its moisture content is still above 18% — all the more reason, therefore, to insist on the proper job.

That's an example of misunderstanding how something works. I've also encountered magical thinking on building sites, in particular dowsing. Anecdotes — like misunderstandings — abound, but as far as I can tell that's all they are. Here's my anecdote.

My own encounter with a dowser was initially impressive as he accurately traced the route of drains. However, when I asked if there were drains elsewhere on the site, he confidently swept the area in question and pronounced it dry. But my own knowledge of how drains are laid led me to speculate that there were in fact drains in this area, and judicious examination of manholes, flushing of loos and turning on of taps revealed that to be the case. I concluded that the dowser was extremely good at detecting drains when he already knew where they were.

It was some time after this encounter that I learned that when dowsers are properly tested under controlled conditions, their results are no better than chance. Unlike the dowsers in the tests, I didn't find this surprising.