I live on a boat. Strange but true. Have done so for the past seven years and although my passion is building, some of my time is spent keeping the rain out of my home.
A steel box, particularly one that isn’t finished yet, is very hard to keep watertight particularly when it’s raining hard. Picture it now. My wife and I sitting on the sofa with the TV on full volume because of the drumming of the rain on the steel cabin roof and we are getting very gently wet. We don’t need umbrellas or anything like that you understand, but the rain drumming down bounces back off the roof and some of that reaches into the underside of the ventilation mushrooms. This fine mist then settles inside the boat onto the seated occupants. Next time when it’s raining and you are sitting watching ‘the Stig’ driving wet laps on ‘Top Gear’, spare a thought for the Hill household who are enjoying the same programme but with added special effects!
People’s houses are their castles and they don’t expect them to leak. They are perhaps not as tolerant as the Hill household or maybe the fear is that the damage may be more permanent or unsightly. It’s not immediately apparent that a danger exists when an external wall suddenly becomes an internal wall. These things do not simply spring to mind. Take an example where a two storey house has a single storey extension attached on the side, a typical gable end with a garage or lounge area off to one side to make the existing accommodation larger. In these circumstances although we don’t think about it the two storey house wall becomes an internal wall when it reaches the roofline of the extension and in effect it is an internal wall for the height of the first storey.
WHAT DO WE DO THEN IN THESE CASES TO KEEP THE WATER OUT?
Externally, we all know that the new roof is flashed into the two storey house wall using a stepped flashing or (for a flat roof) a lead upstand and cover flashing. All laid in lengths not exceeding 1.5m in accordance with the Lead Association guidelines (but you knew that didn’t you?!). Having done all of that there is still a huge opportunity for water to enter the extension. From where? Well an area that you might not immediately think of but an area that still runs the full height of the building irrespective of the presence of an extension; the cavity.
It is for this reason that cavity trays or stepped cavity trays are employed on these junctions to effectively provide a barrier or dam to prevent water from falling below the extension roofline and wetting the internal masonry construction. Cavity trays are also provided with weepholes (at 1metre centres over window heads but again you knew that didn’t you?) to let the accumulated water out to run harmlessly away and not to cause a problem to anyone.
FIT YOUR CAVITY TRAYS AND KEEP YOUR CUSTOMERS DRY
I can hear you shouting now that not much water accumulates in the cavity and certainly not enough to cause a problem. Well, I disagree. I disagree in certain circumstances that is. A visit a long time ago to the Building Research Establishment convinced me that when brickwork on the outer skin of a cavity wall becomes saturated after periods of heavy rain, believe it or not the water can be seen to cascade down the inner face of the cavity in alarming quantities. So fit your cavity trays and keep your customers dry!
It’s a pain isn’t it - all that chopping out coupled with the difficulty of re-pointing the brickwork back into place without detracting from the appearance of the existing house? There are however some good products on the market to make retrospective installation easier. These days (global warming perhaps with wetter summers?) the lack of cavity trays accounts for more and more instances of water penetrating construction, so beware. If you don’t want to keep going back to check those flashings think also, about your cavity trays.
‘The Stig’ has finished now and the Hill household is mopping up until the next episode. Ah well. Life on the ocean wave and all that!
See you next time.
