John , I know exactly where you are coming from with regard to re-furb cavity trays.
Let me tell you what I know about them and what I think of them.
Cavity trays were first introduced in 1920, by Cavity Tray Ltd. It is only over the last 25 years that they have been specified. There was a test carried out over a 4 year period from 2002 to 2006, that found that only 21% of ingress of damp was due to cavity trays. I can not find a copy of that report, though I have tried, therefore I do not know the biggest culprit of what type of cavity tray, or whether the cavities were fully filled with insulation.
Now, on our new builds, we dob the cavity trays in as we go up. but we only use either 300/450mm wide dpc, one edge built into blockwork, one edge built into face at level of top of lead apron, with weeps as required. Then to achieve our U values, we cleverly ram 100mm insulation into the cavity, to act a a giant sponge to soak up Niagara Falls as it supposedly pours down the inside of a fully insulated cavity.
Could some one tell me the genius who decided that was good building practice.
If we were pricing your job for you we would include for cutting in a cavity tray as a P.C. sum, explain our thoughts to you about the cavity tray, and if we got signed of at the end by building control without having to fit one, then we would deduct the sum allowed from final invoice.
How ever saying that our lads have got them down to a fine art. We neatly cut beds and stitch drill perps, so that existing bricks can go back in We start from both ends and the middle, so that we are cutting out12 bricks a day and replacing 8/9 a day, so for a 4m span it takes two days.
Our argument for not thinking they are required is 1.You have only got 1.200 brickwork above lead apron. 2. Normally there is a window above roof, so you do not need tray there. 3. You have got a suspect bed joint right across the opening. You can not pack with semi dry mix as you have nothing to push against, so it is a wet joint fill and slate job, which is never perfect. 4. We had a pig of a B.C.O on one job, so we built a 3 metre long 1.8m high half brick skin, fair face one side only wall. Insisted that him and the chief B.C.O came down and we played a hose pipe on the wall for 30 minutes and not a drop of water came through the wall.5. Look at your window next time it rains and see how much water is running down glass. 6. What is the answer to cav tray when you have a one brick solid wall. How do you fit one then?
Your decision.
old un.
