Type to search


Cavity Walls, Efficiency and The Milk Bottle Conundrum


‘Achieving a profit on any site is all about optimising the amount of work executed every working day’ announced a site agent, ‘and on this site the contract restrictions prohibit weekend working so it’s a tight schedule. It’s like filling up a milk bottle. With just five working days every week, each day’s output fills the milk bottle a little more. Monday, Tuesday Wednesday Thursday…. So, by the time you reach Friday afternoon the bottle is filling to the top. And at the top you find the cream – or to put it another way, the profit. The trouble is, if through circumstances we do not deliver sufficient output every day – we don’t reach the profit level at all. That cream is quite elusive and more often or not these days it’s also semi-skimmed!’   

The site agent had a point. The contractor in question normally returns a steady 5% profit on contracts. But, put into perspective, 5% is just 2/40ths. In a simplistic but sobering way of portraying the obvious, it can be the last two hours in a forty-hour working week that produces the return necessary for financial contentment – and even that’s subject to the weather permitting.

Preformed building components can help site operatives advance the site work programme more speedily. In essence, why attempt to undertake a labour-intensive operation on site if there is an alternative method that additionally ensures a determined quality- controlled detail? And achieves it more cost effectively? Not one but two important gains. There is a strong case for adopting a preformed approach compared with fabricating on site. However, the case is undisputed when comparisons are made with site fabrications that have gone wrong:



DPC used in the cavity is mis-shaped, despite having been built into the inner leaf. Unacceptable relationship between flashing and DPC.






Preformed Answer:         

Type X cavitray. Cannot sag, is self-supporting and protects every course. Flashing also ready attached. Ready to build-in whilst the wall is raised. Suitable for traditional and timber frame construction. Adjustable cavity upstand takes up actual cavity encountered.







DPC is dry bedded and the relationship with the flashing is inadequate. Dampness permeates this unsatisfactory link and is further aided by capillary action.






Preformed Answer:     

Cavitrays with attached lead flashings for horizontal and sloping abutments. The flashing in this horizontal application is already factory bonded to the tray to provide a perfect union. The base of the tray incorporates mouldings to self-gauge mortar bedding. Note how adjoining trays link up to create long runs.






An example of water retention causing staining and eventually spalling of the masonry. The shape of the DPC is suspect and water is trapped within the wall because there are no defined exit routes.





Pre-shaped Cavitrays ensure correct profile and functionality. Preformed dual function caviweeps/cavivents promote water discharge whilst encouraging the cavity to breathe.






Quality of build on site is dictated by the operative with the least ability. But what about the exceedingly competent operative who makes an occasional mistake through a moments lack of concentration?  It can happen and suggests the most proficient of tradesmen can benefit with use of ready-to-use components. A mistake can be out of sight and lay undiscovered until damp penetration problems arise at some time in the future. The preforming of damp control and protection cavitrays is an assured way forward and assists the contractor by speeding up operations on site. If the contractor needs further convincing, he possibly should take a closer look at that milk bottle!

Cavity Trays has published a 2020 Year Wall Planner to accompany its latest Technical Manual.

Both available without charge to readers emailing enquiries@cavitytrays.co.uk  and quoting

“Planner + Manual”.    Limited availability.


Read previous post:
Zumtobel Provide the Perfect Light for Surrey Sports Park

When Surrey Sports Park, one of Europe's premier sites for sport, health, wellbeing and leisure, needed an upgrade to its...