CPD: Dealing with Damp
Author/Issuer: Cavity Tray Standards Added:
Welcome to this CPD section, where we help you gain continuing professional development credits.
This particular study deals with how preformed damp proof courses prevent water penetration in cavity walls.
An investigation into cavity trays, preformed damp courses and dpc’s that arrest water ingress in cavity walls, permits readers to gain study credit through distance learning.
- Study the presented text on the following pages.
- At the end of the study are the multiple-choice questions.
- print off the questions page, write in your answers, sign and date, then freepost to the sponsors (address at the foot of the questions page) for assessment.
if the answers on your test paper are correct, you will receive a study credit of half an hour. such credits are issued by the company sponsoring the study and will be despatched to your postal address within 28 days of being received. his study is open to all readers. we will not enter into correspondence or take phone calls in connection with this service.
In 1920 a West Country family of builders started making their own damp courses, flashings and cavitrays. The fourth generation of the same family continue the tradition. The Company is now called Cavity Trays Ltd and can claim more experience, more case histories and more know-how than any other company in this specialised field.
Approved cavitrays are manufactured to a quality management system conforming with ISO 9001. Cavity Trays Ltd is the only tray manufacturer awarded European Technical Approval. In addition the Company holds LABC Local Authority Building Control Product Type Approval. Approved cavitrays are accompanied with a product performance liability protection for the benefit of Architect, Builder and Client.
THE EXTERNAL WALLS, FLOORS AND ROOF OF A BUILDING SHALL PROVIDE ADEQUATE RESISTANCE TO THE PASSAGE OF RAIN AND SNOW INTO THE ENVELOPE OR THE INSIDE OF A BUILDING.
What are cavity trays?
Cavity trays or cavitrays as they are sometimes known are preformed damp proof courses manufactured in all shapes and sizes to exactly fit and protect specific applications in cavity walls.
At one time damp proof course was supplied only in rolls. Whilst this is easily laid horizontally within masonry, it is difficult to use where a two or three-dimensioned approach is necessary. It is incapable of being cut and formed on site to create (and thus protect) intricate shapes and profiles. The number of failures attributable to DPC being incorrectly, inconsistently or inadequately formed on site still remains alarmingly high.
Fortunately the trend of using preformed cavity trays is growing, eliminating the commonplace failures whilst providing impressive accompanying benefits.
Cavity trays or cavitrays exactly fit and protect specific applications in cavity walls. The use of a ready-shaped three-dimensional cavity tray by definition provides the perfectly matching protective accompaniment. Their use ensures a correct and consistently formed medium is enveloped within the structure to provide long-term damp protection integrity of the building envelope. The use of trays can eliminate errors that still occur regularly on sites throughout the United Kingdom*. Research by Cavity Trays Ltd support such findings. Cavity Trays examined cases over a 50-month period ending 31st March 2006. One of the company’s findings was that 21% of damp ingress problems take place over openings in cavity walls.
*See 50 month survey ending 31st March 2006 carried out by Cavity Trays Ltd and published within manual 17.
It provides data showing where and why typical failures occur.
What happens if you don’t’ use trays?
If one constructs without using cavity trays one places total reliance on site operatives successfully and consistently fabricating using conventional flat material. One is exposed to greater risk and greater costs. And to state the obvious, one is placing dependence on it being feasible to site fabricate intricate shapes and profiles to exact dimensions and for the operative to achieve perfect results – every time.
In reality100% accuracy is rarely possible. Errors occur, materials sag, crack or puncture, shapes fail to form or unite uniformly, and theory that appears so straightforward on the drawing board proves an impossible task to achieve on site.
The most simple and straightforward example of a preformed cavitray is with a lintel over a typical cavity wall opening. The preformed shape of the cavitray ensures perfect harmony with the lintel – it fits like a glove so compatibility is never an issue. When the cavitray is of the self-supporting style, it means the cavity upstand does not need to be returned into the inner leaf. It also means a biased self-adjusting cavity upstand will automatically take-up whatever cavity width is encountered – again ensuring compatibility. Use of a preformed tray also means wastage can be eliminated, and all stock accurately controlled.
Available in any profile to harmonize with all styles of lintel, the Type C cannot sag, distort or be misplaced. The preformed profile extends beyond the lintel ends and accompanying Type L stopends prevent water discharging into the cavity. Instead water is evacuated out of the structure via caviweeps. The NHBC refers to dpc’s over common openings having an external drip, which is difficult if using conventional roll material.
When one studies the more involved applications such as gable abutments and changes in level, the use of preformed tray units transforms both the dpc and flashing undertaking. The risks and high costs of site fabrication are banished and installation time is considerably reduced.
What did builders use before we had them?
A lot of DPC, a lot of labour and a lot of lead flashing – often accompanied with a fingers-crossed approach!
A family of builders from the West Country designed the original cavity tray. They invented the term ‘cavity tray’ that has become a generic term within the industry. Despite the family producing trays since the 1920’s, it is only in the past twenty- five years that widespread use of conventional dpc material has started being abandoned in favour of using preformed cavity trays.
This position is put into perspective if one looks at some old British Standards within which was illustrated a traditional dpc method of protecting a stepped gable abutment. The detail was technically incorrect as well as being exceedingly difficult to fabricate! At the same time the quality of work and the standard of detail witnessed on sites was poor, varying from inadequate to passable whilst the functionality in the long term remained very questionable. Failures were common.
The current position is completely changed. The latest review of BS 5628-3 published at the end of 2005 shows preformed cavity trays with known and assured functionality. This revised British Standard has helped raised awareness and site build quality has risen.
Are cavity trays in any sense required by regulation?
The Building Regulations stipulate those parts of the structure that must be protected from dampness and BS 5628 and BS 8000 highlight the use of trays to achieve robust functional details. Both the NHBC and Zurich Insurances illustrate cavity trays within their technical detail manuals to achieve regulation compliance. It is generally accepted that constructing using preformed trays can elevate and stabilize build quality.
Paradoxically there is currently no minimum standard to which a cavity tray must be manufactured so actual tray quality can vary considerably from manufacturer to manufacturer. Petheleyne is favoured as the material in which trays are produced, as it promotes a long and durable service life. Milled lead flashing to BS EN 12588 is the preferred flashing medium. Of critical importance in trays with attached lead flashings is the actual union between tray and flashing. In good examples the union is secured and compressed within a jaw and then stitched using non-corroding austenitic stainless steel fastenings.
Only one tray manufacturer has been awarded European Technical Approval, which must be favoured by anyone operating within Europe and wishing to secure the best-evaluated products. Interestingly it is the same manufacturer – Cavity Trays Ltd holding European Technical Approval that also accompanies its products with product performance liability protection. Given the general liability exposure of architects and contractors, this commands beneficial significance.
Solutions available today.
There are now preformed cavitrays to suit practically every application imaginable.
From horizontal openings, curved openings and circular openings in cavity walls, to sloping and horizontal gable abutments, changes of level, parapet walls and diaphragm walls. Some cavitrays are specially designed to protect one skin of a cavity wall without actually being built into it. Thus one can preserve the structural bonding, an important but often overlooked aspect when detailing dentils, strings, or changes of skin plane within a cavity. Cavity Trays Ltd holds in excess of 1600 cavitray designs and furthermore undertakes to produce non-standard and bespoke solutions in addition.
Examples of modern cavitray system.
The stepped and staggered gable abutment is an excellent example to highlight the benefits of using preformed cavitrays. The exterior skin of masonry changes status and below the roofline becomes an inside skin of the abutting property. Thus it is necessary to arrest all penetrating dampness before it permeates below the roofline.
This cavitray has a hinged upstand that adjusts to suit whatever cavity width is encountered. The attached lead flashing matches the pitch of the roof and requires only dressing after installation. Suitable for new work build and existing structures, the Type X can be used to satisfy the NHBC demands for trays to be inserted where a conservatory abuts a new home.
The product illustrated within the new British Standard consists of a moulded dpc tray, to which is attached a ready-shaped lead flashing. The mason lays one tray per course, whilst raising the exterior skin. Trays offering self-supporting cavity upstands adjust to suit whatever cavity width is encountered, so do not need building in to the inside leaf. Thus installation is quick and easy and equally at home in traditional build or timber frame construction. Such trays also eliminate any problems of pigs or quarks. (These are regional terms to describe a deviance in level, where inside and outside skin levels drift apart. Often correction of such deviances can be witnessed at plate level when buttering-up of the plate and/or an additional course is introduced to bring both skins to an aligned level). A cavity tray with a self-supporting and fully adjustable cavity upstand is unaffected if levels drift apart.
Every tray has an end upstand that links with the next tray at higher level. Thus one is creating the dpc equivalent of a staircase, with connecting treads and connecting risers. Water cascades from one tray to another, but cannot soak inwardly (into the roof space) because of the end upstands.
The tray adjustable cavity upstands act in a similar manner, preventing water tracking towards the inside skin. The cavity upstands and tray body also receive water that drips after tracking partially across a wall tie. At the bottom of every tray slope is a catchment tray that discharges water out of the structure via a caviweep.
Gable abutment cavity trays are supplied with either long leads to dress directly over profiled tiles or with short leads to dress over the upstand of a secret gutter or soaker. Once the mason has built in the trays (which are flush pointed as the wall is raised) all that remains is the dressing (at a later date) of the lead flashing.
The Type E is for insertion into existing walls and requires only one course of masonry to be removed. (Picture shows larger area for clarity). It is required whenever an extension is added to an existing building. It is also suitable for remedial applications. Each tray is technically a self-contained unit with end upstands that coincide with the perp joints. Trays clip together as insertion progresses to create a long run. Water discharge is via a caviweep in each tray. Tray upstands adjust to suit the cavity width encountered.
The current position
Some manufacturers carry out cavity tray identification and scheduling. This removes from the architect and the contractor the burden to design the important intricate measures necessary to protect the building envelope. Schedules are prepared with each tray application clearly defined. Supply is tailored to suit the build programme and it is not uncommon for individual plots or in the case of multi-occupancy developments, floors, to be delivered on an almost daily basis. This obviously appeals considerably to any contractor as bulk storage is no longer demanded on site. The real benefits however are apparent when it comes to installing the trays, as the dpc installation compliance is transformed into a scheduled and controlled process, at the end of which can be a warranted and defined build detail.
Type K cavitray provides 360 degree protection of frame and prevents damp from passing from outside to inside skin. An example of a three-dimensional preformed cavitray that would be impossible to form on site using conventional damp proofing material.
The cost of rectifying a typical damp penetration problem is typically up to five times more expensive than the cost of executing the detail correctly in the first instance. Where a cavity tray system has been used on a project the total benefits are usually so apparent that continuation of tray use on subsequent contracts is usual.
In comparison, some traditionalists who have the craftsmen to fabricate on site will continue to do so, until perhaps such time their costs become so prohibitive or a major failure is experienced? It is clear the use of preformed trays has transformed the industry, and offers architect builder and client some excellent design options.
CPD Test Paper 1006
Water Ingress and Cavity Walls.
Following reading of the module, please print this page and answer the questions to demonstrate adequate and sufficient CPD study.
1) Research suggests the majority of damp penetration problems take place:
- Through the cellar / basement?
- Through the roof?
- Over openings in cavity walls?
- Through defective mortar?
2) The cost of rectifying damp penetration problems is up to how many times more expensive than building correctly in the first place:
3) Which British Standard illustrates the use of cavity trays?
- BS 5977
4) Cavity Trays have been around since which decade?
5)Which type of tray is designed for use on stepped and staggered gable abutments?
- Type X?
- Type C?
- Type K?
- Type P?
6)What stops water in a cavitray for an existing wall from leaking where trays join?
- Trays are overlapped?
- Trays are stuck together Trays are self-contained units with ends and clip together?
- Ends butted together?
Post to the Sponsors:
Cavity Trays Ltd