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.
Ř Study the presented text on the
following pages.
Ř at the end of the study are the
multiple-choice questions.
Ř 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.
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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.
Sponsor’s note:
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.
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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.
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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 future
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:
q Through the cellar / basement
q Through the roof
q Over openings in cavity walls
q 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:
q 2
q 5
q 8
q 10
3)
Which British Standard illustrates the use of
cavity trays?
q BS1149
q BS5628-3
q BS12588
q BS 5977
4)
Cavity Trays have been around since which decade?
q 1920
q 1950
q 1970
q 1980
5)
Which type of tray is
designed for use on stepped and staggered gable abutments?
q Type X
q Type C
q Type K
q Type P
What stops water in a cavitray for an existing wall
from leaking where trays join?
q Trays are overlapped
q Trays are stuck together
q Trays are self-contained units with ends
and clip together.
q Ends butted together
Post to the
Sponsors:
Cavity Trays Ltd
FREEPOST Administration Centre Yeovil Somerset BA22 8HU
