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Failure of Gable Masonry Structural and Protective Measures

Author/Issuer: Cavity Tray Standards    Added:

Failure of Gable Walls: Structural and Protective Measures

The top triangle of masonry of a gable wall must resist wind suction and be properly tied to the roof trusses.  Paragraphs 2C36 and 2C37 of the Approved Document Part A of the Building Regulations refer, and depict tension straps at not less than 2 metre centres tying the top of the gable wall and also the bottom of the roof trusses. (See diagram 16).  See also BS 5628 / Code of Practice for additional reading.

Commonly cavity insulation is present within a gable wall and it is not unusual for the cavity insulation to terminate horizontally just above the plate level. Thus the gable triangle of masonry rising up to the ridge level not insulated.

Such construction requires the top of the insulating medium to be protected so it does not act as a bridge via which rain penetrating the triangle of masonry can track inwardly to the inner skin. This also applies when a cavity acoustic or fire barrier runs horizontally within a cavity – the top of the barrier must be protected. This arrangement is explained clearly within the NHBC Standards – 6.2 S8.

Incorporating a horizontal protective dpc in the conventional style to protect this detail can adversely affect the structural stability of the gable triangle of masonry.

This is because where traditional roll dpc is used, it must be built in to both the inside skin and built into the outside skin. Roll dpc requires support to both edges, without which it distort and will not hold in shape. It’s horizontal presence in the inner and outer skins means the triangle of masonry above is without the benefit of uninterrupted mortar bond. The highest area of masonry that is most vulnerable to wind suction is effectively separated – it rests on dpc.

An alternative approach can substantially eliminate such shortcomings.

Type CD Cavity Dropcloaks by Cavity Trays of Yeovil are preformed dampcourses manufactured from semi-rigid profiled dpc. They are not built into the otter skin of masonry so the solidity of bond remains uninterrupted. They are ready-shaped and require a minimum of envelopment within the inner skin. Thus the entire outer skin and the majority of the mortar bed of the inside skin remains bonded. The triangle of masonry forming the gable end is not isolated.

In timber frame construction a different version of the ready-shaped Type CD Cavity Dropcloak is available which is simply secured against the inner skin. Again the outer masonry is uninterrupted.

One should not lose focus of the intended purpose of a Dropcloak. Unlike roll dpc that requires cutting and forming on site, the Type CD Cavity Dropcloak provides a preformed solution. It is always the right shape and promotes uniform protection – as well as aiding structural continuity of bond.

Use of Type CD Cavity Dropcloaks within compliant construction can contribute to a raised standard of build and long term integrity of the building. The following provides further reading:

 BRE GBG 44 Part 2
‘Cavity trays should be provided above cavity insulation that does not extend to the top of the wall unless the associated area of external walling is protected by an impervious cladding’.

NHBC Standards 6.2 S8
‘Horizontal cavity barriers (except under eaves) should be protected with a dpc tray. The tray should have a minimum upstand of 100mm’.

Trada Wood information.  Sheet 8. Subject: Timber Frame Construction: Site Control
‘Horizontal external wall cavity barriers should be protected by a cavity tray-‘.

NHBC Standards Extra 43 (Dec 08)
Gable Walls – The Importance of Restraint.

The following article was one within a series of articles originally published in 2004.
Based on the experiences of a surveyor, it reflects on a situation encountered on a building site and the surveyor’s diagnosis.
It is reproduced below courtesy of Cavity Trays Ltd.

Is It The Isle of Wight?

‘This way Governor’. To be called Governor by Percy the foreman was apparently a good omen. He was known for his forthrightness rather than his social etiquette. One felt honoured if he adhered to calling a spade a spade rather than by any other name!

I had been asked to look at a large ongoing housing development, in particular some of the original properties that had now been standing for just over four years. The site looked good, the houses appealing, and the impression gained was one of a high standard of workmanship. This was no bland development, but a good amalgam of social and private housing with courtyards and picturesque streets winding and twisting. The four-year old cottage-style homes featured steeply pitched roofs, timber windows with walls traditionally rendered. Why I had been asked to look at them was immediately apparent.

‘The Isle of Wight is the worst one’ voiced Percy. And I saw what he meant. The render on the cottage before us had discoloured and cracked. And there it was up on the end gable in the shape of the Isle of Wight, a discoloured area some 3 metres across and 2½ metres high. Percy went on to explain he had other examples, but the Isle of Wight as he called it was the worst example of a discoloured and cracked area within any rendered wall. ‘We call that lot the Shetlands’ prompted Percy pointing to some other gables.

I checked the specification. Map cracking usually indicates shrinkage of the render itself, sometimes exasperated if a strong finishing coat is applied over a weak backing coat. But this looked different, and furthermore the Isle of Wight had a plimsoll line or ‘horizon’. Cracking and discolouration extended across the gable at approximately plate level.

An hour later with Percy’s help we had agreed what to do. ‘Better out than in’ prompted Percy, and his direct words perfectly summed up the situation. All work had been executed to a good standard, but there had been a failure to appreciate identical properties will behave in different ways – they are influenced by their exposure to the elements. All gable ends were lacking an important design consideration – but this was currently recognisable only on those in more exposed positions.

The rendered cavity walls incorporated full-fill insulation, which terminated at plate level. A dpc of sorts had been placed across the gable masonry to protect the top of the insulation, but it lacked a good defined profile and furthermore any water arrested by it had nowhere to go. The water remained trapped in the external skin and behind the render coat. But here is the $64,000 question: Should you or should you not provide an escape route for the arrested water? Alarmingly many parties say ‘No’ if it is a rendered wall, on the basis water should not penetrate in the first place! I totally disagree. Such a view is short sighted. It also assumes the render will not suffer any shrinkage, cracking or fissures throughout its life – occurrences that are actually inevitable. Failing to recognise such changes can convey a legacy of potential liability to the property owner.

There are two aspects one should consider when terminating cavity insulation across a gable wall at plate level:
The first is the incorporation of a correctly profiled dpc tray to protect the top of the insulation and arrest the penetrating water. Cavity Trays Ltd manufactures numerous styles, of which Type CD Cavity Dropcloaks and Type Q Arresting Barriers are just two. Unlike roll dpc, such preformed trays cannot sag or distort and offer consistently profiled protection. Additionally preformed trays benefit purpose-made stopends to prevent discharge at dpc tray ends.

The second consideration is the provision of an escape route for the arrested water.
In a rendered wall this can be provided using a small Beak Weep, which promotes the wall to drain rather than act as a water-containing reservoir. Such preformed weeps are hardly noticeable so do not spoil the appearance of the finished wall.

Water penetrating the external skin of a cavity wall does not discriminate.

It must be prevented from using any feature within a cavity (such as the top of cavity insulation) as a bridge across which it can travel and reach the inside skin.  It must also be prevented from pooling and causing retentive damage of the external skin.

Always provide an exit route for penetrating water. In time, severe wet weather conditions can permeate the best of finishes. Discolouration through water retention, cracking through expansion and contraction, and spalling following freezing conditions can otherwise occur.

Think of a rendered wall when new, and ask yourself if that same rendered wall will be impervious to rain penetration and without any cracks or any fissures or any shrinkage in fifty years time?

I understand Percy always has the last word, and I discovered this to be the case a month later when I received a postcard. Percy was on holiday. The postcard was from the Isle of Wight and bore the words: ‘Governor, cracking place but currently wet’.


© Cavity Trays Ltd 2009.


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