Parapet Cavity Wall Movement: Insights from PD 6697
Parapet cavity walls behave differently because both faces are exposed. Such dual exposure dictates protection against damp penetration, taking into consideration more variable elements than are commonly encountered. If the parapet is curved, the behaviour of the masonry itself can be more problematic if certain aspects are ignored.
Behaviour of Parapet Cavity Walls: Dual Exposure Dynamics
When a traditionally constructed parapet wall becomes saturated with rain and in extremes of heat and cold, expansion of the masonry can prejudice its structural continuity. When this occurs, the DPC bedding course in a parapet offers the most miniature line of resistance and acts as a slip plane, so movement at the DPC level in the outer and inner skins is commonly witnessed.
Identifying Movement Patterns: Impact of Saturation and Temperature Extremes
Where the amount of expansion is not fully compensated by contraction, progressive movement over the years will occur due to ratcheting. (Where the expansion movement is not fully matched by the retraction movement and the masonry above the DPC progressively moves out of alignment.) This is identified initially by witness lines and eventually by horizontal cracking along the courses where the parapet DPC has been supported.
Addressing Structural Continuity: Importance of the DPC Bedding Course
Curved parapets can accentuate the problem, especially if the curve is between two resilient structural points. The thrust direction can then move at 90º to the arc, and a radius eyebrow ledge can result. Such movement is commonly accompanied by coping dislodgement, where coping pinching occurs.
Mitigating Challenges in Curved Parapets: Strategies and Solutions
The British Standard detail depicts an outward-stepping DPC supported in the parapet wall’s inner and outer skins. This results in the entire parapet masonry above the DPC being separated from the masonry below the DPC. It is not the ideal structural arrangement for a highly exposed wall at the top of a building.
Optimising Structural Arrangements: Role of Type P Parapet Cavitray
However, by using a preformed Type P parapet cavitray that is self-supporting and requires building into one skin only, the outward stepping profile is achieved, but the opposite masonry skin can remain continuously bonded and uninterrupted. Consequently, the solidity and structural integrity of the parapet wall as a whole is considerably improved.
As with any wall, in long runs and especially when constructing a curved parapet, expansion provision is necessary in the form of expansion joints.
Historical Perspectives: Evolution of British Standards for Parapet Walls
Interestingly, if one looks back at earlier British standards, an inward-stepping DPC crossing the cavity was stipulated rather than an outward-stepping DPC. Initially, parapet walls built to that detail functioned correctly, but after a few years, cracking along the bedding course provided an entry route for penetrating water to enter.
Significance of Outward Sloping Cavity Trays: Debunking Misconceptions
The water could then track inwardly on the underside of that inward-stepping DPC. As soon as the tracking water reached the inner skin, the damp problem became visible.
Our approach was the opposite. We designed an outward-stepping arrangement: the concept subsequently appeared within the revised British Standard. However, by further developing our Type P Cavitray so it builds into one skin only, we improve structural integrity and minimise cracking opportunities. The correct parapet specification ensures a correctly protected construction in which structural continuity is enhanced and the protective tray arrangement is supported independently and steps outwardly.
How does exposure affect the behaviour of parapet cavity walls differently than standard walls?
- Dual exposure leads to more variable elements for protection against damp penetration, especially critical in curved parapets.
What are the main factors contributing to movement in parapet walls, and how does this impact structural integrity?
- Saturated masonry, extreme temperature fluctuations, and inadequate DPC bedding courses contribute to movement, compromising structural continuity and integrity.
How does the design of the DPC bedding course influence movement and potential damage in parapet walls?
- The DPC bedding course, when compromised, acts as a slip plane, leading to movement in the outer and inner skins, ultimately resulting in horizontal cracking and alignment issues.
What are the specific challenges associated with curved parapets, and how can they be addressed?
- Curved parapets exacerbate movement problems, often leading to coping dislodgement. Using self-supporting Type P parapet cavitrays can mitigate these challenges.
Why is there a difference in recommended cavity tray slopes between PD 6697 and the practice advocated by Cavity Trays Limited?
- Cavity Trays Limited recommends outward-sloping trays regardless of cavity fill status to enhance structural integrity and minimise cracking opportunities, contrary to PD 6697’s recommendations.
Important
PD 6697: 2010 states ‘While a cavity tray may direct water towards the inner or outer face, when cavity insulation is installed, the tray should always slope or step towards the outer leaf’.
We believe this notion to be misleading and have reported our concern.
The presence of cavity insulation does not remove a parapets receptiveness to crack along the DPC support beds and admit water.
We recommend trays always slope outwardly.
Never inwardly regardless of the cavity fill status.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
T: 01935 474769
F: 01935 428223
www.cavitytrays.co.uk
