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Our primary aim is to focus on preventing water from entering the parapet wall structure. In the event that water does enter the parapet, the construction must include appropriate measures in place to discharge any water ingress from the wall and ensure it does not affect the building fabric.
This article introduces the principle of adopting a ‘3 lines of defence’ approach to creating robust and executable detailing at this critical area of building fabric.
Definitions have been used from BS 5642 - Sills, copings and cappings
The proportion of claims relating to water ingress due to failed coping detailing is extremely high compared to other claims and therefore considered a high risk element of construction that needs careful detailing on site. Parapets are more exposed to the elements, regardless of the climatic exposure of the site and hence more prone to failure.
Principal failings of coping installations continue to be:
Section 1 - The ‘3 Lines of Defence’ Principle
1.1 Introduction
1.2 Risk Strategy
Section 2 - Parapets to masonry and framed structure
2.1 Identifying and investigating failures
2.2 Defence line 1: The coping
2.3 Defence line 2: The horizontal damp proof course
2.4 Defence line 3: The cavity tray
2.5 Variations to the ‘3 Lines of Defence’
2.6 Additional Considerations for terrace guarding
Section 3 - Raking parapets to masonry and framed structure
3.1 Identifying and investigating failures
3.2 The ‘3 Lines of Defence’ principle in raking parapets
3.3 Defence line 1: The raking coping and capping
3.4 Defence line 2: The raking damp proof course
3.5 Defence line 3: The stepped cavity tray
Section 4 - Workmanship and Quality Assurance
This is a supplementary guide to the Premier Guarantee Technical Manual and relates to all ‘new build’ parapet wall constructions including flat roof, terrace and balcony parapet walls, irrespective of height.
The primary aim is to design and provide an effective construction to prevent water from entering the parapet wall. However, with such high risk types of construction, additional measures must be incorporated for warranty purposes should water enter the parapet. The additional measures should prevent water from coming into contact with the inner leaf and reaching the habitable parts / internal finishes of the building.
1.3 The ‘3 Lines of Defence’ principle
When considering the parapet wall, we are faced with a structure that is exposed to the effects of weather on both sides and at its head. Often incorporating a capping, coping or balustrade system, the designs and methods adopted to weatherproofing this critical element are placed under an increased performance requirement.
In the following detail, the ‘3 lines of defence’ principle is highlighted as three distinct areas of focus in a parapet wall construction:
2.1 Identifying and investigating failures
In the event of water ingress through parapets, invasive investigations often reveal failings in the detailing and overall execution of these critical areas of construction.
Commonly occurring issues are highlighted below, categorised under their position in the ‘3 lines of defence’ principle.
Failure of defence line 1: Copings and capping
The first image above represents a situation where the DPC and DPC support has been omitted. As a result mortar bedding is not present and the copping has detached. The second image above represents a situation where the DPC has been omitted and the capping has been attached using mastic. Adhesion has failed and the lack of mechanical fixing has resulted in detachment and water ingress.
The images below depict a project that is less than two years old. The powder coated aluminium parapet capping has been shaped to encase the base channel of the frameless glass panels. This base channel has been fixed into the structure below creating penetrations. The drainage provision has been blocked and this has diverted water into the structure below.
The second image below also shows a glazing sealant has then been applied which has also failed in service.
The third image below is the resulant invasive checks that were carried out during the investigation.
Failure of defence line 2: Horizontal DPC
Failures in the horizontal DPC are common place. In the first image below, we can identify a DPC of incorrect width, a lack of support to the DPC as it crosses the cavity and a lack of sealed joints in the DPC. The second image below shows a complete omission of the DPC, which has compounded the cavity tray issue (described below).
Failure of defence line 3: Cavity trays
The cavity tray provision is the third and final line of defence in parapet construction. The failures range from poor cavity tray joint detailing, inadequate support and incorrect or lack of adequate provision for removal of trapped from the cavity.
In the images above, the cavity tray has been incorrectly set to fall from the outer leaf to the inner leaf to avoid installing weep holes through the render. As a result, water entered the cavity and reached the building fabric below.
2.2 Defence line 1: The coping
Firstly, a coping is a construction that protects the top of a wall, balustrade, or parapet and sheds rainwater clear of the surfaces beneath. Copings can be formed using masonry or pressed metal profiles.
All parapet walls should be provided with a suitable coping construction, which:
Where masonry coping units are being used, they must:
Where horizontal masonry copings are required to be mechanically fixed by the design engineer i.e. where they are forming part of the guarding, or where they may become displaced by lateral loads or vandalism or maybe subject to high wind pressures, they should be fixed using stainless steel fixings / dowels / bracketry. An A4 grade is required in coastal locations.
Where pressed metal coping units are being used, they must:
Bi-metallic corrosion must be considered between the fixing and any pressed metal system. Consideration must be given to any aggressive environment affecting the site e.g. coastal locations, industrial zones, etc.
Aluminium coping systems must not be installed in contact with copper or its alloys, or the runoff from them – notably, attention should be given to the effect of any lightning conducting fittings attached to or in proximity to pressed metal copings.
Aluminium copings should not be bedded into mortar or concrete.
Where gaskets are part of the system jointing, manufacturers must confirm the life expectancy of the gasket achieves a minimum 15-year durability period. Any maintenance of gaskets should be relayed within any ‘Operations & Maintenance Manual’ for the property, in order to ensure the required maintenance regime is met.
Where the specified system relies on overlapping sections or joints that utilise an anti-capillary methodology e.g. drainage gaps at joints, the developer must prove and demonstrate through testing that sufficient weather tightness can be achieved.
The pull-out resistance of the fixings must be checked for wind uplift by a suitably qualified engineer. Adhesive bonding of pressed metal copings alone is not considered acceptable for warranty purposes.
2.3 Defence line 2: The horizontal damp proof course (DPC)
The DPC is positioned directly below the coping provision. It is considered as the second line of defence against water penetration into the parapet wall, guarding the building fabric and any cavities below.
Directly beneath the coping, all parapet walls should be provided with a horizontal DPC which:
Where masonry coping stones are being used, the DPC must:
Where pressed metal copings are being used, the DPC must:
In both instances, any penetrations through the horizontal DPC e.g. from coping stone fixings, balustrades, balcony guardings, must:
OR
2.4 Defence line 3: The cavity tray
Within parapet walls, cavity trays are considered the final line of defence. Their function is to intercept, and divert any water entering the cavity to a suitable external face via weep holes.
A cavity tray must be incorporated into all designs incorporating a masonry outer leaf and must always must be positioned to prevent ingress of water into the roof structure. Where the external wall has cavity insulation installed, the cavity tray should always slope down towards the outer leaf.
Where the parapet is constructed in facing masonry, including rendered masonry, a DPC cavity tray must be provided at a height of not less than 150 mm above the top surface of the adjoining flat roof, which links directly with and laps over the flashing to the roofing to provide continuity.
Where the inside face of the parapet is entirely protected by the flat roofing membrane or cladding, a DPC cavity tray must be installed as close to the coping as practically possible, draining towards the outer leaf.
All cavity trays must:
2.5 Variations to the ‘3 Lines of Defence’
Where less than three lines of defence are proposed, these should only be accepted in exceptional circumstances on the basis that the following are in place:
Situation A - 2 Lines of Defence
Where a pressed metal coping/jointing system is proposed with no horizontal DPC beneath or roofing membrane, then the coping/jointing system used must hold a valid third party accreditation (BBA or similar) which adequately demonstrates weather tightness and which is deemed acceptable to the warranty provider.
No reliance must be placed on silicone sealant in the coping system in any situation.
A cavity tray is still required to be installed in this instance. The third party accreditation must be carefully reviewed by the architect, developer and installer in order to demonstrate to the warranty surveyor that all conditions and details within the certificate are appropriately satisfied.
Situation B - 2 Lines of Defence.
Where the roofing membrane is to extend up and over the head of the parapet wall and be covered by a pressed metal coping, the fixings will penetrate the roofing membrane. As a result, penetrations should be appropriately sealed as per the membrane manufacturer’s guidance. A cavity tray is still required.
Situation C – 2 Lines of Defence
In framed construction, where cladding forms the outer surface of the wall and the roofing membrane is to extend up and over the head of the parapet which will then be covered by a pressed metal coping:
Situation D. 2 Lines of Defence.
Where framed construction is used and either side of the parapet wall is to be clad, which incorporates a drained and vented cladding:
In this situation:
2.6 Additional considerations for terrace guarding and balustrade systems
The constituent parts of the ‘3 lines of defence’ principle are only as strong as their weakest detail.
The penetrations created by structural connections and fixings into and through various layers of the parapet wall carry significant risks of water penetration.
Resultantly, the design for the guarding must be identified at an early stage and appraised to ensure the weather tightness of the parapet construction is protected and that all lines of defence are present and not compromised by fixings or other aspects of the design.
Examples of some typical approaches are provided alongside the guidance below:
Section 3 - Raking parapets to masonry and framed structure
3.1 Identifying and investigating failures
In the event of water ingress through raking parapets, invasive investigations often reveal failings in the detailing and overall execution of these critical areas of construction.
Commonly occurring issues are highlighted below, categorised under their position in the ‘3 lines of defence’ principle.
Images depict a raking parapet with stone copings. Various failings of the 3 lines of defence are present such as a horizontal DPC of incorrect width which lacks support across the cavity, a lack of mortar bedding across the full width of the wall and missing mechanical fixings leading detachment of the coping, and the omission of the cavity tray.
3.2 The ‘3 Lines of Defence’ principle in raking parapets
When considering the raking parapet wall, we are faced with a structure that is as equally exposed to the effects of weather as its horizontal counterpart, but with the added complication of constructing to a slope.
The 3 lines of defence principle highlights 3 distinct areas of focus in a parapet wall construction:
A correctly installed, weathered coping or capping with robust weatherproof connections.
A fully supported DPC, with sealed joints at laps, intersections and any penetrations.
A correctly installed stepped cavity tray, with sealed joints at laps, complete with weep holes.
This guidance document provides a detailed insight on our technical requirements for the ‘3 Lines of Defence’ principle which must be incorporated into parapet design.
3.3 Defence line 1: The raking coping or capping
Firstly, a raking coping is a construction that protects the top of a wall, or parapet and sheds rainwater clear of the surfaces beneath. Additionally, as raking structures are set to slope, any rainwater will also move downwards on the slope.
Copings can be formed using masonry or pressed metal profiles.
In some instances, a raking capping may also be used. These capping sections protect the top of a wall from water ingress but do not project beyond the walls outer surface and therefore can lead to staining of the wall beneath.
Capping arrangements are typically formed from pressed metal profiles.
All raking parapet walls should be provided with a suitable coping or capping construction, which:
Where masonry coping units are being used, they must:
Where the coping stone is porous, manufacturers may recommend a proprietary sealant is applied to the coping, this may include a primer. The coping stone manufacturers’ advice should be sought in such instances.
Where pressed metal coping or capping units are being used, they must:
Bi-metallic corrosion must be considered between the fixing and any pressed metal system. Consideration must be given to any aggressive environment affecting the site e.g. coastal locations, industrial zones, etc.
Aluminium coping or capping systems must not be installed in contact with copper or its alloys, or the runoff from them – notably, attention should be given to the effect of any lightning conducting fittings attached to or in proximity to pressed metal copings.
Aluminium copings and capping systems should not be bedded into mortar or concrete.
Where gaskets are part of the system jointing, manufacturers must confirm the life expectancy of the gasket achieves a minimum 15 year durability period. Any maintenance of gaskets should be relayed within any ‘Operations & Maintenance Manual’ for the property, in order to ensure the required maintenance regime is met.
Where the specified system relies on overlapping sections or joints that utilise an anti-capillary methodology e.g. drainage gaps at joints, the developer must prove and demonstrate through testing that sufficient weather tightness can be achieved.
The pull-out resistance of the fixings must be checked for wind uplift by a suitably qualified engineer. Adhesive bonding of pressed metal copings alone is not considered acceptable for warranty purposes.
3.4 Defence line 2: The raking DPC
The raking DPC is positioned directly below the coping provision. It is considered as the second line of defence against water penetration into the raking parapet wall, guarding the building fabric and any cavities below.
Directly beneath the coping, all raking parapet walls should be provided with a horizontal DPC which:
As an alternative to a DPC material being used, a lead detail could be adopted. In such a cases installation must be in accordance with the ‘Rolled Lead Sheet guidance’ and the guidance within Section 11 of the Technical Manual with regards the execution of flashing details.
Where masonry coping stones are being used, the DPC must:
Where pressed metal copings or capping sections are being used, the DPC must:
In both instances, any penetrations through the horizontal DPC e.g. from coping and capping sections must:
3.5 Defence line 3: The stepped cavity tray
Within parapet walls, cavity trays are considered the final line of defence. Their function is to intercept, and divert any water entering the cavity out of the fabric to a suitable external face via weep holes.
Where the external wall has cavity insulation that terminates at a horizontal axis e.g. coincidental to the insulation within an adjacent cold pitched roof, then a horizontal cavity tray will be located directly above the insulation and should slope down towards the outer leaf.
Where the external wall has insulation to the full height e.g. where the roof structure contains habitable space, then a stepped cavity tray must be incorporated into all designs. Where the parapet is constructed in facing masonry, including rendered masonry, a DPC cavity tray must be provided at a height of not less than 150 mm above the top surface of the pitch roof, which links directly with and laps over the flashing to the roofing to provide continuity.
Where the inside face of the parapet is entirely protected e.g. encapsulated in lead, a DPC cavity tray must be installed as close to the coping as practically possible, draining towards the outer leaf.
All stepped cavity trays must:
Section 4 – workmanship and quality assurance
As ever workmanship is key and the developer must demonstrate to the warranty surveyor the ‘Quality Assurance’ (QA) procedures for the installation of the copings, DPC’s and cavity trays to the parapets.
The QA procedure must prove and demonstrate the attainment of weather tightness and durability of the construction ensuring that the parapet arrangement represents a standard insurance risk. As the warranty surveyor will not be able see every parapet being completed the site manager should keep a detailed set of photographs at key risk stages (before and after each defence line stage for example) and these would need to be provided to the warranty surveyor. This should be discussed with the warranty surveyor before works on the parapets begin.
In order to reduce claims on parapet wall constructions, it is of paramount importance that all aspects of the parapet detailing are discussed at the ‘Site Risk Assessment’ stage, to ensure the design incorporates the lines of defence we require.
This article may not cover every situation you are likely to encounter on a site, however for each situation you encounter, please ensure as many of the lines of defence as discussed in this article, are incorporated into the design and correctly constructed, in order to reduce the potential of a claim occurring.
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Every care was taken to ensure the information in this article was correct at the time of publication (February 2022). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.
Ref: KM-3150