Structural Components Built onto Waterproofing Systems
Following work undertaken by a Working Group of the Liquid Roofing and Waterproofing Association and Bauder's own review, this update highlights the issue of building structural components directly onto waterproofing systems and explains why the industry does not condone this practice.
The Issue
Specifiers frequently request that exposed vertical waterproofing wall flashings, which seal beneath the damp proof course (DPC) of a wall, are concealed by dressing the waterproofing at slab level.
This may involve dressing the waterproofing beneath both leaves of a wall, or beneath the exposed leaf before taking it vertically within the cavity behind the inner leaf. In effect, this results in the wall being built directly off the waterproofing system.
These requests are typically driven by aesthetic considerations where it is desirable not to see exposed waterproofing, particularly on podium decks, balconies and smaller roof terraces.
However, this approach is contrary to the technical guidance contained within British Standards, specialist trade association guidance and warranty provider requirements.
Alternative detailing solutions are available that can conceal or counterflash waterproofing upstands whilst retaining a compliant waterproofing detail. Eliminating the upstand flashing altogether is not considered an acceptable option.
Even where a waterproofing upstand is correctly dressed vertically by a minimum of 150mm above finished roof level, there remain important reasons why structural walls should not be built directly on top of a waterproofing system.
The Consequences
Whether the intention is to conceal the waterproofing flashing or to dress the waterproofing beneath one or both leaves of a wall, the following considerations apply:
- Most waterproofing systems are not designed, certified or tested to act as a damp proof course. Hot Melt systems and reinforced bitumen membranes with torch-on, self-adhesive or bitumen-bonded backings are susceptible to cold creep (a form of deformation and compression). This can reduce membrane thickness, potentially affecting waterproofing performance and causing issues for the wall construction above.
- Where waterproofing is dressed beneath one or both leaves of a wall, future access for inspection and maintenance is eliminated. Any requirement to inspect the waterproofing would result in significant disruption and cost.
- As waterproofing systems can be susceptible to cold deformation, there is a risk that a wall could move or slip horizontally. In severe circumstances this may lead to structural instability, cracking or, in extreme cases, collapse.
- If moisture enters the wall cavity it may become trapped, allowing freeze-thaw cycles to damage lower masonry courses. Whilst introducing weep holes may improve drainage, these can also create the risk of stagnant water collecting within the cavity.
The Experts' Opinion
The Liquid Roofing and Waterproofing Association (LRWA) is incorporating guidance within its Hot Melt Code of Practice relating to Structural Components Built onto Hot Melt Waterproofing Systems. Whilst focused on Hot Melt waterproofing, much of the guidance is equally applicable to other waterproofing systems that are susceptible to cold deformation.
The Brick Development Association (BDA), when consulted on the subject, commented:
"Essentially you need to have continuous waterproofing for the podium (or balcony) but avoid loading any brickwork onto the Hot Melt waterproofing system. Any compression of the waterproofing could result in the brickwork cracking."
The NHBC Standards 2021, Chapter 7.1 Flat Roofs, Terraces and Balconies, Section 7.1.17 Accessible Thresholds and Upstands states:
"Generally, where a flat roof or roof terrace abuts a wall, the waterproofing layer should extend up the wall to form a minimum 150mm upstand."
NHBC also states that the waterproofing material forming the upstand should link directly beneath a cavity tray to ensure that the cavity drains fully.
The NHBC guidance further advises:
"Designs which continue the waterproofing layer horizontally through/under the outer leaf of a cavity wall and form an upstand against the inner leaf within the cavity are not acceptable."
Chapter 7.1, Section 7.1.18 relating to parapets and guarding to terraces and balconies also includes the following relevant guidance:
"In balcony/roof terrace walls (especially long ones) the structural stability should be checked, as cavity trays and DPCs in the wall can create a slip plane that can seriously limit the ability of the wall to resist horizontal forces."
Local Authority Building Control (LABC) Warranty also published guidance relating to Podium Decks in November 2020, updated in April 2021.
Although focused on podium decks, the guidance is equally relevant to balconies and roof terraces. It reinforces the importance of linking waterproofing to the cavity tray and ensuring that waterproofing extends a minimum of 150mm above finished surface level.
The LABC guidance additionally recommends that structures, including planters, should be built from a 150mm monolithic kicker formed as part of the deck slab.
BS 6229:2018 – Flat Roofs with Continuously Supported Flexible Waterproof Coverings – Code of Practice is also referenced throughout industry guidance and continues to uphold the requirement for a vertical waterproofing upstand extending a minimum of 150mm above finished roof level, including any landscaped finish.
Bauder Policy
Bauder's position is that there is a risk of cold deformation sufficient to contribute to wall movement, slipping or cracking where walls are built directly onto Reinforced Bitumen Membrane systems (torch-on or self-adhered) or Hot Melt waterproofing systems.
For this reason, building directly onto these waterproofing systems is not recommended.
Wall flashing waterproofing should instead be carried vertically up the outer face of the wall and terminated beneath the damp proof course (DPC).