Technical Note – Waterproofing External Areas

BSBG Junior Architectural Technologist Tom Dryburgh presents a Technical Note on the process of waterproofing external building areas.


External areas in a building such as roofs, balconies and terraces, have to be waterproofed to protect the structure and other building elements from damage by water. If these areas are left unprotected it could lead to serious problems affecting the structural integrity of the building and effectiveness of the building’s envelope.WaterproofFlat roofs, balconies and terraces will generally have a finish such as tiling, screed or ballast which will help prevent surface water from reaching the structure. However, these finishes by themselves will not stop surface water from penetrating the structure. Surface water can remain under the finish if not sufficiently drained, and this can then seep into the structure. A waterproof membrane is required to fully enclose and protect the structure from this.

A waterproof membrane is a sheet or coating that stops water passing through the envelope of a building. Waterproof membranes on roofs, balconies and terraces should continuously connect to other waterproof elements in a building, such as external walls, rigid foam insulation, curtain walling and other external openings to ensure the building is water tight.

Types of Waterproof Membranes

Waterproof membranes on external areas can be applied either by liquid coating or in sheet form.

Liquid Applied Waterproofing Membranes are liquid coatings that are sprayed or brushed onto the surface. The advantage of using this construction method is that it can be easily and quickly applied to any shaped surface, it can expand and contract with the structure’s surface and the coating is seamless and joint-free – so it reduces the risk of water penetration. When applying, consideration has to be taken as to the thickness of the coating, as the membrane can break if it’s too thin. The membrane must also be able to adhere to the concrete surface.Sheet Based Waterproofing Membranes are sheets of rubber or plastic that are rolled out over the surface. They overlap and are sealed at the seams by tape or liquid. Traditionally, bituminous waterproofing membranes were used to waterproof roofs, and this membrane is adhered to the surface by tar which is then heated by a blowtorch. Other common types of sheet membrane are EPDM or PVC, which can use adhesives, mechanical fasteners or hot-air to attach to the surface. The advantage of sheet based waterproofing membranes is that the quality of the waterproofing is consistent over the covered surface. After the sheet is applied, care has to be taken to avoid creating punctures.

Construction Methods

Generally, waterproofing should be applied to the external side of the structure, or on top of the insulation if applicable. The line of the waterproofing should then join to the drainage system of a building to allow for any water to be removed.When a water proof membrane reaches another building element, attention to detail is required to ensure that the membrane laps and joins with the element to maintain a continuous water tight line.

If a horizontal sheet waterproof membrane meets a vertical surface, like a parapet for example, good practice is always to place a fillet at the junction to reduce bending stresses. A chase should also be cut into the parapet 150mm above FFL for the sheet membrane to be dressed into. If the vertical surface has a corner, the membrane must be folded and overlapped.Flashings can be used at parapets and eaves to cover the waterproof membrane; refer to some typical flashing details below that are used on construction projects to protect any potential waterproofing weak points:Further Reading

BS 8000-4: 1989 – Workmanship on building sites. Code of practice for waterproofing

BS 8217: 2005 – Reinforced bitumen membranes for roofing – Code of practice

Code of practice – Specification and use of liquid waterproofing systems for roofs, balconies and walkways. Liquid Roofing and Waterproofing Association, 2010

Design-Tech: Building Science for Architects. 2nd Edition, 2014

Waterproofing of Reinforced Concrete Flat Roof

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