The Six Ds of Decking: Waterproofing flat roof assemblies

All images courtesy Duradek
All images courtesy Duradek

By John Ogilvie
When waterproofing a walkable roof deck, there are a number of important principles to consider in determining the ultimate performance of not only the decking assembly, but also the waterproofing and the overall building envelope. These can be thought of as the Six Ds:

● deflection;
● drainage;
● drying;
● durability;
● detailing; and
● deconstruction.

Each plays a key role in the performance of the deck, waterproofing system, and building envelope.

Deflection refers to a deck’s ability to control surface water. This is affected by numerous factors, including slope, drip edges, and diverters. In multi-storey buildings where decks or balconies are stacked, the structure below will benefit from the overhang provided by the balcony above.

When a deck is correctly designed, water will flow where it is supposed to go—that is, away from the building into a gutter system or over a drip edge away from the exterior wall system. A proper deck slope of ¼:12 (i.e. two percent) helps reduce the possibility of ponding water, even with building shrinkage or settlement.

‘False fascia’ details can be used to hide gutter, fascia, railing mounts, and deck slope. Given the proper slope, a simple drip edge detail is typically the most expedient way of removing surface water from the deck surface.


Once the deck has been designed with the proper slope, it is critical water be deflected away from deck transitions at adjoining walls. Even a rainscreen wall system would have difficulty handling the volume of water flowing from a deck surface that is not properly diverted.

Simple pre-manufactured diverters can be installed on the deck surface and waterproofed to easily deflect water away from any critical areas. Figure 1 shows three simple, but effective, cricket and flap water diverters. Figure 2 depicts how severe structural damage can occur when water is not properly diverted.

To help ensure success, walkable patio decks such as this one should be treated as roofi ng assemblies with respect to waterproofi ng.
To help ensure success, walkable patio decks such as this one should be treated as roofing assemblies with respect to waterproofing.

Drainage refers to the building assembly’s ability to redirect any water that enters the building envelope and allows it to drain to the outside. Drainage depends on the proper sequencing of moisture barriers and detailing of wall openings and posts. Proper wrapping of rough door opening jambs and sloping of door sill framing members will help deflect any trapped moisture to the outside.

Flashing (Figure 3) is a critical component of the deck’s waterproofing system and must be correctly integrated with the polyvinyl chloride (PVC) waterproofing membrane. Flashing may be used to tie a new PVC membrane into the existing wall membrane and wall cladding. Alternately, the PVC membrane could be brought up the existing wall and covered with metal reglet flashing or through-wall flashing as required.

Curbs or knee walls greater than 75 mm (3 in.) in height, or higher than any wall opening, result in a ‘trapped’ deck system that requires special drainage. For these cases, it is important to ensure the top plate of the curb or knee wall slopes back toward the deck surface (at about 1:6). Additionally, the membrane should extend to the line of the outside drip edge, which should extend well out from the building wall surface.

Incorrect detailing of flashing elements will lead to leaks and water damage. Proper sequencing of moisture barriers is also critical (Figure 4). For example, on a knee and pony wall, a peel and stick capping membrane should overlap two layers of building paper, which in turn overlay the PVC membrane. Installing the PVC membrane after the wall assembly has been completed will almost always lead to failure. It is also recommended the membrane extend a minimum of 150 mm (6 in.) up the vertical surface, behind the water shedding membrane.

A deck assembly’s durability is greatly affected by the waterproofi ng membrane’s selection and then overall maintenance.
A deck assembly’s durability is greatly affected by the waterproofing membrane’s selection and then overall maintenance.

Posts are another deck element where proper construction and sequencing of moisture barriers is essential. To waterproof posts, they must be covered with a moisture barrier and then finished with siding. For applications where the dramatic effect of a rough-hewn wood post is required, special consideration must be made for post attachment. Similarly, all curbs, knee walls, posts, and wall openings must be constructed and detailed properly to ensure the membrane’s performance.

Whether modifying an existing door opening or constructing a new door opening, the sill should be sloped toward the deck surface. The waterproof membrane should continue into and waterproof the rough door opening saddle in order to deflect any moisture intrusion.

In addition to a properly sized deck drain(s), building authorities may require overflow drains or scupper boxes on trapped decks to provide an additional path for water to drain from the deck. Acrylonitrile butadiene styrene (ABS) shower drains are typically not suitable for any roof deck or multi-family building applications. Only PVC-coated drains, or those with a positive clamping system, should be used to allow water to drain into the building’s stormwater system. With PVC-coated overflow devices, the decking membrane can be welded directly to the surface without caulking.

Drains and scuppers should be sloped toward the outside, and scuppers should be let into the deck surface so as not to allow ponding to occur (Figure 5). Generally, scuppers are very difficult to detail in such a way as to make them truly impervious to wind, rain, snow, and structural deflection.

With multi-family projects where space is at a premium, walkable roof decks can be especially important.
With multi-family projects where space is at a premium, walkable roof decks can be especially important.

This aspect of the Six Ds refers to any features of the building assembly that speed the drying of materials that have been exposed to moisture. Drying can be aided by the use of a rain screen wall assembly or by the venting of trapped air spaces. Venting of the air space underneath decks, balconies, and walkways with a closed soffit or ceiling is especially important for adequate drying.

If the underside of a roof deck, balcony, or walkway has a closed soffit or ceiling, venting is required to reduce moisture buildup and prevent mould, mildew, and rot. The designer should follow local building codes for venting requirements.

Durability refers to assemblies and materials tolerant of foot traffic, moisture, mildew, chemicals, and environmental conditions. The assembly’s durability will be affected by the selected waterproofing membrane. Durability will also be affected by the overall maintenance of the membrane.

In terms of durability, modern waterproof PVC membranes can be expected to last for 15 to 20 years, depending on the environment, ultraviolet (UV) exposure, service use, and maintenance. The quality of the ingredients can change the price of a similar-looking membrane by 25 per cent, and the life expectancy by 75 per cent.

Figure 6 shows accelerated testing samples (per ASTM G90, Standard Practice for Performing Accelerated Outdoor Weathering of Nonmetallic Materials Using Concentrated Natural Sunlight) of two different materials with 840 MJ/m2 UV—an amount equal to three years of Florida sun exposure. The testing samples have test strips on the left (exposed) and control strips on the right (unexposed).

When deemed required, roof membranes are tested in accordance with nationally recognized standards, such as Underwriters Laboratories of Canada (ULC) S107, Methods of Fire Tests of Roof Coverings, as per the National Building Code of Canada (NBC), for Classes A or B.

A new aspect of sustainable design is to consider the potential for deconstruction. A walkable deck and railing assembly that can eventually be dismantled and used anew will reduce environmental impact.
A new aspect of sustainable design is to consider the potential for deconstruction. A walkable deck and railing assembly that can eventually be dismantled and used anew will reduce environmental impact.

PVC roof and walking deck membranes must conform to the same material standards as PVC roofing and waterproofing membranes. In other words, walking surfaces in these applications must meet the same material standards as other roofing membranes. NBC recognizes Canadian General Standards Board (CAN/CGSB) 37.54, Polyvinyl Chloride Roofing and Waterproofing Membrane, to act as a basis for evaluating PVC membranes used on decking subject to pedestrian traffic.

PVC roofing membranes specifically designed to sustain pedestrian traffic can be accepted as long as they demonstrate compliance with the performance features of the applicable code. In Canada, PVC sheet-applied waterproof decking membranes must comply with Canada Mortgage and Housing Corporation’s (CMHC’s) Canadian Construction Materials Centre (CCMC) technical guide, falling under MasterFormat Section 07 54 19.01−Polyvinyl-chloride Roofing.

In terms of quality control, products should be manufactured under an approved quality control program with visits by an inspection agency accredited by the International Accreditation Service (IAS).

It is also important to ensure all components of the waterproofing system are compatible and will not adversely affect performance (Figure 7). This includes the membrane, along with:

● tapes;
● sealants;
● adhesives;
● drains;
● scupper boxes;
● flashings; and
● perimeter fastening devices.

While asphalt paper (building paper) is compatible with PVC membranes, asphaltic membranes are not. Protective separation can be provided with an aluminum foil tape, thin sheet of metal, or asphalt-compatible thermoplastic membrane.

Waterproof PVC roof deck and flooring systems are highly durable and easy to maintain with periodic washing to remove surface dirt. In areas of environmental pollution or heavy service use, membranes may require washing with a scrub brush and mild detergent or pressure washing. Regular inspection of caulking and immediate attention to any lose seaming will ensure the PVC membrane lasts for years to come.

Detailing refers to the workmanship involved in installing materials and assemblies such that they are watertight and esthetically appealing. Simply put, a good or bad detail can often mean the difference between a good and a bad installation; proper workmanship and detailing are critical to the long-term performance of a waterproof walkable roofing system.

Waterproofing systems should always be installed by manufacturer-approved and/or trained technicians. PVC membranes can be hot-air-welded to provide a completely waterproof seam, making them well-suited to fine detailing (Figure 8).

Deconstruction refers to the use of waterproofing solutions that offer long-term life expectancy, as well as ease of replacement. This concept is a relatively new concern that has become increasingly important for sustainability-minded construction, and manufacturers are starting to have programs in place for recycling deconstructed materials.

Demountable flashing systems are one way to reduce the amount of deconstruction required to repair or replace a waterproofing membrane. These systems are easy to remove and allow the PVC membrane to be repaired or replaced quickly and easily with minimum destruction.

Considering the Six Ds—deflection, drainage, drying, durability, detailing, and deconstruction—helps the design team significantly increase the longevity of a roof deck. Each of these elements demands a degree of attention individually to work together cohesively.

Most manufactures will provide detailed drawings and building information models (BIMs) for common installation features or elements to accommodate best-practice recommendations.

john_ogilvieJohn Ogilvie has been involved with waterproofing decks and balconies with his company, Duradek, since 1976. He has been involved in the development of vinyl waterproofing products and installation across North America, and was a key figure in developing walkable roof deck polyvinyl chloride (PVS) assemblies. Ogilvie certifies ‘journeymen’ installers, and runs deck waterproofing seminars for building envelope specialists throughout the continent. He can be reached at

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