Flat Roof Decks – The Flat Roof of the Fourth Kind

We recently came across an article on This Old House’s website titled, “The 4 Kinds of Flat Roofs”.

The author, Max Alexander, did an excellent job of translating the complexity of roofing systems into a concise, easy to digest comparison. He looked at:

  • Built-Up Flat Roofs (BUR)
  • Modified Bitumen Flat Roofs
  • Rubber Membrane Flat Roofs (EPDM)

…but where was number 4?

There was no ‘next’ or ‘more’ buttons to click. That was it…only 3 kinds of flat roofs. We were left in suspense. Perhaps the editor mistakenly cut a very important option from the article… or maybe just checking that we were paying attention? (Yes, we are poking a little fun at This Old House’s post, but we are actually really big fans and found this article to be very valuable and inspirational).

So we are going to presume to discuss the mysterious flat roof of the 4th kind…Flat Roof Decks.

The 4th Flat Roof – Roof Decks

Flat roofs do not always need to be utilitarian in nature. They are ideal locations to extend livable space by creating outdoor leisure areas. With the caveat that safe access is available and the roof is structurally sound to bear the weight of traffic, flat roof decks are a value-added feature to incorporate into building or remodeling plans.

Flat Roof Decks Completed with PVC Sheet Membranes.

Duradek PVC Flat Roof Deck
Duradek’s PVC Sheet Membranes are an attractive option for roof-grade waterproof performance.

PVC Sheet Membranes (commonly referred to as ‘vinyl decking’) allow you to gain the waterproofing advantages of other flat roof options, but with the added benefit of also acting as a pedestrian traffic coating. They are all relatively comparable with cost and life expectancy. However, the right PVC membrane allows you to turn a flat roof into usable outdoor living space in the form of a luxurious roof deck. The beauty is, it can accomplish this in a single product solution.

How Other Flat Roof Options Compare to PVC Sheet Membranes

Built Up vs PVC Membranes Flat Roofs

This flat roof (deck) with an asphaltic material and tar is not an ideal place to spend leisure time.

The traditional hot-tar-and-gravel roof build-up requires multiple steps. The alternating of a waterproof material with hot tar, which is then ballasted by a layer of river stone. While this is one of the lower cost options, and some might even consider gravel attractive to look at, it is a very heavy system. Not to mention, it is smelly and messy to install and not recommended for occupied homes. The gravel can clog gutters and scuppers and it is hard to find the source of leaks. With a complicated installation and multiple producthttps://duradek.com/vinyl-decking requirements, Built Up Flat Roofs still do not add any usable space.

PVC Sheet Membranes, on the other hand, are a much simpler installation. Both options require professional installation, but PVC sheet membranes are completely acceptable to install on occupied homes. A roller grade adhesive and hot air welding techniques allow for a much cleaner and safer installation.

Modified Bitumen vs PVC Membranes Flat Roofs

Torch on flat roof
A traditional torch-on flat roof is not a desirable surface for a functional flat roof deck.

The Modified Bitumen option is a single-ply rolled roof available in traditional torch-down systems and newer peel-and-stick systems. The peel-and-stick method is available as a do-it-yourself option. The torch-down method is a fire hazard and not recommended for occupied homes. Neither are good options as a walkable surface to make a flat roof usable space as a roof deck. If that is the goal of a flat roof, a modified bitumen roof would require an extra build up and product application to create useable deck space.

PVC Sheet Membranes are also a single ply application. While there are some brands available as a do-it-yourself option, the waterproofing reliability demands specialized installation techniques and professional application is recommended. However, in the case of the PVC membrane single-ply system, no additional product is needed to have it function as a leisure roof deck. It is designed as a pedestrian traffic coating in addition to its roofing properties and does two jobs in a single product solution. It is important to be sure to choose a PVC membrane that has been tested for use as both a roofing membrane and a pedestrian traffic coating.

Time, Tested and Approved – a checklist of product testing requirements for walkable roof deck membranes (.pdf)

Rubber Membrane (EPDM) vs PVC Membranes Flat Roofs

This rubber material resembles an inner tube and can be mechanically anchored with fasteners, ballasted with stone, or glued. It is relatively lightweight and durable, however, the standard black material absorbs heat. There are light-colored options available but they are about 30% higher cost. Comparatively, it is a higher cost option, but it is something a homeowner can install. It does not, however, make a good surface for a flat roof deck and would require an additional assembly to make the flat roof acceptable for that purpose.

PVC Sheet membranes have similar performance expectations, although PVC membranes have 2 distinct advantages.

1 – when installed by a professional applicator, PVC membranes are integrated into the building envelope and come with a sound warranty.

2 – PVC sheet membranes once again are the only product of these comparisons that provides an attractive walking surface in addition to its roofing membrane performance in a single product application.

Walkable, PVC Sheet Membranes for Flat Roof Decks

A waterproof membrane and a pedestrian traffic coating in one product solution can be the most cost effective way to turn a flat roof into a low-maintenance leisure deck area.

Just be sure to select a professionally installed product that has been tested to meet building code requirements for both a roofing membrane and pedestrian traffic coating.

When you are comparing options for flat roof deck projects, feel free to use this handy checklist on product testing requirements.

Time, Tested and Approved – checklist of product testing requirements for walkable roof deck membranes (.pdf)

orchon-before-pvc-after flat roof deck
Before with torch-on and After with Duradek PVC Membrane.

If you would like more information on how you can incorporate a walkable, flat roof into your projects to create private, outdoor leisure space, contact an authorized Duradek dealer in your area to discuss your options.

See how one asphaltic tar-over-concrete roof deck was transformed into an attractive, low-maintenance roof deck in Case History #156 – Roof Deck Renovation, Washington, DC.

And while we poked a little fun at the missing 4th flat roof on the article from the This Old House website, we are big fans of This Old House too. For all of you other This Old House fans, you might like this story of how PVC membranes contributed to the renovation and waterproofing solutions for a 200-year-old Home.

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