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How To Understand The Anatomy Of Your Flat Roof

How To Understand The Anatomy Of Your Flat Roof

Most adults have 206 bones in their bodies. Some of us have more, some have fewer. Why a varying number? From infancy at 300 bones to adulthood, some bones of our axial skeletons fuse. Human anatomy is complicated, no doubt. Flat roof anatomy, by contrast, should be easy. However, many facilities managers would fail the one-question quiz, “What layers are part of your flat roof?” 

Roofs Vary, Too

Just as the human body has variations (wormian bones, for instance), flat or low-slope roofs have variations as well. Your commercial property’s roof could have all of the layers described below; it could also have only some of them. No fear — your local, trustworthy commercial roofer can not only visually inspect your roof, but your contractor can also accurately analyze its anatomy. 

Roof construction has changed markedly over the decades, so if you happen to own a legacy building, its roof may have built-up roofing (BUR) which looks nothing like the sleek new warehouse next door. Many modern industrial roofs take advantage of single-ply membrane over corrugated steel deck plates. If your building relies on plywood sheathing and wood roof deck members, that’s fine, too. Good roofing contractors are prepared to work with any type of flat roof anatomy they encounter. 


Decking is the horizontal layer on a low-slope roof supporting all the other layers (and sits on the steel bar joists or wooden joists). Decking can be made from many materials:

  • Steel — light-gauge, cold-rolled sections welded or screwed to bar joists or wooden joists
  • Wood sheathing — most commonly plywood or oriented strand board (OSB), but dimensional lumber can also be a good deck in older buildings
  • Concrete — poured on-site or precast
  • Gypsum — precast or poured on-site
  • Cementitious wood fiber — these provide sound insulation, and when combined with various types of foam, can provide valuable thermal insulation
  • Composite decks — lightweight insulating concrete on corrugated steel or form boards 

Vapor Barrier

Sheets of thin, flexible plastic are rolled out over the decking to form a vapor barrier or vapor retarder. A vapor retarder is a material or system which substantially reduces water movement from inside your building up into the roof system, where it can condense. Condensation within your roof’s layers significantly shortens your roof’s lifespan the same way a water leak from outside to inside will.


No “flat roof” is truly flat. All roofs have to slope to drain water from their surface before it finds its way inside your building. Rigid board insulation, which provides sound and thermal insulation, accounts for most of the slope. Your skilled roofer shapes the insulation. The insulation is then placed on top of the vapor barrier so your roof slopes down to parapet scuppers or internal drains. 


Anywhere on your building where two unlike materials meet, flashing will provide sturdy, weatherproof protection of the joint. This relatively thin metal is flexible enough to be shaped to fit around building elements, but strong enough to withstand year-round rough weather and years of service. Flashing is usually mechanically fastened and its edges sealed with caulk. 

Top Coating

When facilities managers or building owners think of commercial roofs, they usually have only the top coating in their heads. This is the visible, uppermost layer of an industrial or commercial roof. It is the ultimate protector of all the rest of your business. It must hold strong against weather, wind, and abrasion from workers’ boots and equipment. 

Choices in top coatings allow building owners to select the best option for their buildings based on wear, budget, and looks:

  • PVC — Polyvinyl chloride (PVC) is a single-ply plastic membrane attached to a reinforcing scrim material; its many advantages include its high strength, durability, energy efficiency, low weight, and its resistance to chemical, wind, and water damage.
  • TPO — Thermoplastic Polyolefin (TPO), an unusually bright white plastic single-ply membrane, came to the United States from Europe in 1991; it appears to be as durable as PVC and is noted for its energy efficiency and high reflectivity.
  • EPDM — Ethylene propylene diene terpolymer is a synthetic rubber membrane roofing material which is rolled out and seamed in place; it comes in widths from 7 ½ feet to 50 feet and has a 50-year track record of durability, longevity, and easy installation.
  • BUR — Built-Up Roofing (BUR), one of the oldest commercial roofing materials, is alternating layers of a reinforcing mesh or fabric and bitumen (asphalt) finished with a top layer of ballast. 
  • Connect with us today at Division 7 Roofing in Columbus, Ohio, and the surrounding area. We are the region’s oldest and most trusted commercial roofer. We stand ready to help you better understand your commercial roof, maintain and preserve it, and replace it when absolutely necessary.