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Anatomy of a Commercial Roof

Anatomy of a Commercial Roof

From the outside, a low-slope commercial roof looks like a simple installation – that’s the idea. The low profile and simple design build a durable roof while making installations easier. But on the inside, a commercial rooftop is significantly more complex, made out of several layers that can vary based on the materials used and the intention of the installation. To give you an idea, let’s go over the primary components and the roles they play.

Structural Deck

The structural deck is the bottom layer that needs to support everything else. This is the most essential layer for structural integrity, resisting serious damage and supporting the installation of materials above. Structural decks can also vary significantly. Metal and concrete are two common materials, but a professional roofer may recommend a specific material depending on the nature of the roof, load-bearing mechanics and other considerations.

Vapor Control Layer

This is a basic moisture barrier, a thin sheet of plastic that goes down to prevent moisture from passing through and reaching the more vulnerable roofing materials. Without this layer, moisture can build up and cause big problems, including rot and mold. It’s especially crucial in more humid, rainy areas, or situations in which inside and outside humidity differ greatly. 


Insulation helps protect building temperatures from being affected by external conditions. For low-slope commercial rooftops, insulation is usually made of rigid boards created from gypsum or materials like polystyrene. This helps keep the roof flat while still providing important protection.


The roof needs to be protected by a membrane or sheet that will keep the materials safe from the weather. Still, this membrane doesn’t bond easily to the insulation alone. That’s why it needs some help with a layer of adhesive. This step can vary, but in many installations, the adhesive is applied as the membrane is installed, section by section. It’s often a rubber-based bonding agent that is carefully applied and smoothed over to ensure no bumps or indentations develop.

Of course, there are some exceptions. For many metal rooftops, this step is replaced by mechanically attaching metal panels to the roof, which is common when rooftops need more durability or protection from certain kinds of weather.

Anatomy of a Commercial Roof

Weatherproof Membrane

On the top, a weatherproof membrane is installed on the adhesive layer, often by rolling it out in a long sheet that allows for speedy installation. This membrane can be made from many different materials depending on the local climate and expected weather, frequently with specific types of protective plastic or rubber.

With some kinds of commercial rooftops, a final layer of substrate, like fine gravel, is also added on top, instead of a membrane. In certain circumstances, additional layers may be installed to add more protection or better installation.

Not sure what kind of commercial roof materials you're dealing with, or interested in a potential replacement? Contact MG Roofing Inc. today, and we can arrange an inspection to learn more and provide the information you need!