(316) 927-2233Any day you have to think about your roof is probably not a good day. For anyone who only occasionally has to discuss their flat (in the roofing industry, more typically defined as “low-slope”) roofing system, we’ve put together a glossary of terms to know. These definitions are geared toward building owners/managers, maintenance personnel, and investors, rather than roofing industry professionals.
Coatings
“Coatings” refers to a wide variety of thick, paint-like substances that are applied over the entire existing roof, creating an additional layer of protection from sun and weather. They can be applied to a wide variety of roof surfaces, including shingles, modified bitumen, single-ply waterproofing membranes like TPO and PVC, as well as various types of metal roofs. Coatings are used as a cost-deferring (we are careful not to say cost-saving) alternative to replacing the entire roof system just yet. Sometimes this works to extend the life of an aging (but still in good condition) roof system and push the expensive, cash-flow-demanding roof project down the road a few years. We don’t think that roof coatings are the best solution for most flat and low slope commercial roof, but there are a few situations where it makes sense. Learn more in our Coatings Overview and Applications article. The next ‘step up’ from a coating may be a layover – see below under ‘Layover.’
Decking
Roof decking refers to the structural part and base layer of a roof system roof. Insulation (if present), cover board (if present), and weatherproofing roofing material is attached on top of the roof deck. Roof decking can be wood sheets (oriented strand board (OSB) or plywood), metal, or concrete. The roof decking is installed on the rafters or trusses of the building.
Sheet decking is installed with staggered joints to prevent parallel seams from touching. This reduces the chances of a leak or sag, and adds structural strength to the roof.
Drains
Flat roofs, especially large roofs, often have some form of internal drain located at intervals where runoff collects. Drains are very useful when the design or use of the building prevents the use of gutters. Drains can be located virtually anywhere runoff collects, so they are a versatile solution to drainage, especially on large roofs. Smaller buildings tend to use scupper/gutter systems (defined below, or we have a more thorough comparison in a separate article about when Drains vs Scuppers are more appropriate drainage design.
Fully Adhered (Membrane)
Fully adhered flat roof membranes (the top waterproof layer of a flat roof system) is a method of attaching a single-ply membrane (like TPO, PVC, or EPDM). The entire surface of the membrane is adhered to the roof deck, insulation layer, or cover boards. Fully-adhered membranes are installed using adhesives, creating a stable and long-lasting roof surface, but that is more difficult to repair or remove. so removing a fully adhered roofing membrane can be a real chore. See much more detail in Mechanically Attached vs Fully Adhered Roofing Systems
Hot Mopping (Mop-down Roofing, Torch-down Roofing)
Hot mopping is the oldest flat roof technology still in use today. Hot mopping, as the name implies, involves mopping hot tar, or asphalt, or bitumen directly onto the roof deck and then sealing layers of roofing onto the roof. The pitch can be combined with a ballast material, like gravel, to add wind uplift resistance.
Hot mopping is an effective, but dangerous and messy project. The pitch produces VOCs, or volatile organic compounds, which are the toxic fumes emitted by the boiling asphalt. Some roofs still have the same hot mopped roofs they have had for decades, albeit with regular maintenance. Hot mopping is not often used for new installations today, but many existing buildings are still operating in good condition so hot mopping is used to repair or patch existing roofs.
Layover
A layover (also called a re-cover) roof refers to the process of “laying over” a new membrane over an old one. Sometimes additional insulation or cover-board is installed over the existing roof system as well, in addition to a new surface membrane. When an old roof is in need of extensive repairs, it is possible to lay a new membrane over the old one, without removing the old material. This method is faster and cheaper than a complete replacement, but it does have limitations. Some municipalities allow for two roofs on a structure, as the additional weight will not harm the structure of the roof. If appropriate, a layover can add life to the roof, and have minimal impact on the activities of the building. A layover is more expensive and more durable than a coating, and still limited, but is appropriate in more situations, but still limited. See more detail in Layover vs Remove-and-Replace On a Membrane Roof.
Mechanically Attached
Mechanically attached roofing refers to the use of mechanical fasteners; namely cleats and screw anchors, to attach membrane roofing (like TPO, PVC, or EPDM) to cover board or decking below. Mechanically attached roofing membranes are cheaper (than fully adhered systems) to install and easier to repair, but they tend to leak more due to the perforations made by the fasteners, and have a shorter expected lifespan because of movement & fluttering of the membrane. See more detail in Mechanically Attached vs Fully Adhered Roofing Systems.
Scuppers
Scuppers are the drainage termination points at the exterior of a flat roof. They can tie into a gutter system (or flow freely to the ground). Typically, flat roof systems rely on either an internal drain system or a scupper system – though occasionally very large roofs will have both. Gentle slope on the roof surface itself will lead drainage to the scuppers around the perimeter. Scuppers often pass through parapet walls at the perimeter as a way for water to get through. We have a more detailed article about when Drains vs Scuppers are more appropriate drainage design.
