Flat Roof Curb Height

What’s the recommended flat roof curb height according to roofing professionals?

Flat roofs have curbs installed at a variety of different types of openings and/or mechanical equipment.  For example skylights on flat or low slope roofs must be installed on a curb, they can’t be installed directly against the flat roof deck.   There are several reasons why these skylights must be elevated away from or above the roof deck, one of the main ones is that water must flow and be able to run around the skylight as water travels and runs on it’s pathway from the higher end of the roof to the lower end of the roof.

Flat Roofs Are Not Actually Flat 

It’s a misnomer, but flat roofs are never actually flat, there’s always some intentional grade or slope built into the roof intentionally so that the roof can shed water appropriately.  We should explain further though by clarifying that when we say there are never any spots that are actually flat, we really mean to say that there are no spots that are actually allowed to be flat, per building code, by extension, as required by almost all UL labeled flat roofing systems.  So, we often talk about this clarification and distinction, related to flat vs. low-slope roof systems and the common misnomer used in the industry.

Rooftop Skylight   

The picture below shows a rooftop skylight, designed for installation angles as low as zero-grade, flat.  Some skylights, by comparison, are not designed or allowed, by the manufacturer and supported by the building code, to be installed at zero-degree angles.  In contrast the one shown here is allowed to be installed flat.  Often though, we may suggest a slight grade be added by setting a congruent curb directly on the slightly sloped roof decking, as was done in this case to create a slight grade to passively drain water.

rooftop skylight

This curb was built with 2″ x 12″ framing lumber.  This dimension, 2×12, is a nominal dimension, because the lumber, after milling and kiln shrinkage, is actually about 1.5″ x 11.25″.   Several different types of materials can be used to frame the curb, but we recommend, that in this climate zone, depending on the exact type of roof system, location, and code context, that the curb be at least 7.5 or 8″ high.

flat roof curb height

In an upcoming article we will talk more about the differences between modern and contemporary versus historic rooftop curbs.

Maintaining Flat Roofs

Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building. 

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, contact us or fill out the webform below and drop us a line.  We will be in touch if we can help.

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