Flat Roof Curb Height – Part II

What is the recommended flat roof curb height according to professionals?

Historically, curbs, for mounting skylights and other devices such as equipment and or associated roof penetrations, were used but commonly built much lower than modern curbs.  The historic builders did not heed the impact of snow loads and ice-damming with the same reverence as modern builders and code writing organizations / AHJ’s.

Curb Height Should Be Higher In Case of Skylights

In the case of the roof and skylights below, some relatively recent roofing contractors came and replaced the skylights and the roof system without building the curbs higher.  This isn’t really correct, but not all renovation contracts implicitly require all existing conditions be brought up to meet the requirements of the modern code.  There is a gray area here because it wouldn’t make sense to assume every time a repair or new installation is done, that all building systems would also be modernized.  That would make a repair job cost many multiples more than it would normally cost, so there’s an understandable gray area which requires a line to be drawn.  In fact, some systems are actually allowed, by DCRA and other AHJ’s to remain as they were built, even if outside of modern tolerances, if considered to be a grandfathered condition. 

higher curb height skylights

The picture below shows an example of a curb that is so low that the overlapping concentric edge of the skylight almost touches the flat roof membrane.   In this case, that membrane is a single-ply modified bitumen roof system.  You should note, as you learn more about roofing from our weekly blogs on roofing principles and best practices, this particular roof was not well installed, as is evident by the lack of bleed-out at the seams of the roll-roof membran. 

Why Bleed-Out Matters

Bleed-out is a term, similar to run-out, used to describe the torch melted emulsion on the underside of torch-applied modified bitumen roof materials which oozes out from under the edge of the concentrically or successively laid sheets of roofing material.   Not only is bleed-out acceptable but it’s actually required to show, as a quality standard, to indicate that the roof membrane was heated (torched) to the appropriate temperature at the time of installation.   A slight bleed-out also has a tendency to help the roof membrane edge resist delamination. 

torched roof membrane skylight flat roof curb height

In the picture above, we might be able to assume that this is a APP modified bitumen roof, and not a SBS system.  Once installed, with a granulated ceramic protective covering, it’s hard to discern the difference between APP and SBS systems.  Of the two sites types, APP has less bleed-out.

The next picture shows historic skylights, somewhat unique to the area, as built at the time of original construction. In the decades before modern air-conditioning, the louver skylight, in the background, could passively vent the building to significantly reduce temperatures, even on the hottest summer days.

The curbs, busy historic standards, are relatively ample at these particular skylights.  In modern times, they’d be even taller though. 

curb height historic building

Flat Roof Maintenance

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, fill out the webform below and drop us a line.  We will be in touch if we can help.

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