Building Code Required Minimum Curb Height – Part I

Learn What Is The Minimum Curb Height Allowed By Buiding Code of Washington, DC

You can learn a lot more about the building code requirements regarding curb height in our area from the two past articles in the links below:


learn about building code requirements

An outline of this two-part article follows:

  • Why curbs are required by the Building Code
  • Why rooftop curbs were not used in historic construction 
  • Installation methodology
  • Best practices 

Why curbs are required by the Building Code

 Curbs are used at all types of different openings and/or penetrations or mounting of different devices or installations on flat rooftops. Curbs are often not required or used at pitch rooftops for similar types of installations but flashing requirements are different with pitch roofs versus flat roofs. This is one of the many reasons it’s important to have and use a contractor who is knowledgeable, trained, and understands of the differences and specialty requirements of flat roofs.

Two of the most common types of curbs used on flat roofs are built especially custom to fit 

  1. Skylights, and 
  2. Access hatches

The picture below shows an area of a low s**** historic rohom roof, covered with a modified b**** you can remember him. There is an access hatch that goes from the inside to the rooftop and in the picture below you can see a picture of the access hatch lid or cover.  This particular access hatch was built with regular framing lumber and then wrapped with a thin layer of modified Benjamin membrane to make it watertight from the top or exterior size.

The building Code has different requirements for the minimum curb heights, depending on the exact location of the property come up but by a large in our area it’s generally required to have a curb over 8″ above the adjacent horizontal or low-slope membrane. 

minimum curb height

Without a curb, the device or element of rooftop installation such as this access hatch or a skylight would sit at the level of the roof itself. These openings, although built with a cover, have an underlying interstitial space which can allow air to flow from the outside to the inside and therein cause the wicking of water through capillary action.  One of the Building Code’s main concerns is potential snow loads and associated ice damming. Ice damming is a condition that occurs at temperatures hovering around 32゚F, coincident with precipitation over time.  These conditions cause water to build-up below a surface of ice and then wick or suck that water (through pressure differentials) into areas of the building interior through capillary action.

The picture below shows the same access hatch with the lid removed and flipped upside down and set aside. You can see that the earlier contractor had put insulation on the inside of the access hatch lid, this was an intelligent move. Installation of insulation can prevent condensation on the inside of the relatively thin hatch lid.

access hatch

Overall though, the installation did nothing to preserve or increase the longevity of the access hatch. Because the wood framing members were almost in direct contact with the adjacent horizontal surface of the roof membrane.  Over time precipitation events caused the hatch framing to become hydrated, repeatedly, over several years. This led to the complete rot and deterioration of the access hatch framing.

access hatch 2

In our next installment, part II of this article will look at the history related to rooftop curbs and contemporary vest practices.

Maintaining A Flat Roof System

Smart proactive replacement, construction, upkeep and maintenance of low slope roof and mansard roof systems requires an enthusiastic interest and understanding of historical methodologies, 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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