Building Code Required Curb Height – Part II

Is the required curb height by the building code necessary?

In a recent article, we talked about the importance of rooftop curbs at areas of installations related to access portals, skylights and a variety of other rooftop mounted devices.  In this article we pick back up with that topic focusing on the historic reasons behind the absence of curbs at many similar types of installations and best practices related to building and flashing rooftop curbs properly. 

Why rooftop curbs were not used in historic construction

Most of the rowhomes built-in Washington DC were built over 100 years ago. At the time, the technology and materials available were very different and therefore directly affected the means and methods of construction options.  The earliest rowhome flat roofs in urban areas in America were made from a few different types of materials which are rarely used today: sheet metal and BUR, otherwise known as built-up Roofing.

built up roofing  

Low slope or flat sheet metal roofs on historic buildings were mainly built with alloys of metal which contained high amounts of lead.  This type of lead alloy cause toxic pollution in manufacturing, and for a variety of related reasons these materials are no longer available today.  Modern alternatives may advertise comparable durability, but actual lead materials are highly resistant to oxidation.   

Built-up roof alternatives used a stone aggregate, as an alternative to the full adhesion we are accustomed to with modern roll-roofing membranes.  However, built-up roofing did not work well on vertical surfaces, for the reasons explained: poor adhesion to substrates which required a ballast to keep materials in a horizontal position.  That method does not work to ballast or pin down, by the pressure of bearing weight, materials in a vertical position. 

The pictures below show close up views of the rotted wood at the framing of the perimeter of the access hatch lid.

rotted wood on built up roofing

wood rot on wood

Installation methodology

Modern installation methodology is relatively simple: in the framing stage of construction, at a new construction, dimensional lumber can be installed to raise the curb above the horizontal plane of the flat roof. That framing can be fastened directly down to the substrate roof membrane at the time of installation of the roof.  In most cases that framing, unless in a retrofit, will be installed prior to installation of the underlayment board.

A modern single-ply membrane can then be installed from the flat portion of the rooftop deck and run vertically upwards over a curb.  It’s important to install areas of the lower side of the curb first so that the sides and then lastly the higher side of the curb membrane overlap the lower sides in sequential order. 

required curb height

Best Practices

The list of suggestions below are considered best practices for installation of curbs:

  1. All overlaid membrane elements and flashing should be laid in successive order starting with lower sides as the roof is intended to drain from high to low areas.
  2.  Edges of overlaid flashing elements should be rounded at corners without a 90゚ or sharp corner. Sharp corners have a tendency to delaminate as they are not supported on each side, compared to a continuous edge of a membrane which each small area, for example has another area of membrane next to in continuous form. 
  3. Crickets should be used where need to avoid excessive ponding or build-up of water at the upper side of the curbs, perpendicular to the flow of rain water.
  4. Curbs should be installed at least 8″ above adjacent low-slope surfaces.

membrane elemends

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, 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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