Re-Roofing Projects in NYC

If you are considering a roofing project on your building that involves going over the existing roofing membrane (rather than completely replace the current membranes) section 1510 of the NYC Code provides guidance. There are 2 key take aways:

1. If the area to be re-covered is less than 50 percent of the roof area and less than 500 square feet then the requirements of section 1504.9 shall NOT apply. (Section 1504.9 covers Reflectance and Thermal Emittance

2. Re-roofing over the existing membrane is NOT allowed where any of the following conditions occur:

If the existing roof is water soaked;

If the existing roof covering is wood shake, slate, clay,  cement or asbestos-cement tile;

If the existing roof has two or more applications of any type of roof covering.

Commentary:

 The standard to avoid switching to a ‘white’ roof is very high. As item 1 states, other than re-roofing a very small area, your new overlay must be white.

 Item 2 is important. Many Owners are unaware of the multiple layers of various systems in place on their roofs. Many wood framed structures have several layers in place, the accumulated load from which can reach a level detrimental to the structure. The statement of ‘two or more applications’ is interpreted to mean two separate membrane applications – the quantity of layers per installation is incidental.

Code section 1504.9 is shown below – for reference.

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1504.9 Reflectance

 Roof coverings on roofs or setbacks with slope equal to or less than two units vertical in 12 units horizontal (17 percent) shall have:

  1. A minimum initial solar reflectance of 0.7 in accordance with ASTM C 1549 or ASTM E 1918, and a minimum thermal emittance of 0.75 as determined in accordance with ASTM C 1371 or ASTM E 408; or
  2. A minimum SRI of 78 as determined in accordance with ASTM E 1980.

Exceptions:

  1. Terraces on setbacks comprising less than 25 percent of the area of the largest floor plate in the building.
  2. Any portion of a roof covered by a green roof system, including such a system with agricultural plantings, in compliance with Section 1507.16.
  3. Any portion of a roof used as outdoor recreation space by the occupants of the building that is landscaped, covered by wood decking or covered with a walking surface or other protective surface, provided that such walking surface or protective surface has a minimum initial solar reflectance of 0.3 as determined in accordance with ASTM C 1549 or ASTM E 1918.
  4. Ballasted roofs, provided that the ballast has a minimum initial solar reflectance of 0.2 as determined in accordance with ASTM C 1549 or ASTM E 1918.
  5. Any portion of a roof that is under mechanical equipment, flush mounted solar panels lying directly on the roof surface, duckboarding, decking, platform, roof tank, cooling tower or any other rooftop structure or equipment exempted by rule by the commissioner.
  6. Any roof or portion of a roof composed of glass, metal, clay or concrete tile or plastic/rubber intended to simulate clay or concrete tile, wood, or slate.
  7. Any portion of a roof used by a school or daycare center as a playground for children.
  8. Any roof, if the amount of rooftop space not subject to exceptions 1 through 7 is in the aggregate less than 100 square feet

The Unintended Consequences of White Roofs

The recent push in NYC and elsewhere to make roofing membranes white, must have been thought of during the summer.  Because in the winter, black is the way to go.
My recent investigations of roof deflections due to snow accumulation yielded an interesting observation….white roofs had more snow on them than black roofs.  Even in a literally ‘side by side’ situation of white versus black….the white roof had more snow.
We know that the color white reflects heat and the color black absorbs heat, so it stands to reason that in the winter, snow will accumulate faster on a white roof, since there is little or no latent heat.  A sunny day after the snow event would require the solar energy to penetrate more snow depth on a white roof than a black roof.  And, since most melting begins at the edges, this can’t happen on a white roof since the membrane is turned up the parapet.

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Snow Loads on Roofs

Here is a quick primer on snow loads for flat or low slope roofs, and whether or not you should be concerned:
  • Water weighs 5.2 lbs per inch of depth.
  • Fluffy snow, the type we get during very cold temperatures, is the lightest and is about 3% water.  12” of fluffy snow is equivalent to 0.36 inches of water or 1.9 pounds per square foot
  • Wet snow may contain 33% water.  Using the same math, wet snow 1 foot in depth may weigh 20.6 lbs.
Wet snow is common in our area and 12” of snow weighs 20.6 lbs per square foot.
Two feet of wet snow will weigh approximately 41.2 psf.
In NYC, your roof has a capacity of 40 psf for Live Load.   Large retail spaces are afforded a Live Load Reduction  which results in a capacity less than 40 psf….closer to 30 psf.   (For the purposes of this discussion, snow may be considered Live Load, because of its expected duration.)

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Forensic Engineer Looks UP

In New York City, there is plenty to see: museums, parks, broadway shows and interesting people.  But with over 11,000 buildings taller than 6 stories, there is also plenty to see by looking up.  That is where Eric W. Cowley, PE, and his firm Cowley Engineering, comes in.  “Since 1980, the City of New York has had a Façade Ordinance requiring the exterior inspection of all buildings taller than 6 stories.  My firm has been doing these inspections for over 20 years, so nothing surprises me anymore,” says Cowley.  Cowley is providing his expertise to smaller cities, and helping them customize an effective façade ordinance.  “The objective is proactive maintenance to ensure safety.  The first step is to craft a law that suits the local area, and is sensitive to the area’s economic viability.  The second step is to set up the controls and procedures to implement and monitor the program.  And the third step is to track the repairs.  We provide the consultation and our own database software that monitors and charts the process,” says Cowley.

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