Roofs

2018-2019 Flat Roof Replacement

The three materials generally recommended for flat roofs are EPDM, TPO, and PVC. This site offers an introduction to the pros and cons of the three systems:

http://www.rtnroofing.com/learning/article-library/perspectives/77-which-is-a-better-roofing-system.

2015-2016 Pitched Roof Replacement

The pitched asphalt shingle roofs of the complex were all replaced at one time along with flashings, anchor points for workers, and a dozen or so ridge vents that had never been installed. This work was undertaken in part to facilitate the installation of a 40kva photovoltaic system on the East and West Stack roofs. The Roof Circle Minutes chronicle the decision making process of this work (see pdf below or here: https://sites.google.com/site/cambridgecohousing/board/projects/roof-replacement).

Loads:

• pitched roof: 40 psf

• membrane roof on commonhouse: 60 psf (was planned for a roof deck)

• our townhouse and unit 215 roof decks: 60psf (exceptions: 105 and 106 are engineered for much higher hot tub loads)

• our balconies as rebuilt: 100psf

Question: At what point should we shovel off what areas?

Perhaps the most vulnerable roof is the flat roof and then only when it rains after a significant amount of snow has accumulated. In the past, we have directly tested the weight of snow. Here is how:

1. Get access to the roof (Jean Mason has a key).

2. Bring a shovel less than 12 inches wide and a trash bag.

3. Select a location of average snow depth (our roof loadings are based on large areas not single spots).

4. Being careful not to damage the membrane, cut out a 12 inch by 12 inch square (one square foot) from the top of the snow down to the roof.

5. Put this snow and its water content in the trash bag.

6. Take it down to the scale in the Exercise Room and weigh yourself holding the trash bag and then without.

7. Subtracting the lower weight from the higher weight gives a direct measurement of the weight per square foot.

8. Compare this weight to the rated load above (60 pounds per square foot for the flat roof)

In the major snow-rain storm of 2011 this process yielded the result: 26 pounds or about 40 percent of the load rating of the roof.

That said, the second reason to shovel a roof is to prevent ice damming (see 9 Buildings > Ice Dams).

Our Structural Engineer:

Leonard J. Morse-Fortier, Ph.D., P.E. (MA, NY, others) Staff Consultant, Simpson Gumpertz & Heger 41 Seyon Street, Bldg. 1, Suite 500 Waltham, MA 02453 781.907.9370 direct 781.424.3965 mobile 781.907.9009 fax www.sgh.com

— Bob Cowherd 2015