square meters 20 to 35 lbs per 100 square feet before installing vapor
retarder. Lay first ply of 2 ply system with each sheet lapping 480 mm 19
inches over preceding sheet. Lap ends not less than 100 mm 4 inches.
Stagger laps a minimum of 300 mm 12 inches. [For vapor retarder consisting
of one layer of asphalt base sheet, provide side and end laps not less than
100 mm 4 inches and stagger laps a minimum of 300 mm 12 inches. Cement
base sheets together with a solid mopping of asphalt.]
][3.2.3
Vapor Retarder on Wood Decks
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NOTE: A vapor retarder should be specified only
where:
1. The outside, average January temperature is below
4 degrees C 40 degrees F, and the expected winter
interior relative humidity is 45 percent or greater;
2. The roofing system will be subject to continuing
excessively high interior humidity; and
3. The results of a detailed analysis indicate a
potential roofing problem resulting from water-vapor
infiltration.
Delete contractor's option for a single-layer vapor
retarder for roofs over areas having excessively
high interior humidity such as swimming pools and
laundries. Generally, the vapor retarder should be
in direct contact with deck. However, compute
dewpoint temperature and location and locate vapor
retarder below dewpoint location. Avoid use of
vapor retarder over steel decks unless vapor
retarder is determined to be absolutely necessary.
Do not specify wood fiberboard or any other type of
insulation sensitive to moisture for the layer
directly on steel deck. Glass mat gypsum roof board
is not sensitive to moisture and may be used in such
situations. Specify installation of insulation over
vapor retarder using hot asphalt to avoid puncturing
the vapor retarder. The combination of two or more
layers of insulation shall be of such thickness that
the dewpoint temperature will occur above vapor
retarder located directly over first layer of
insulation. Topside venting should always be
provided by insulation vents and perimeter edge
vents when a vapor retarder is used. Specify
venting requirements in roofing membrane
specification section.
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Lay first ply of 2 ply system dry with each sheet lapping 50 mm 2 inches
over the preceding sheet. Lap ends not less than 100 mm 4 inches. Stagger
laps a minimum of 300 mm 12 inches. Nail felt at 150 mm 6 inch intervals
along side laps and install two rows of nails approximately 275 mm 11 inches
apart down longitudinal center of each sheet, with nails staggered at 450
mm 18 inches O.C. [For vapor retarder consisting of one layer of asphalt
base sheet, lap each sheet 100 mm 4 inches over the preceding sheet.
Provide end laps not less than 100 mm 4 inches and stagger laps a minimum of
SECTION 07220
Page 21
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