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ROOF DECKS |
4.11.4 Following are requirements for the application of a JM roofing system over plywood or OSB decks:
A. |
Must be American Plywood Association (APA) labeled, CD Exposure 1, Struc 1, 5 ply and minimum
thickness of 1/2 " (13 mm). |
B. |
Minimum thickness of plywood or OSB considered acceptable by JM for a roof deck is 1/2 " (13 mm). JM
only accepts selected brands of OSB for guaranteed roofs; contact a JM representative for acceptable
OSBs. |
C. |
All joints between plywood or OSB sheets should be blocked with wood framing. Ply-clips are not
acceptable. Square edge joints should be loosely butted to prevent buckling of the panels. |
D. |
Securement must be in accordance with APA recommendations. |
E. |
Install plywood so the face ply grain is perpendicular to the joists. OSB should be installed with the long
direction perpendicular to the joists. |
F. |
Plywood or OSB, as well as all other components of the roofing system, must be kept dry prior to the
application of the roofing system. |
G. |
Spacing of joists or purlins should conform to the specifications shown in the table below. |
H. |
To prevent condensation on the lower side of the roof membrane, especially over buildings where
insulation has been placed beneath the plywood deck, add enough roof insulation above the deck to
raise the temperature above the dew point so condensation will not occur. For this reason, underdeck
insulation is not recommended. |
Refer to the roof deck fastening chart in this section for recommendations regarding the proper fasteners to use in attaching the membrane or
insulation to wood and plywood decks.
4.11.5 IT IS THE RESPONSIBILITY OF THE OWNER OR THEIR TECHNICAL REPRESENTATIVES TO DETERMINE IF THE DECK
STRUCTURE CAN ADEQUATELY SUPPORT THE ROOFING SYSTEM OF CHOICE.
4.12 Lightweight Insulating Concrete
4.12.1 Lightweight insulating concrete resembles poured-in-place structural concrete in that it starts with a mix of cement and water. The end
result however, is quite different from structural concrete. Lightweight insulating concrete can provide a suitable base for built-up, modified and
single ply systems when designed, specified and installed properly.
4.12.2 There are two basic "families" of lightweight insulating concrete: aggregate and cellular. Both types have relatively low densities and
are considered to be nailable substrates. Both can be poured by themselves to create slope and provide insulation or they can be poured over
a special expanded polystyrene insulation board to increase the thermal efficiency of the system. Both systems must be poured over some
type of structural substrate. The most common structural substrates today include metal and structural concrete.
4.12.3 Because of the amount of water involved in mixing and pouring, the use of either system is more sensitive to climatic conditions than
many other deck systems. Extreme care should be taken when utilizing this deck system in areas that can experience freezing temperatures.
JM will not accept any type of lightweight insulating concrete fill in Region 1 that has been poured between October 1 and May 1 without
specific prior written approval of a JM Technical Service Specialist.
4.12.4 Lightweight insulating concrete (aggregate base) is a mixture of portland cement and water with a vermiculite or perlite aggregate. The
perlite or vermiculite helps reduce the overall density and improve thermal efficiency of the material. A minimum pour thickness of 2" (51 mm)
is normally required by the manufacturer. A typical 1:4 or 1:6 mix (cement: vermiculite/perlite by volume) contains as much as 94% water by
weight. Because of the high percentage of water, to be eligible for a JM Roofing System Guarantee, this fill must be poured over a slotted
metal deck. JM will not accept standard lightweight insulating concrete mix which has been poured over a non-venting substrate. If a JM
Roofing System Guarantee is required, a Ventsulation venting base sheet may be used as the base sheet. FP-10 One Way Roof Vents must
be installed at a rate of one vent per 1000 square feet. Contact a JM Technical Service Specialist for specific information.
4.12.5 In recent years, lower water content aggregate fills have been developed. These products utilize a 1:3.5 volume ratio (cement:
vermiculite) and because of the higher strength, can be poured in a minimum 1" (25 mm) thickness. Contact a JM Technical Service Specialist
for regional requirements involving this deck system.
4.12.6 Lightweight insulating concrete (cellular) is a mixture of portland cement and either a chemical to create bubbles in the cement, an air
entraining agent that "foams" the cement, or a preformed foam that replaces the mineral aggregate in the cement. This type of lightweight
insulating concrete typically uses 75% less water in the mix than traditional aggregate based lightweight insulating concretes. "Bubbles" in the
concrete reduce the density and improve the thermal performance.
4.12.7 Cellular lightweight insulating concrete may be poured over non-venting substrates such as metal and structural concrete. Because of a
rapid drying time, some manufacturers of these products recommend that a roof be installed not more than 5 days after pouring the deck.
Care should be taken to see that the cellular, light-weight concrete is dry, but not severely cracked due to rapid drying.
4.12.8 There are several types of expanding fasteners available for the securement of built-up or modified bitumen sheets to lightweight
insulating concrete fills. Fasteners must be treated to resist corrosion. The current Factory Mutual Research Corporation Approval Guide
should be consulted for approved types and fastening rates. When no outside agency approvals are required, the fasteners should provide a
minimum 40 pounds (178 N) or more of pullout resistance.
