An Association of Contractors and Manufacturers Providing Engineered Roof Deck Systems
Frequently Asked Questions
Answers to frequently asked questions about engineered roof deck systems are provided in this section. The questions and answers are divided into specific roof deck systems for ease of locating your questions.
Light Weight Insulating Concrete Roof Deck Systems
The R-value of any LWIC is increased by adding expanded polystyrene insulation
board during application of the LWIC. A LWIC system (LWIC plus EPS
insulation) can easily achieve 50 R's for the installation.
The equilibrium moisture content of cellular concrete is typically 14%-18% by weight.
Roofing may begin 2-4 days after placement. The base sheet fastener should attain a
minimum 40-pound withdrawal value.
After a LWIC deck is roofed, it is highly resistant to weather conditions as has been
demonstrated for over 60 years of continuous applications in all geographic areas of
theUnited States and in other parts of the world.
Place the slurry coat so that the bond holes are full of material. It is not necessary to
slurry coat the board surfaces. A slurry coat on the board is acceptable for next day
topping without any special treatment such as a bonding agent.
Built-up Roofing, Modified Bitumen, and Single Ply Membranes of various approved
compositions have been successfully installed over LWIC.
Typically, hairline cracks are caused by differential drying shrinkage of the top surface that dries out more quickly than the bottom due to sun and wind. Cracks may also originate off the corners of rooftop penetrations due to differential curing shrinkage. The presence of surface cracking is not detrimental to the roofing membrane or its long term performance. Additionally, the cracks do not affect the physical properties of the insulating concrete system.
Venting to relieve vapor pressure is dependent upon the structural substrate, the specific
LWIC, and/or the membrane manufacturer.
A surface finish free of ridges and protrusions is an acceptable finish for roofing
The minimum slope is 1/8 inch per foot to provide positive slope and to insure that
water will drain. Consult local building codes.
The advantages of LWIC include fire resistance, wind uplift, and seismic ratings,
positive drainage, dimensional stability, moisture resistance, and reroofable. LWIC is
permanent, non-combustible, strong, economical, approved, and roofing compatible.
Drying shrinkage is a normal occurence of cellular concrete and will not adversely affect the
performance of the insulating concrete roof deck.
"Cold joints" are not determental to the performance of an insulating concrete roof deck.
Whenever possible the "cold joint" should be squared off to the depth of the insulating concrete.
Portland cement based roof decks should be patched with a Portland cement material
and gypsum roof decks should be patched with an approved gypsum material.
Both aggregate and cellular products are placed at a minimum thickness of 2 inches.
One aggregate product can be placed at a thickness of 1 inch. LWIC systems (LWIC
plus EPS Insulation) may be placed at a thickness greater than 18" depending on the
specified slope (1/8 or ¼ inch per foot).
If possible use heated mix water and/or High Early Strength cement (Type III) to
accelerate the setting time before snow occurs.
In many instances, it may be acceptable to cast LWIC directly over an existing
membrane and insulation system. If the membrane is removed and there are base sheet
nails of holes left on the deck, they shall be handled per specific job conditions. Unlike
insulation board systems, just remove the existing membrane and install a new
membrane per local requirements for acceptability. The LWIC system is permanent and
does not need to be replaced.
Per structural engineer approval, cast new LWIC over existing LWIC and then apply the
roofing membrane. Follow the procedure outlined in NRDCA Standard 500 for
If maintained in a watertight condition, LWIC should last for the life of the building.
For system information, contact the appropriate supplier.
All products have FM (wind uplift) and UL (fire resistance) listings in addition to
national and local code approvals.
Yes. LWIC has appropriate credits for these requirements.
LWIC has been successfully pumped from the ground to 550 feet. This is dependent on
the local contractor's equipment capability.
Differing amounts of Portland cement, water, aggregates (vermiculite, and/or perlite) and/or preformed cellular foam are mixed together dependent upon specific project requirements.
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