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.
Table of Contents
Light Weight Insulating Concrete Roof Deck Systems
- What are the typical physical properties of LWIC?
- What is the equilibrium moisture content of cellular concrete?
- How do you increase the R-value of LWIC?
- When can LWIC be roofed?
- How resistant to weather is LWIC?
- If EPS is in place and a threat of rain exists, what is the best procedure to follow?
- What types of roof membranes can be installed over LWIC?
- Are control joints needed for thermal movement in a LWIC system
- Are expansion joints needed for thermal movement in a LWIC system?
- Do hairline cracks affect the lightweight insulating concrete?
- Is venting required in a LWIC system?
- What is an acceptable finish for LWIC?
- What is the minimum slope required of LWIC?
- What are the advantages of LWIC over rigid insulation board?
- Is there a concern about drying shrinkage cracking of cellular concrete?
- Are “cold joints” acceptable in an insulating concrete roof deck?
- With what materials should various roof decks be patched?
- How thick is LWIC poured?
- What precautions should be taken for top coat casting if snow is forecast?
- What is involved in re-roofing over LWIC?
- Is it possible to cast over an existing LWIC roof (re-cover)?
- How long does LWIC last?
- How long is the guarantee for LWIC?
- What regulatory approvals exist for LWIC?
- Is LWIC a green product?
- How high can you pump LWIC?
- What is the mixing process for lightweight insulating concrete?
- Is there an NRDCA/UL accreditation program for LWIC contractors as noted in my project specification?
What are the typical physical properties of LWIC?
Air Dry Density
Weight per Inch
R Value per Inch
|Aggregate||44-60 pcf||22-28 pcf||1.9 psf/in||1.49||125+ psi|
|Aggregate||60-68 pcf||35-41 pcf||1.9 psf/in||0.90||300+ psi|
|Cellular||34-42 pcf||28-36 pcf||2.5 psf/in||1.20||200 psi|
|Cellular||42-50 pcf||38-46 pcf||3.0 psf/in||1.00||250+ psi|
How do you increase the R-value of LWIC?
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.
What is the equilibrium moisture content of cellular concrete?
The equilibrium moisture content of cellular concrete is typically 14%-18% by weight.
When can LWIC be roofed?
Roofing may begin 2-4 days after placement. The base sheet fastener should attain a
minimum 40-pound withdrawal value.
How resistant to weather is LWIC?
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
the United States and in other parts of the world.
If EPS is in place and a threat of rain exists, what is the best procedure to follow?
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.
What types of roof membranes can be installed over LWIC?
Built-up Roofing, Modified Bitumen, and Single Ply Membranes of various approved
compositions have been successfully installed over LWIC.
Are control joints needed for thermal movement in a LWIC system?
LWIC roof decks do not require control joints and control joint filler at the perimeter or vertical projections such as walls and curbs. The initial drying shrinkage of Portland cement based LWIC is many times greater than any possible thermal expansion. Only poured perlite decks require control joints.
Are expansion joints needed for thermal movement in a LWIC system?
Expansion joints in a LWIC roof deck system are only required when there is an expansion joint in the building structure.
Do hairline cracks affect the lightweight insulating concrete?
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.
Is venting required in a LWIC system?
Venting to relieve vapor pressure is dependent upon the structural substrate, the specific
LWIC, and/or the membrane manufacturer.
What is an acceptable finish for LWIC?
A surface finish free of ridges and protrusions is an acceptable finish for roofing
What is the minimum slope required of LWIC?
The minimum slope is 1/8 inch per foot to provide positive slope and to insure that
water will drain. Consult local building codes.
What are the advantages of LWIC over rigid insulation board?
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.
Is there a concern about drying shrinkage cracking of cellular concrete?
Drying shrinkage is a normal occurence of cellular concrete and will not adversely affect the
performance of the insulating concrete roof deck.
Are “cold joints” acceptable in an 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.
With what materials should various roof decks be patched?
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.
How thick is LWIC poured?
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).
What precautions should be taken for top coat casting if snow is forecast?
If possible use heated mix water and/or High Early Strength cement (Type III) to
accelerate the setting time before snow occurs.
What is involved in re-roofing over LWIC?
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.
Is it possible to cast over an existing LWIC roof (re-cover)?
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
How long does LWIC last?
If maintained in a watertight condition, LWIC should last for the life of the building.
How long is the guarantee for LWIC?
For system information, contact the appropriate supplier.
What regulatory approvals exist for LWIC?
All products have FM (wind uplift) and UL (fire resistance) listings in addition to
national and local code approvals.
Is LWIC a green product?
Yes. LWIC has appropriate credits for these requirements.
How high can you pump LWIC?
LWIC has been successfully pumped from the ground to 550 feet. This is dependent on
the local contractor’s equipment capability.
What is the mixing process for lightweight insulating conctete?
Differing amounts of Portland cement, water, aggregates (vermiculite, and/or perlite) and/or preformed cellular foam are mixed together dependent upon specific project requirements.