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

  1. What are the typical physical properties of LWIC?
  2. What is the equilibrium moisture content of cellular concrete?
  3. How do you increase the R-value of LWIC?
  4. When can LWIC be roofed?
  5. How resistant to weather is LWIC?
  6. If EPS is in place and a threat of rain exists, what is the best procedure to follow?
  7. What types of roof membranes can be installed over LWIC?
  8. Are control joints needed for thermal movement in a LWIC system
  9. Are expansion  joints needed for thermal movement in a LWIC system?
  10. Do hairline cracks affect the lightweight insulating concrete?
  11. Is venting required in a LWIC system?
  12. What is an acceptable finish for LWIC?
  13. What is the minimum slope required of LWIC?
  14. What are the advantages of LWIC over rigid insulation board?
  15. Is there a concern about drying shrinkage cracking of cellular concrete?
  16. Are “cold joints” acceptable in an insulating concrete roof deck?
  17. With what materials should various roof decks be patched?
  18. How thick is LWIC poured?
  19. What precautions should be taken for top coat casting if snow is forecast?
  20. What is involved in re-roofing over LWIC?
  21. Is it possible to cast over an existing LWIC roof (re-cover)?
  22. How long does LWIC last?
  23. How long is the guarantee for LWIC?
  24. What regulatory approvals exist for LWIC?
  25. Is LWIC a green product?
  26. How high can you pump LWIC?
  27. What is the mixing process for lightweight insulating concrete?

What are the typical physical properties of LWIC?

Material Type

Cast Density

(pcf)

Air Dry Density

(pcf)

Weight per Inch

(psf)

R Value per Inch

(BTU Units)

Compressive

Strength (psi)

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
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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.

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What is the equilibrium moisture content of cellular concrete?

The equilibrium moisture content of cellular concrete is typically 14%-18% by weight.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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What is an acceptable finish for LWIC?

A surface finish free of ridges and protrusions is an acceptable finish for roofing

application.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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

additional information.

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How long does LWIC last?

If maintained in a watertight condition, LWIC should last for the life of the building.

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How long is the guarantee for LWIC?

For system information, contact the appropriate supplier.

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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.

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Is LWIC a green product?

Yes.  LWIC has appropriate credits for these requirements.

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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.

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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.

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