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.

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Light Weight Insulating Concrete Roof Deck Systems

  1. What are the typical physical properties of LWIC?
  2. How do you increase the R-value of LWIC?
  3. When can LWIC be roofed?
  4. How resistant to weather is LWIC?
  5. What types of roof membranes can be installed over LWIC?
  6. Are control joints needed for thermal movement in a LWIC system?
  7. Is venting required in a LWIC system?
  8. What is an acceptable finish for LWIC?
  9. What is the minimum slope required of LWIC?
  10. What are the advantages of LWIC over rigid insulation board?
  11. How thick is LWIC poured?
  12. What is involved in re-roofing over LWIC?
  13. Is it possible to cast over an existing LWIC roof (re-cover)?
  14. How long does LWIC last?
  15. How long is the guarantee for LWIC?
  16. What regulatory approvals exist for LWIC?
  17. Is LWIC a green product?
  18. How high can you pump LWIC?

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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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

theUnited States and in other parts of the world.

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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?

No.

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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,

positivedrainage, dimensional stability, moisture resistance, and reroofable.   LWIC is

permanent, non-combustible, strong, economical, approved, and roofing compatible.

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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 is involved in re-roofing over LWIC?

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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