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.
Frequently Asked Questions
1. 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 |
2. WHAT IS THE EQUILIBRIUM MOISTURE CONTENT OF CELLULAR CONCRETE?
The equilibrium moisture content of cellular concrete is typically 14%-18% by weight.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. 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.
12. WHAT IS AN ACCEPTABLE FINISH FOR LWIC?
A surface finish free of ridges and protrusions is an acceptable finish for roofing application.
13. WHAT IS THE MINIMUM SLOPE REQUIRED OF LWIC?
The minimum slope is 1/8 inch per foot to provide positive slope and to insure thatwater will drain. Consult local building codes.
14. 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.
15. IS THERE A CONCERN ABOUT DRYING SHRINKAGE CRACKING OF CELLULAR CONCRETE?
Drying shrinkage is a normal occurance of cellular concrete and will not adversely affect the performance of the insulating concrete roof deck.
16. 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.
17. 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.
18. 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).
19. 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.
20. 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.
21. HOW LONG DOES LWIC LAST?
If maintained in a watertight condition, LWIC should last for the life of the building.
22. HOW LONG IS THE GUARANTEE FOR LWIC?
For system information, contact the appropriate supplier.
23. 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.
24. IS LWIC A GREEN PRODUCT?
Yes. LWIC has appropriate credits for these requirements.
25. 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.
26. 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.
27. 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.