We have a new building in design with a low sloped roof. Our options are using a sloped rigid insulation or LWIC. Which do you recommend?

Asked by Patrick Barclay
Hayward, CA

The roof deck construction is metal deck and we are in San Francisco Bay area.


Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

August 9, 2012

You have probably already checked with your structural engineer as to whether the the deck was designed to support the Light Weight Insulated Concrete System (LWIC). 

Next this is a budget and schedule question.

  • The LWIC System is a permanent thermal solution and therefore is more costly.
  • Also the System can not be immediately roofed as you must wait for the concrete to properly cure.
  • Curing could take up to a month, so you would really need to check your Farmer’s Almanac to see that you are outside of the rainy season.

Sloped Rigid Insulation System

The Sloped Rigid Insulation can be screwed directly to the decking. If the deck has a low-slope then you may not have to create a slope in the top sheet of insulation.

  • To this you could immediately apply the waterproofing membrane.
  • I would recommend a single ply membrane (with a CRRC Label – Cool Roofing Rating Council).

Light Weight Insulated Concrete

LWIC systems are made up of a combination of components.

The thermal properties of the concrete are not enough to satisfy the R-Values required of roofs, so therefore the system includes Expanded Polystyrene (EPS) insulation sheets which are encapsulated in the concrete between the base and finish coat forming a monolithic unit.

  • A slurry coat is first applied to the substrate (in your case the fluted deck) with an embedded sheet of EPS. Thicknesses of 1” to 16” of EPS board can be stepped to create positive slope-to-drain.
  • As your roof is low sloped you may not need to step the layers of EPS.
  • Varying the topcoat thickness of LWIC creates the final positive slope.
  • Finally, a topcoat of LWIC is applied to create a smooth sloped uniform surface for application of the roof membrane.

Due to the fact that an insulating concrete roof deck is a cast-in-place cementitious product, it cannot be roofed the same day it is installed. As a result, the deck is exposed to the elements for several days until the roofing membrane is applied. Consult with the manufacturer for the required wait period.

Light Weight Insulated Concretes are non-structural with the less dense cellular concretes better suited for thermal applications. Consult with your structural engineer as to what weight is recommended for your structure.

Benefits of a LWIC system

These are some of a LWIC system's significant benefits. 

  • A better substrate for roofing: Sloped rigid insulation on top of the corrugated deck can easily deform with foot traffic which could cause splits and tears to the waterproofing system.
  • Creates an ideal slope to drain.
  • A permanent insulation solution. Inexpensive re-roofing can be done without affecting the insulation.
  • Creates a better diaphragm for seismic and up-lift from wind forces. As you are in CA the former is most important. For that reason you should consult with your structural engineer with this question.
  • A dimensionally more stable system with varying temperature and moisture conditions. Rigid insulation board systems alone have joints that may open under changing environmental conditions.
  • Non-combustible and properly designed systems meet UL listings for fire rating. This means that fireproofing under the steel deck will not be required. Also, since the flutes are filled with the concrete slurry this rectifies a dangerous fire channel if left unfilled. Some UL listings may require the incorporation of a wire mesh.


In calculating the thermal value of the LWIC one of course must factor in the values of the other materials.

  • The light weight insulating concrete will have an R-Value listed per inch.
  • To this you would add the number of layers of EPS required to achieve R-30 which is the California’s required R Value for roof insulation.

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