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?


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

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

Answer this question


Cynthia Phakos's picture

You have probably already checked with your structural engineer as to whether the thedeck 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 thermalsolution and therefore is more costly.
  • Also the System can not be immediately roofedas you must wait for the concrete to properly cure.
  • Curing could take up to a month, soyou would really need to check your Farmer's Almanac to see that you are outside of therainy season.

Sloped Rigid Insulation System

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

  • To thisyou could immediately apply the waterproofing membrane.
  • I would recommenda 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 ofthe concrete are not enough to satisfy the R-Values required of roofs, so therefore thesystem includes Expanded Polystyrene (EPS) insulation sheets which are encapsulated inthe 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) withan embedded sheet of EPS. Thicknesses of 1" to 16" of EPS board can be steppedto 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 surfacefor application of the roof membrane.

Due to the fact that an insulating concrete roof deck is a cast-in-placecementitious product, it cannot be roofed the same day it is installed. As a result, thedeck 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 concretesbetter suited for thermal applications.Consult with your structural engineer as to what weight is recommended for yourstructure.

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 caneasily deform with foot traffic which could cause splits and tears to the waterproofingsystem.
  • Creates an ideal slope to drain.
  • A permanent insulation solution. Inexpensive re-roofing can be done without affectingthe insulation.
  • Creates a better diaphragm for seismic and up-lift from wind forces. As you are in CAthe former is most important. For that reason you should consult with your structuralengineer 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 changingenvironmental conditions.
  • Non-combustible and properly designed systems meet UL listings for fire rating. Thismeans that fireproofing under the steel deck will not be required. Also, since the flutesare 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 thevalues of the other materials.

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