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You're LightWeight Concrete Questions Answered

If you have any questions, check out this compilation that we have created with the most frequently asked queries to make your concrete roofing process a little easier. If a question is not listed here, feel free to reach out.

LWIC stands for LIGHTWEIGHT INSULATING CONCRETE. LWIC is a product derived from a combination of Portland cement, water, and preformed cellular foam. These ingredients get mixed together on-site and poured in place to form a lightweight insulating concrete roof deck.

LWIC systems produce a monolithic insulation system with increased wind uplift and fire ratings. It is considered a Green Building Material and can be used in eco-friendly buildings. 

Yes, LWIC provides a monolithic system that does not allow the same amount of stress on a roof membrane that you would typically see from a rigid insulation system. The joints between the boards in a typical rigid insulation system impart mechanical stresses on the attached membrane during the heating and cooling of the day, weakening the membrane significantly over time.

NO! Once LWIC achieves its initial set, the product does not expand to its original wet volume; therefore control joints and/or filler are NOT required.

NO! Expansion joints are not required for the LWIC itself. However, when required to contain structural movement, structural expansion joints should be built to extend through the LWIC and not “buried” in the LWIC pour.

In Florida, LWIC has approvals over metal roof decks, poured in place concrete, precast concrete, and existing roof systems. In the case of metal roof decks, the deck should have a minimum G-90 coating per Florida Building Code.

While the Florida building code states the required venting for metal decks is between 0.5% and 1.5%, BOAF has released a letter allowing for the use of non-vented metal decks when a sufficient alternate means of venting the lightweight is used.

The suggested equilibrium moisture content of a LWIC system is typically between 14% and 20%. However, moisture contents that exceed 30% are typically not an issue to the roof system. As a side note, it should be taken into consideration that typical wood products can have an EMC of 40% or more yet are considered a “dry product.”

NO! The LWIC application creates a chemical bond with the zinc in the galvanized coating. LWIC does not bond well with painted surfaces and may compromise the overall system uplift rating.

LWIC bonds to a metal deck in 2 ways. The first is by mechanical attachment to irregular surfaces such as vent slots, sheet laps, weld washers, etc. The second is by a natural reaction of the galvanized coating with the alkaline components of the Portland cement that creates a chemical reaction that binds the two components together.

The suggested waiting period prior to installing roof membrane over LWIC is 2-4 days, or generally, once the deck can handle foot traffic and can achieve a minimum of 40lbs of withdrawal from a base ply fastener. Typically, most LWIC manufacturers will want to see their product dried-in within 15 days of installation to avoid warranty issues.

NO! There are several differences between sampling and testing these two products. With LWIC, the sample is taken from the point of placement (roof level), vs. the point of delivery (concrete mixer), and the samples should not be rodded or vibrated in any way prior to curing. Additionally, LWIC should not be artificially dried prior to testing. Provided these criteria are strictly followed, testing procedure ASTM C 495 should be used to test for LWIC compressive strength.

Normal installation techniques can be used when the expected temperatures are at or above 40 degrees. When temperatures are expected to be below 32 degrees within the first 24 hours of the initial pour, the installer should use his discretion in proceeding with the installation. In temperatures between 32 and 40 degrees, there are alternate means with which you can successfully install an LWIC deck. These alternates include heated mix water, increased cement content and/or chemical accelerators.

Once roofed, the LWIC is extremely resistant to weather events, including hurricane conditions. Prior to roofing, the LWIC can be susceptible to weather events within the first 24 hours of the pour, and precautions should be taken to avoid any damage. After the LWIC achieves its initial set and prior to roof membrane installation, LWIC can absorb excess water in the form of rain. Depending on the extremes of the condition, excess water can be removed by simply allowing the deck to dry out or by other mechanical means.

If the threat of rain is imminent, the LWIC contractor may make the decision to not pour for the day or pour the LWIC in part. Depending on the temperature, a finished LWIC deck can take as many as 6 hours or even more before the surface could handle a significant rain event. If the EPS insulation is installed and cannot be “topped out” before the rain, LWIC should be poured over the insulation in a thin layer to fill the bond holes; this helps keep the water from settling below the insulation and separating it from the substrate.

There are a multitude of Florida Product Approvals for roof membranes over LWIC decks. These include both adhered and mechanically fastened systems. 

Typical roofing systems that are tested and in compliance with LWIC are Single-ply, Modified Bitumen, and Built-up Roofing systems.

NO! While not aesthetically pleasing, these cracks do not effect the physical characteristics of an LWIC deck. Hairline cracks are the result of the surface of the LWIC deck drying at a different rate as the rest of the “body” of the deck and typically do not carry through the full thickness of the system.

While some types of roof membranes (such as a single-ply) may require a smoother finish than others, the finished surface should not exhibit extreme ridges or tool marks, “bird baths”, or protrusions that would damage the membrane or broadcast through the finished roof.

Slope is obtained by the use of stair-stepping insulation boards to create the overall drainage pattern of the finished system. The LWIC topping is added to cover the insulation and provide the final, smooth and monolithic surface suitable for roofing. In new construction projects, the minimum slope required is ¼” per Florida Building Code. 1/8” per foot slope may be used in cases such as reroofing projects, provided the local municipalities are consulted for acceptance.

The use of a “cold joint” is an acceptable means of terminating a pour for the day. However, when doing so, the cold joint should be squared off cleanly the full depth of the topping layer and should not be placed along the seams of a row of insulation.

Many LWIC manufacturers have specific materials and/or procedures for patching their product. Where a specific product is not available, LWIC should be patched with cementitious material of equal or greater strength.

Total deck thickness is dependent on many factors, especially the specified R-Value for the system. Most LWIC systems require a minimum 2” thickness of LWIC over a given thickness of EPS insulation to achieve a total system R-Value. Contact your FloriCrete to determine what thickness of LWIC may be required to achieve the specified R-Value of your project.

YES! In all cases, an existing LWIC deck should be evaluated for its condition and strength prior to the commencement of reroofing. Suppose the existing LWIC is found to be a suitable substrate for reroofing. In that case, the old membrane is removed, any holes in the LWIC are quickly patched, and the reinstallation of the new roof system may commence immediately.

Provided that the LWIC has not been physically damaged or has fallen victim to water intrusion, a properly installed LWIC can last the typical lifetime of a commercial building (40-60 years). The LWIC deck itself does not contain any products that deteriorate easily when properly protected. Therefore you can rest assured that your deck should last a very long time. LWIC has been placed in the U.S. for well over 60 years. There are still fully functional buildings standing today with the original LWIC decks in place.

On a square foot basis, typical R-30 LWIC decks weigh in the range of 9 lbs per sqft (utilizing a standard 2” thick pour). As the EPS insulation weighs in at less than 1 lb per cubic foot, the impact of any additional weight of increased thickness of EPS needed to achieve a higher R-Value rarely impacts the overall weight of the system.

NO! It has been determined that LWIC is an insulating product only and is not structural in nature. Although the use of LWIC does provide additional rigidity diaphragm action to the overall roof structure, LWIC is not considered a structural component.

NO! There is no specific license required by the CILB for LWIC installers, although most of Florida’s LWIC installers hold licenses such as General Contractor, Registered Building Contractor, Registered Roofing Contractor, etc.

LWIC decks poured over metal roof deck may allow the omission of the costly spray-applied fireproofing of the underside of the metal deck. Structural beams, bar joists, etc., are NOT exempt from fireproofing in these situations. Contact FloriCrete to determine if this option is right for your project.

Yes! Unlike Rigid Board installations that require a complex set of metal fasteners that can transfer heat and cold from the inside of the building to the outside, LWIC systems fasteners typically do not penetrate deeper than 2” in the surface of the lightweight insulating concrete. Therefore a thermal bridge is NOT created.

The concept of quality control during the placement of a LWIC deck is based on measuring and controlling the wet density of the LWIC during the installation. Wet Density Checks should be made frequently throughout the duration of the LWIC pour. These density checks entail weighing a specific volume of the LWIC and ensuring the weight derived is consistent with the weight equal to or greater than the determined weight of a specific mix design. These Wet Density Checks should be logged consistently and kept as part of the job record.

The concept of quality control during the placement of a LWIC deck is based on measuring and controlling the wet density of the LWIC during the installation. Wet Density Checks should be made frequently throughout the duration of the LWIC pour. These density checks entail weighing a specific volume of the LWIC and ensuring the weight derived is consistent with the weight equal to or greater than the determined weight of a specific mix design. These Wet Density Checks should be logged consistently and kept as part of the job record.

Making LWIC can be a rather complicated process for a simple result. Typical installations are fully “batched” on a jobsite using (at a minimum) 3 main components, dry Portland cement, water, and a foaming agent. The three main components are mixed at predetermined volumes in a combination mixer/placer. Once thoroughly mixed to a smooth, homogenized consistency, the mixer unloads its contents into the placer (or pump). The pump then pushes the mixture through as a series of connected hoses to the roof or other point of placement.

Water used for mixing LWIC must be potable, possess no excess chlorides and derived from a source that can handle the volume of water needed to provide the volume of water necessary for a continuous and seamless install. Typical installations would require the use of an active fire hydrant to maintain the volume and pressure needed. At up to 50 gallons of water required per yard of LWIC, the volume of water required would not allow the use of a simple ¾” hose bib as an adequate source of water.

You can find more information about LWIC by contacting the office for Flori-Crete. Barring that, the Florida Roof Deck Association (FRDA) is a Florida-based association created solely to tackle the concerns of the Lightweight Insulating Concrete Industry in Florida. With its own Building Code, expansive coastlines, increased exposure to hurricanes, etc. Florida has some unique challenges that you don’t find elsewhere in the country, and FRDA assembles local applicators, manufacturers, and vendors to meet the challenges befall the industry in the Sunshine State.

There are many tested and approved roofing systems utilizing LWIC decks for Enhanced Hurricane Protection Assemblies (EHPA). These assemblies are accepted for use in Florida Public Schools and other Public Hurricane Shelters. In fact, a survey done by members of the FRDA found that the damages sustained over the past 20 years to LWIC decks were SIGNIFICANTLY lower than the Rigid Board insulated decks when faced with hurricane-force winds. In many cases, the LWIC decks and their attached roof membranes were left perfectly intact, while a neighboring building with a Rigid Board system sustained $10,000’s of damage.