By: Madelin Mule
May 26, 2021
Roof deck substrates on low slope and steep slope roof can be comprised of a variety of possible materials, including concrete, wood, terra cotta, metal panel and Tectum. On a low slope roof system it is common that the deck is dead level deck without any positive pitch to promote drainage. Often a concrete deck surface presents this condition that requires a tapered insulation system, an inverted roof assembly or possibly a lightweight insulated concrete (LWIC) system. The LWIC can be a great option for a new or replacement assembly since the weight of the system is user-friendly when deadload capacities are limited. The LWIC consists of two key components: lightweight concrete and polystyrene foam insulation boards. Typically, a layer of lightweight concrete is installed over an existing roof membrane followed by the polystyrene insulation boards. Another layer of the lightweight concrete is then installed to fill in any gaps or holes in the insulation boards, creating a bond between the layers and a permanent insulated substrate for a new roof assembly. The lightweight system will add a load of approximately 9 psi while wet and 5-7 psi when fully cured.
Due to the workability of the lightweight concrete and the various thicknesses of the insulation boards, the desired roof slope can be easily achieved upon installation. The thickness of the insulations boards will decrease in a step format towards the roof drain(s), creating an outline for the new slope. The top layer of concrete will then refine and smooth out the slope to facilitate proper drainage.
LWIC systems also provide excellent R-values. The whole system generates an approximate R-4 value per inch, making the minimum required R-value simply attainable. The current required R-value in New York City for roof assemblies is R-33, as per section R402.2 of the 2020 Energy Conservation Code. The LWIC System is a non-combustible system with average fire ratings ranging from 2-3 hours. This system can contribute towards achieving a Class A roof assembly, in that it is “effective against severe fire test exposure,” as defined in section 1505.2 of the 2014 NYC Building Code. To ensure the system is a Class A assembly, look for an Underwriter Laboratories approval of the system.
Although there are many benefits to using a LWIC system, there are a few items to be aware of when choosing to move forward with this system. As the R-value and fire rating increases, the thickness of the overall system increases. This increase in thickness will affect the height of the existing rooftop elements, such as parapet walls, railings, skylights and door saddles. Under the current NYC Building code, protective guards are required to be a minimum height of 42” above the finished roof surface. Roofing manufacturers require a minimum flashing height at door saddles and skylights that must be met in order to obtain a warranty for the new roof assembly. Such rooftop elements may have to be raised or adjusted to meet these code and manufacturer requirements. It may be wise to account for this by including an alternate in the project proposal form. Another item to keep in mind would be the roof’s load capacity. Although the system is lightweight, an assessment of the roof’s structural integrity should be performed to ensure that it is capable of carrying the additional dead load of the new concrete system.
The LWIC is also a great substrate for a future roof replacement. The membrane system may be removed and replaced over the existing LWIC system and potentially bring a significant cost savings in the future.