Retrofitting LWIC


Roof decks consisting of galvanized high tensile steel sections covered with concrete consisting of cement and either perlite, vermiculite, or foam glass aggregates or cellular foam are called lightweight insulating concrete (LWIC) roof decks.   Expanded polystyrene may be used as an insulation that is contained within the poured concrete to add additional insulating values and is referred to as holey board.  This name is due to the holes formed in the boards to allow concrete to fill the holes and form small structural columns to support gravity loads.  LWIC is poured a minimum of 2” over the top of the steel sections or over the holey board to form the structural top of the assembly.  In the past roof coverings were attached by spot mopping with hot asphalt or were nailed to the LWIC with special expanding nails.

LWIC roof decks are structural composite assemblies that create their structural qualities by the chemical bond that occurs between the zinc finish on the galvanized high tensile steel sections and the cement in the LWIC.  This chemical reaction structurally bonds the two components together and under loads they act as a single structural unit to support gravity loads, wind uplift loads and diaphragm shear loads from wind or seismic loads.  As long as this bond exists the assembly exhibits structural properties that comply with building code requirements.  However, should that bond be broken or the monolithic quality of the LWIC slab be fractured the assembly returns to the strength of the individual components.  Collectively they will not provide the strength required to meet code and stabilize the structure under the required loads.  Should this occur, steps must be taken to immediately correct the situation since the building enters a condition defined as “dangerous” by the IBC and one in which the walls of the building could collapse under wind loads.

Prior to retrofitting LWIC roof decks, the roof deck must be examined to determine its structural condition.  If the slab is fractured and broken into small pieces or if the slab has come loose from the steel section, structural composite strength is gone and retrofit action must be taken to restore it.

Additionally, if the building uses conditioned air inside  the structure, the new energy code requires that all conditioned air structures that are retrofitted have the insulation value of the roof deck increased  up to R = 20, 25, 30 or 35 depending on the buildings location.  For either or both of the above conditions, Loadmaster recommends the use of the following procedure:

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  1. Have an authorized Loadmaster Dealer inspect the building and take cuts to determine the exact makeup of the existing roof deck and roof coverings..
  2. Submit inspection data to Loadmaster Technical Department for analysis, engineering design and recommendations with associated engineering reports and specifications to correct the structural composite deficiencies using Loadmaster construction in conjunction with the existing steel deck section.
  3. Taking care to only expose portions of the interior of the building that can be dried-in the same day, remove the roof coverings from the LWIC roof deck.
  4. If necessary, using saws cut the LWIC into manageable squares and remove them from the steel deck section.
  5. As necessary to achieve the required composite strengths, install additional screw fasteners in the side laps and the bearing surfaces of the steel deck sections. Replace any damaged or deteriorated steel sections as necessary.
  6. In a thickness as required, install Loadmaster insulation over the existing steel deck sections followed by a layer of Duraflex Mineral Board and attach both simultaneously to the steel section with screw fasteners.  Attachment to be an engineered pattern to develop the required structural composite strength of the assembly.
  7. If required, tape all board joints with Loadmaster Joint Tape prior to roofing application.
  8. Install roofing membrane as specified.

This process will decrease the dead load on the structure, improve the insulating values of the roof deck and restore the required structural composite properties of the roof deck assembly.  In combination with the new roof covering the building’s roof assembly will be structurally operational as required to provide long term service.

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The selection of the correct roof covering is vital to achieving IBC compliance for the roof assembly.  Only those roof coverings that have been tested as required by the IBC regulations can complete the assembly and make it a lawful assembly. The use of untested and unqualified roof coverings over Loadmaster roof decks place the roof assembly outside the building code and reduces it to an unlawful assembly.  To comply with code regulations the tested and legal roof coverings and manufactures of each are as follows:

Built Up Roofing —————————  GAF – Johns Manville – U, S. Ply, Inc.

Modified Bitumen ————————– Siplast – GAF –  Johns Manville – U. S. Ply, Inc.

PVC Membranes  ————————-  Carlisle – GAF – Johns Manville

TPO Membranes  ————————-  Carlisle – GAF – Johns Manville

EPDM  Membranes  ———————- Carlisle

NOTE:  All roof coverings are to be adhered to the Loadmaster roof deck.  Loose laid or mechanically attached membranes are not code approved.

For more information on roof coverings contact your local Loadmaster Dealer or call the Loadmaster Technical Department at 800-527-4035.

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Any work performed on a Loadmaster roof deck must be performed by a trained and licensed Loadmaster Dealer.  Untrained and unlicensed roofing contractors are not authorized to work on Loadmaster installations. Should that occur the structural composite strength of the assembly will most likely be compromised and the assembly is no longer a Loadmaster assembly and Loadmaster withdraws any and all strength and performance claims for the resulting non-Loadmaster assembly which also will no longer comply with building code regulations.  For a listing of authorized Loadmaster Dealers contact Loadmaster at 800-527-4035.

Market studies and prior bids indicate this process to be competitive in cost to other means of retrofitting LWIC roof deck assemblies. This method however, offers several advantages.  It is quicker to accomplish, it reduces weight on the structure, it cannot lose structural composite strength unless misused and a leaking roof is allowed to decompose the materials.  And the most important quality – IT COMPLIES WITH INTERNATIONAL BUILDING CODE REGULATIONS where most other approaches are unlawful and do not comply!

Read More Here (PDF): Things You Should Know Reroofing Lightweight Insulating Concrete Roof Decks