Substrate Boards

The most common use of a substrate board is on steel and wood decks.

The most common use of a substrate board is on steel and wood decks.

STORAGE AND ATTACHMENT

Proper onsite storage of materials is always important, and the substrate board is no exception. The manufacturer’s instructions should be followed. Typically, this parti is to prevent the moisture contamination of the materials. Substrate boards are often hydroscopic (a material that attracts and retains moisture) and need to be protected from free moisture vapor, as well as rain and snow. The manufacturer’s packaging is designed for shipping and not for long-term exposed onsite storage. Manufacturers often require the shipping wrap be removed and the material be covered with breathable tarpulins. In locations of high relative humidities, I suggest at the time of installation that the moisture content of the boards be taken. Those with unacceptable levels should be discarded.

Another item to consider is the pallet weight of many substrate boards: It can approach 1,920 pounds, or 60 pounds per square foot. Therefore, the roofdeck capacity should be checked prior to lifting the palletized substrate boards onto the roof. My firm often requires the pallet to be “broken” in half prior to placement on the roof deck for storage. Substrate boards need to be attached to the roof deck they cover. For steel roof decks, screws and plates work well. I do not recommend the attachment of substrate boards to steel decks with polyurethane bead foam because the propensity for adhesive to de-bond from the steel deck due to the presence of manufacturing oils, wind uplift and/ or construction traffic is too great for my peace of mind.

The number of fasteners needs to be considered, designed and specified, as well. Johnston, R.I.-based FM Global or roof system manufacturers’ requirements are base guidelines and often dependent on the roof system’s wind-uplift design requirements. However, the designer should give consideration to the substrate’s thickness, dimensional stability, and its potential to cup or buckle when subjected to moisture before selecting the correct number of fasteners. It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance. For low-height buildings of all sizes in the Midwest, we have been very successful using a rate of one fastener for every 2 square feet of substrate board. Additional fasteners in the corners and perimeters may be necessary for roof-system performance.

Substrate boards need to be attached to the roof deck they cover. For steel roof decks, screws and plates work well.

Substrate boards need to be attached to the roof deck they cover. For steel roof decks, screws and plates work well.

When installing substrate boards in adhesive, such as asphalt or polyurethane foam, to solid roof decks, the key is to be sure the substrate board is fully engaged with the adhesive. To achieve a full bond, the substrate board should be placed and then weighted in the center and corners—yes, five points of pressure—until the adhesive is set. Only 4- by 4-foot boards should be used for adhesive applications.

A full understanding of the limitations of substrate boards, as well as their attributes, can give the designer a confident feeling that his or her roof system, as designed, will perform as intended.

Hutch’s Pointers

Choose a substrate board that:

  • Can withstand the job-site conditions.
  • Withstand moisture drive.
    Withstand construction-crew foot traffic.
    Can be supplied by the roof-system manufacturer for inclusion into the warranty.
    Is compatible with the vapor-retarder adhesive and material.
    Is compatible with the insulation adhesive.
  • Design and specify the proper attachment methods.
  • Ensure the edges running parallel to steel roof decks rest on top, not the middle, of the roof deck spans.
  • Be sure the attachment of the vapor retarder or thermal insulation above is positive; weighting of the insulation until adhesive is set is not a common specification requirement.

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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About Thomas W. Hutchinson, AIA, FRCI, RRC, CSI, RRP

Thomas W. Hutchinson, AIA,FRCI, RRC, is principal of Hutchinson Design Group Ltd., Barrington, Ill., and a member of Roofing’s editorial advisory board.

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