Mark Zehnal, CPRC, FRSA’s Director of Technical Services
We are regularly asked where someone can find information about what kind of underlayment needs to be used for a particular roof tile or roof tile installation? First, what is the definition for underlayment? Here are three:
■ Asphalt Roofing Manufacturers Association (ARMA), Residential Asphalt Roofing Manual 2014; Underlayment: Asphalt saturated felt or other material typically delivered in a roll, used beneath roofing to provide additional protection for the deck.
■ 2015 Concrete and Clay Roof Tile Installation Manual; Underlayment: A water shedding membrane installed over the roof sheathing, rafters or trusses. The underlayment may be rigid or roll form.
■ FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual Fifth Edition Revised; Underlayment: one or more water shedding layers of roofing applied to a sloped roof prior to the installation of the roof tiles.
These definitions only tell us basic and limited information: that underlayments must be water shedding and in place prior to roof tile installation. There is much more to consider when selecting a roof tile underlayment.
In an asphalt shingle installation, the underlayment serves as the primary water shedding layer until the asphalt shingles are installed and not as an anchor to which the shingles would be primarily attached. Once the asphalt shingles are in place the underlayment functions as a secondary water barrier just in case the water is able to get past the asphalt shingles.
Mechanically fastened roof tile systems are similar to the asphalt shingle installation in that the roof deck is protected by the underlayment until the roof tile are in place. However, as an added protection the roof tile fasteners are generally sealed with an approved roof mastic where asphalt shingle fasteners aren’t. Additionally, the underlayment used with mechanically attached tile roof systems must meet the uplift design pressures for the field, perimeter and corner of the building. During a high wind event roof tiles can be damaged, resulting in the underlayment being exposed to extreme uplift pressures that the now damaged roof tile had been resisting.
For the adhesive set roof tile systems, this is where the similarities end. The underlayment performance qualities for an adhesive set roof tile system require the underlayment to become part of the load path. The load path of a roof system begins at the anchor point or roof deck. Each component of the roof system that shares the load is like a link in a chain that work together to resist being
pulled apart by the load placed upon it, which in this case is wind uplift. The required testing of roof systems reveals the limitations of use based on uplift resistance or design pressures in compliance with the structural chapters in the building code.
The underlayment product approval should provide the installation information listed with accompanying maximum design pressures. Each installation application description should include at a minimum the following information;
■ Deck type (plywood, OSB, primed or unprimed)
■ Anchor sheet if required (fastening pattern provided or direct to deck)
■ Attachment method to anchor sheet or direct to deck (hot asphalt, self-adhered, cold adhesive)
■ Approved roof tile adhesive (compatible)
■ Maximum design pressure
Remember, underlayment used in either an adhesive set or mechanically fastened roof tile system needs to meet the design pressures specific to that building.
Previous Article