Roof System Fasteners

by: Alvin Nunnikhoven, RRC

The use of mechanical fasteners in the roofing industry has substantially increased over the past 20 years. One reason for their increased use relates to Factory Mutual's rejection of asphalt bitumen as an acceptable insulation attachment method to steel roof decks. The popularity of lightweight, mechanically fastened single-ply roofing systems is another reason.

A wide variety of fasteners are also available for use with other deck types, i.e., concrete, wood, gypsum, cementitious wood fiber, lightweight concrete, etc. This article will focus on the various types of fasteners available for use in roofing applications.

Steel and Wood Deck Fasteners
These fasteners are typically #12 or #14 self-drilling, self-tapping screws with a low profile head. The #12 screws are generally referred to as "standard" fasteners for use on 24-gauge to 20-gauge decks. The #14 screws are "heavy duty" fasteners designed for use on 20-gauge and 18-gauge steel decks, but may also be used on lighter gauge decks when increased strength is desired. The fastener should be sufficient length to penetrate through steel and plywood decks a minimum of 1/2 inch, and penetrate a minimum of 1 inch into wood plank decks. The fastener should be manufactured using carbon steel and have a corrosion-resistant coating that meets the requirements of FM Approval Standard 4470.

With the more stringent wind uplift testing procedures implemented by Factory Mutual, several single-ply membrane manufacturers have worked closely with the fastener manufacturers in designing fastener enhancements to meet these new wind uplift requirements. By reducing the drill point size and increasing the number of threads per inch on the fasteners, these new membrane fasteners have exhibited increased pullout strengths over the traditional fasteners. These fasteners are typically for use on steel roof decks only.

Wood Deck Fasteners
The mechanical fasteners discussed above, may also be used for attaching roof insulation and/or single-ply membranes to wood plank or plywood decks. When securing base sheets or conventional roofing felts, cap nails are usually used. The nail is manufactured from 12-gauge wire with an annular groove shank and a minimum 1inch cap. The quantity and spacing of the nails is determined by the pullout strength of the nail, the anticipated wind speeds, and the roof height above the ground.

Concrete Deck Fasteners
Threaded screw-type fasteners and compression-fit fasteners are available for insulation or membrane securement into concrete roof decks. The fasteners range from 3/16 inch to 1/4 inch in diameter, and typically provide pullout strengths ranging from 1,000 to 1,500 pounds.

A concrete deck must be pre-drilled before the fastener can be installed. Proper embedment of the fasteners into the concrete is critical in order to provide the proper pullout resistance. Fastener manufacturers recommend a minimum 1 inch embedment into the concrete. To achieve minimum fastener embedment, they also recommend the pre-drilled hole depth be a 1/2 inch greater than the desired embedment depth.

Before deciding to mechanically fasten materials into a concrete deck, the specifier, owner, or roofing contractor must make certain the deck thickness will accommodate pre-drilling. To prevent concrete spalling, the concrete should be a minimum of 1-1/2 inches to 2 inches thick below the pre-drilled hole. Several types of pre-cast concrete deck panels cannot be fastened into due to their thin profile and the damage inflicted upon the panel by the fastener installation. The building owner must also take into consideration the noise levels associated with pre-drilling into the concrete.

Gypsum, Lightweight Concrete, and Cementitious Wood Fiber Deck Fasteners
Several types of fasteners are available for use on these deck types. A variety of fasteners are designed to attach base sheets and roofing felts to these decks, such as rectangular split-shank fasteners, hollow cone-shape fasteners, serrated split-shank fasteners, or two-piece tube with nail insert fasteners. The quantity and spacing of these fasteners is dependent on the pullout strength of the fastener, the anticipated wind speeds, and the roof height above the ground.

Specialty fasteners are also available for securing insulation and/or roof membranes to these deck types. One such fastener is a toggle bolt. With the toggle bolt, 3/4 inch holes must be pre-drilled through the insulation and decking. The toggle bolt must be of sufficient length to allow the toggle wing to expand once it clears the bottom of the deck. When initially marketed for use, toggle bolts experienced less than desired performance. The toggle wings would turn loose when exposed to dynamic movement from wind forces. The inclusion of wire loops or low-melt adhesives (on fastener threads) has helped to prevent turning of the toggle wings.

The plastic non-thermal bridging fastener is another specialty fastener used for securing insulation and roof membranes to these deck types. This fastener also requires a hole to be pre-drilled into the deck. The fastener is installed with a special fastener tool. Once the fastener is set into the pre-drilled hole, the tool drives steel pins enclosed within the fastener that exit through the bottom of the fastener. These steel pins form two hooks that bite into the deck and provide much improved pullout resistance. With this fastener, the specifier, owner and/or roofing contractor must make certain that the deck will accommodate the required pre-drilling.

With the variety of fasteners available for use in today's roofing market, it is imperative that the proper fastener be selected for securing the roofing materials to the roof deck substrate.