Mechanical Fasteners

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 57 October 2006

Mechanical Fasteners

by Chad Kaminski, RRO

Incorrect fastener installation, when used for securement of the insulation and/or roof membrane, can cause serious roof problems, including membrane blow-offs.

It is a widely accepted practice to use an approved fastener and plate to secure the roof insulation and/or roof membrane to the substrate. However, problems occur when the wrong fastener is used or improperly installed as discussed in the following scenarios.

During a routine roof survey of a large industrial complex, a partial blow-off of a modified bitumen roof was discovered. The initial review of the damaged roof components showed the insulation plates and screws were properly placed in the steel deck and damage likely occurred due to high winds, which exceeded the roof’s design requirements. The fasteners and plates were no closer than six inches from the edges of the insulation, evenly spaced, and did not appear to be over driven. However, while extracting a fastener counterclockwise to verify its length and condition, it simply spun but did not back out. The fastener was apparently stripped out or damaged, so the plate was lifted to aid in the extraction. As the plate was lifted, the screw also lifted approximately 1/8”, then stopped and was backed out for examination. The screw measured 6” long, showed no corrosion or damage and looked like new. This was the perfect screw if the insulation was 4.5” to 5” thick, but as this insulation was less than 3” thick, it didn’t hold.

On longer screws the threads do not extend all the way up the shank, as only 3/4” of deck penetration through the top deck flange is recommended, so engagement of the bottom rib does not occur. While rib depth does vary, typical depth is 1.5”. Not installing threads up the entire shank also saves money. The contractor had utilized fasteners long enough to engage the top and bottom ribs, and in doing so, the fastener was so long that when the screws penetrated the top flange, there were no threads to provide constant tension and compression of the insulation to the deck. Winds had been lifting the roof system up off the deck in an ongoing ratcheting motion until the insulation failed at the fastener locations. This roof was less than two-years old when it failed.

During the design phase of a failed mechanically attached single-ply system, incorrect fastener installation again appeared to be the culprit. This mechanically fastened reinforced single-ply roof system was secured to a cast-in-place concrete deck with steel spikes and 2” barbed plates at 18” on center. The fastening rate met the manufacturer and code requirements for the timeframe when it was installed.

This particular roof had also survived previous storms and hurricanes, but eventually failed during such an event. It failed because the fasteners were pulled from the deck. The system used 3” drive pins to secure 2” insulation, resulting in 1” fastener embedment. This also met the roofing manufacturer’s criteria, but a closer look at the drive pin revealed the paint had not been scraped off the shank of the three fastener compression points, but rather only the lower two. This indicated that the fastener was not fully driven into the predrilled hole. This could have been caused by many things:

When a hole is predrilled it must be 1/2” deeper than the fastener penetration to ensure drilling debris does not restrict penetration. The fastener must be fully set (i.e. hammered into the hole), and the fastener must be of sufficient length. A check of the hole depth showed the predrilled hole was indeed deep enough. The fastener size met the issued specification, and a check of existing fasteners revealed they were properly driven, thus ruling out the three requirements described above.

The fastener failed because in fact it was not long enough. Although the membrane manufacturer stated 1” embedment was adequate, the fastener manufacturer required 1 1/4 - 1 1/2” embedment. Fastener withdrawal tests revealed the existing 1” fasteners had 450 to 500 pounds of pull-out resistance, while a 1 1/4” fastener would have 1100 pounds of pull-out resistance and 4000 pounds for a 1 1/2” fastener. This is a huge difference in strength, and ultimately contributed to this roof failing during a high wind event.

In both cases, the correct fastener was used, but it was not the correct length. When designing and installing a roof system, the proper size and type of fastener must be used or failure can result.