Our typical focus on roof assemblies is the quality of the waterproofing layer and the insulation directly under the membrane. Attachment requirements for membranes are typically addressed by a reference in the project documents pointing to the FM Global Loss Prevention Data Sheet 1-49, or by a reference to fastener withdrawal pounds force 'in all directions'. Unfortunately, there is usually little or no information provided to effectively secure this critical component to meet wind loading requirements.

FM Global Property Loss Prevention Data Sheet 1-49 (LPDS 1-49) states:

"The majority of roof covering failures resulting from windstorms involve improperly designed or constructed perimeter flashings."

As the perimeter of the roof assembly sustains the highest wind loads during wind events, the proper application of perimeter woodblocking will serve to strengthen the roof system by providing a strong nailing base for the attachment of metal flashings.

Woodblocking is also the point of mechanical attachment for the roof membrane. This is potentially more critical for ballasted systems where winds can scour gravel, minimizing or eliminating ballast load at critical areas of the roof. Adhered and mechanically attached systems have the added advantage of physical bonding or attachment to the deck. However, all of these roof assemblies depend on woodblocking strength to resist wind loads on flashings and perimeter membrane. The loss of attachment to the woodblocking can potentially mean the loss of the roof system, at least at the perimeters and corners.

In general, there are three types of loads perimeter woodblocking anchors must withstand when securing a roof system and perimeter flashings (See Figure 1):

  1. Tensile Load: Applied parallel to the axis of the anchor
  2. Shear Load: Applied perpendicular to the axis of the anchor
  3. Oblique Load: Also known as a combination load, it applies stress with the qualities of both a shear load and a tensile load

Load conditions at the perimeter vary depending on wind speed, perimeter conditions and substrate material.

Recommendations for Securing Perimeter Woodblocking

Both FM Global LPDS 1-49 and many roofing manufacturers provide recommendations and guidelines on woodblocking construction and attachment. FM Global issues recommendations only for concrete, masonry and steel decks in the Loss Prevention Data Sheets. For concrete and masonry, FMG recommends a minimum ½" diameter corrosion resistant anchor, combined with a minimum 1" diameter bearing washer embedded into the structural substrate. It is further recommended that the anchor and washer be recessed into woodblocking at least 1 ½" thick, spaced at a maximum of 48" o.c. (24" at corners). For steel decks, FM Global recommends ¾" diameter bolts tapped into a structural steel member or bar joists spaced between 48" and 72" o.c. depending on perimeter conditions.

Though roofing manufacturer guidelines do address a variety of attachment methods and formulas, they do not address safety factors related to specific deck types, instead providing general recommendations for spacing and attachment. The phrase most commonly found in guideline specifications is "attachment of woodblocking to resist a minimum pull-out resistance of 175 lbf/ft in all directions". In some specifications, this recommendation has been increased to 350 lbf/ft in all directions.

While these guidelines might provide a useful starting formula for attaching woodblocking, the following recommendations addressing specific deck types, and based on laboratory and field testing, provide detailed guidelines for woodblocking attachment.


For buildings with concrete decks, the fastener design load should not be less than 250 lbf/ft after application of a 4:1 safety factor. For ballasted systems, this value should be increased by 20% (300 lbf/ft). The pull-over value should not be less than 125% of the design load and, if necessary, a larger bearing washer should be utilized to achieve this requirement.

A variety of different fasteners and anchors can be utilized to achieve these recommendations, though certain conditions, such as concrete with a compressive strength of less than 2,500 psi or thickness less than 2 ½", will require on-site performance testing to ensure design criteria are being met.

Table 1: Anchor type, size and spacing criteria for concrete decks

 Anchor Type 




 Wedge Anchor


 40" o.c.

 32" o.c.

 Sleeve Anchor


 30" o.c.

 24" o.c.

 Threaded Concrete Anchor


 12" o.c.

 9" o.c.

 Drive Anchor


 12" o.c.

 9" o.c.

Power actuated (PAT) fastening is not recommended for woodblocking attachment to concrete as achieving the industry accepted safety factor of 8:1would require too many fasteners to meet design criteria. Moreover, dynamic load conditions at woodblocking attachment points and other safety issues preclude PAT fastening from being an effective alterative.


Woodblocking attached to 18 to 22 guage steel roof deck can be carried out using standard diameter roofing fasteners including #12 (0.209") and #14 (0.240") fasteners. These fasteners have an average withdrawal resistance of 425 lbf and 480 lbf from 33 KSI, 22 gauge steel decking, respectively. The industry accepted margin of safety is 1.5:1 with a pull-over value of not less than 125%, if additional bearing washers are not employed.

Where woodblocking is attached perpendicular to the deck flute, it is recommended that fasteners be positioned over the high flanges of the deck at 12" o.c. (6" in corners) intervals. Where woodblocking runs parallel to the flutes, similar spacing is recommended, with the addition of ¼" diameter self-tapping screws through the woodblocking deck and bar joist, spaced not greater than 7' apart. Also, since the possibility exists that the deck is poorly attached to structural components, especially on re-roofing projects, the decking can be mechanically attached to the bar joists with either a self-tapping fastener or a #5 pt. self-driller with a minimum 7/8" bearing washer.

For those metal decks using light gauge metal (less than 22 ga.), the following formula can be utilized to determine fastener spacing: Xmn × FS = Xfst ÷ MS (Xmn = Minimum withdrawal resistence = Known (1); FS = Fastener spacing = Unknown; Xfst = Average fastener withdrawal = known; MS = Margin of safety = 2). As with standard metal decks, woodblocking attached parallel to the ribs should be secured to steel angles, or mechanically secured to bar joists using self-trapping or self-drilling fasteners.

"Lightweight" Decks (Gypsum, Tectum, Lightweight Insulating and Cellular Concrete)

Due to the low density of these deck materials and the load combinations they sustain, attaching woodblocking to lightweight decks is somewhat of a challenge. In general, the deck should not be used as an attachment substrate unless the chosen anchor can clamp to the underside of the deck or attach to a structural member, and achieves not less than a 425 lbf ultimate load.

In order to determine fastener spacing, the following formula with a 4:1 margin of safety should be used: Xmn × FS = Xfst ÷ MS (Xmn = Design withdrawal resistance = 250 lbf/lineal foot; FS = Fastener spacing = Unknown; Xfst = Average fastener withdrawal resistance = Known; MS = Margin of safety = 4). Toggle bolts are not recommended for fastening as they require a large hole for installation and rely on a trunion nut to hold the toggle rod to the wing, creating the potential for horizontal movement of the woodblocking and greater internal pressure to the underside of the woodblocking and the membrane at the point of attachment.

Woodblocking can also be attached to vertical walls (See Figure 2). In these cases, the woodblocking should have a minimum thickness of 1 ½" with fasteners spaced not greater than 12" apart. Each anchor should have a minimum withdrawal resistance value of 800 lbf. Larger diameter threaded concrete anchors or hammer-in anchors are preferred in order to draw the blocking tight to the substrate.

Following recommended guidelines for woodblocking attachment is critical in preventing catastrophic damage caused by poorly maintained or improperly installed roof components. If installed to the proper specifications, perimeter woodblocking can play a crucial role in strengthening and protecting a building's roof system, no matter what design or material is being utilized.

March 2008
Issue Two, Volume Three

This article by Colin Murphy.

Colin Murphy is a founder and managing partner of Trinity | ERD.

Trinity | ERD


"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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(1) For Standard Systems: Xmn = 250 lbf/lineal foot; For Ballasted Systems: Xmn = 300 lbf/lineal foot