Sep 2016

Update for Retrofitting Standing Seam Roofs with External Seam Clamps for Enhanced Wind Uplift Resistance

After our March 2016 article entitled “Retrofitting Standing Seam Roofs with External Enhancement Elements: Clamps,” FM Global updated and revised their Property Loss Prevention Data Sheet 1-31, Panel Roof Systems. The data sheet was completely revised and re-titled from the previous edition and now includes information regarding the use of external seam clamps to increase the wind resistance of standing seam roofs for new installations and retrofitting of existing roofs.

As we previously noted, the wind-uplift capacities of standing seam roofs vary considerably depending on the components used and their means of attachment. Wind uplift in the roof corner and perimeter areas can be significantly higher than that experienced in the field of the roof; hence, additional securement is typically needed in these areas.

While there are several differing failure modes that can occur with standing seam roofs during a severe windstorm, one of the most common is the standing seam panels bowing between the panel ribs and releasing from the internal clips that are attached to the secondary structural steel members (i.e., purlins or joists). Other failure modes include pullout of the screws securing the internal clips to the purlins or joists or breakage of the internal clips themselves.

The wind uplift resistance of a standing seam roof assembly may not increase linearly with a decrease in the area per internal clip. Comparative test data by FM Global shows that, in some cases, cutting the clip spacing in half results in only a one-quarter to one-third increase in wind uplift resistance.

To optimize the wind resistance of the standing seam roofing panel, it should stay attached to the internal clip until approximately the same pressure that causes the clip to break or the clip screws to pull out of the secondary structural member. There are two methods of enhancing the securement of the standing seam panels to the internal clips; installing external seam clamps or through-fastening of the panels to the secondary structural members.

Installing external seam clamps is the preferred way to enhance wind-uplift resistance of standing seam roof systems as it does not compromise their thermal movement capabilities. In addition, if through-fastening is used, added precautions must be taken to prevent roof leakage.

Selecting the correct external clamps is essential to ensure that they fit properly on the deck seams. The clamps used should be based on the roofing system and the type of seam arrangement (vertical or horizontal) in place. The device may be a proprietary assembly supplied by the roof system manufacturer. In some cases, clamps originally intended to secure snow guard systems can be used.

Tests by FM Global have shown that, in some cases, properly fitting external seam clamps can double or triple the wind pressure resistance of the same SSR system without the clamps, all else being equal.

If your facility has a standing seam roof and you would like to confirm that it is properly secured to prevent against wind uplift, contact Risk Logic.