Wind uplift is what causes roofs to “blow off”. As wind speed across a roof increases, negative air pressure above the roof is created. This negative air pressure, or vacuum, pulls the roof system up.

Wind up lift ratings for roof systems have been created to quantify how much wind up lift a given roof system should withstand. Here is how it works. A roofing material manufacturer sends a piece of its material to Factory Mutual’s testing laboratory. The membrane is installed over roof insulation and a steel deck, intending to replicate a real world installation. Air pressure beneath the roof membrane is increased until attachment failure. The results are recorded and the given membrane along with its attachment fastening pattern receive a wind up lift rating.

FM ratings are typically I-60, I-90, I-120, I-150. FM I-90 means the membrane/ attachment sustained 90 lbs. per square foot pressure. This is equivalent to a 110 mile per hour wind. I-60 is typical across most of the United States. I-90 is common along coastal areas while I-120, and above is now routinely specified in places like the Caribbean.


Ballasted
Often the cheapest, always the most basic method of attachment. To install simply “loose” lay insulation on the deck, roll out the membrane then weight it down with inch and a half, round, washed river rock weighing ten pounds per square foot. The cost savings may be reduced by the need to “beef up” the structure to accommodate the added weight.

Adhered
Simply glue the single ply to the insulation. Typically the insulation is held with screws into the deck, however, you may adhere the insulation to avoid sometimes unwanted screws through the deck.

Mechanical
Insulation is held in place with screws into the deck. Membrane is also attached with screws through the insulation and into the deck.

Equal Air Pressure
Membrane is loose laid without attachment. Specially designed vents are installed in the corners and along the perimeter to equalize negative pressure above the membrane caused by wind speed across the roof. This method is by far the least used.


As it relates to roofing, insulation has two purposes. One is to create a substrate over which the roofing material is installed. The other is to add a thermal barrier (measured as R-Value) to reduce heating and air conditioning costs.

Insulation used to create a substrate is characterized by its low cost, and minimal thickness (normally ½”). Perlite, wood fiber, and high-density recovery board, are all names of products which fit into this category.

When R-Value is the objective the common insulation used today is polyisocyanurate insulation. This material offers an R-Value of approximately 7 per inch of thickness, making it a popular choice.

Most single ply roofing membranes can be installed directly over polyisocyanurate insulation, however, built ups and modifieds – i.e. asphalt systems require new “substrate” insulation.


When is it necessary to “tear off” an existing roof?

Most often when past leakage has caused a significant amount of insulation to become saturated. There are three reasons to replace wet insulation:

1. Wet insulation loses its R-Value.
2. Wet insulation accelerates the destruction of any roofing installed over it, and
3. Wet insulation ultimately causes wood decks to rot and steel decks to rust. (Tectum, lightweight concrete, and gypsum decks are also damaged in the presence of wet insulation).

Occasionally, a complete tear off is necessary to reduce the dead load (weight) to the structure, prior to re-roofing (e.g. If a building presently has 2 or 3 built up roofs in place, it may be necessary to “tear off” before the weight of another roof is added to the structure). Complete tear offs add about 50% to the price or re-roofing exclusive of R-Value insulation replacement.

When a third or less of the roof insulation is wet, consideration should be given to replacing only that which is wet. This will reduce cost but won’t reduce the quality or R-Value of the new roof.