What Are DP Window Equivalents?
Design pressure (DP) is a measurement in pounds per square feet of the amount of wind stress a window can sustain. In areas prone to hurricanes and hurricane-force winds, homes contain windows with different DP ratings. These ratings measure how well a window will sustain high-force winds. Homes without DP-rated windows must have other protective measures in place, such as shutters and plywood.
DP ratings are determined by the amount of stress a window can sustain. The ratings range from 0 to 100, with most windows falling within the 20 to 60 range. The type of window that homeowners choose is determined by local building codes, which are determined by the geographic area of the home, the prevailing wind speeds and the size of the window. Large corner windows in areas where wind speeds can exceed 160 miles per hour should have a DP-45 rating while smaller corner windows should have a DP-70 rating. Knowing the required rating for the area helps determine what alternative materials can be used in place of DP windows.
Certain areas of the country, such as the Texas and Florida coasts, enforce strict building codes. Hurricane- and wind-tolerant structures and designs are required for residential as well as commercial structures. DP windows are constructed with a thin layer of polymer sealed between two window panes. When pressure, wind or debris compromises the window glass, the polymer prevents the glass from shattering. DP windows can be expensive, but there are other measures, such as mesh, plywood and shutters, that are more economical and fulfill building code requirements. Contact your local building code enforcement agency or university cooperative extension for building codes specific to your community.
There are several types of shutters that can be installed over windows and doors to prevent wind and debris damage. Accordion shutters are installed vertically alongside windows. The shutters are pulled back during good weather and, when high winds threaten, can be pulled across windows. Roll down shutters are similar in function and are permanently placed horizontally above windows then rolled down when danger threatens. Temporary storm panels are not attached to the building. Instead, these galvanized steel or aluminum panels are placed in grooves outside of window sills and can withstand high winds.
The most temporary and least costly DP window equivalent is 5/8-inch-thick plywood. Plywood sheets should be installed by a contractor but can be installed by the homeowner, with building code standards in mind. Fasteners, such as lag bolts and anchors, should be used instead of nails. Bolts should be embedded to a depth of 1 to 2 inches, depending on the size of the window. The plywood should have an overlap of 4 inches from the frame to the building's siding. Once installed, these barriers should provide the same amount of protection as a DP window. Another alternative material is window film that prevents glass from shattering; however, the film will not protect against heavy debris.