Most South Florida homeowners understand the importance of securing windows before an impending hurricane. Even though hurricane conditions are well known in advance, what about the rest of the time? Florida has a notoriously unpredictable climate. High winds and inclement weather can whip up at any given time. So the question remains, how much wind can a window withstand? For example, can 70 mph winds break windows?
What risks do high winds pose to windows?
Hurricanes aren’t the only windy weather events that South Florida faces. Afternoon rainstorms or even tropical disturbances hundreds of miles away can cause gusts or sustained high winds. According to the National Weather Service, a wind gust is a sudden, brief increase in wind speed that peak at least 18 miles per hour and have lulls that vary by at least 10 miles per hour. A gust usually lasts less than 20 seconds. Even though Hurricane Dorian recently missed the Florida coastline, tropical storm-force winds were still felt in Miami-Dade and Palm Beach Counties.
When windows fail during a hurricane, broken glass, debris, and huge amounts of water enter the home, causing catastrophic damage, or worse, injuries or death. Older windows are especially at risk as they may not be properly mounted or framed. A 50 mph wind will apply 5 to 7 pounds of force per square foot, but this increases exponentially as winds get stronger. At 100 mph, that figure jumps from 20 to 28 pounds of pressure per square foot, and at 130 mph, 34 to 47 pounds per square foot of pressure are applied.
How strong does wind have to be to break windows?
Hurricane wind speeds start at 74 mph, but here’s the damage that winds can do before and up to that point, according to the National Severe Storms Laboratory:
- 19 mph: small trees begin to sway
- 25 mph: umbrella use becomes difficult, whistling can be heard through wires and branches
- 32 mph: whole trees sway and walking into the wind becomes difficult
- 39 mph: branches can be broken from trees, cars can veer on the road
- 47 mph: light structural damage
- 55 mph: trees can be uprooted, considerable structural damage is possible
- 64 mph and up: widespread structural damage is possible
According to the Saffir–Simpson hurricane wind scale (SSHWS), tropical storm-force winds begin at 39 mph. The SSHWS is the well-known scale that classifies hurricanes into five categories based on wind speed:
Category 1: 74-95 mph
Winds at this speed will mostly cause damage to mobile homes and trees, but well-constructed frame homes may see minor damage to the roof or gutters. Shallowly rooted trees may topple and damage to power lines can cause power outages that last for days. A storm surge of 4-5 feet above normal can cause coastal road flooding.
Category 2: 96-110 mph
These winds are extremely dangerous; homes may sustain roof, siding, window and door damage. Trees will sustain considerable damage and some are uprooted, blocking roads. The storm surge is usually 6 to 8 feet above normal. Coastal and low-lying areas are evacuated.
Category 3: 111-129 mph
Mobile homes and poorly constructed buildings are destroyed. The storm surge is typically 9 – 12 feet higher than normal. Electricity and water may be unavailable for days to weeks after the storm passes, major trees are snapped or uprooted, homes may incur major damage.
Category 4: 130-156 mph
Catastrophic damage; homes can suffer severe damage including the loss of the roof structure, windows, doors and/or some exterior walls. Most of the area will be uninhabitable for weeks or months. The storm surge is generally 13 to 18 feet above normal levels. Land that is less than 10 feet above sea level may be flooded.
Category 5: 157 mph or higher
These storms destroy most homes, causing roof failure and wall collapse. The storm surge is usually more than 18 feet higher than normal. Fallen trees and power lines will isolate areas, with many shrubs, trees, and signs blown down. Lower levels of all structures sustain major damage and there are massive evacuations of residential areas. Power outages can last for weeks or months, and most of the area will be uninhabitable.
So with all that said, at what wind speed do windows break? A Design Pressure or DP rating measures the strength of a window. Standard residential windows have DP values between 15 and 50. A DP 15 window can reasonably be expected to sustain winds of roughly 77 mph before shattering.
A DP 50 window is expected to sustain winds up to 173 mph. The windows installed in your home will depend on what the building code dictated at the time of installation. After Hurricane Andrew, Miami-Dade has one of the strictest building codes in the country. Note that unlike impact glass, these values only account for wind speed, not projectiles.
What to do if a window breaks during a hurricane
If you aren’t required to evacuate and decide to ride out the hurricane, be sure to take all the necessary precautions beforehand. This includes making sure to secure and brace external windows and doors, remove projectiles, trim bushes, and shrubs. Then stock up on supplies such as water, food, and batteries, and listen to a battery-operated radio or television for storm updates.
Do not tape windows in an effort to prevent shattering, and never, ever leave the windows slightly open to attempt to stabilize the pressure. Taping windows does not make them more shatter-resistant, it just ensures that larger pieces of shattered glass are flying around in a storm. Inviting a violent wind into your home by way of open windows will destroy the structure from the inside.
If a window breaks during a hurricane and you are home, immediately move to a room with few or no windows, if you aren’t there already. Be sure to close the doors to the rooms with broken windows when you leave, and don’t try to make any repairs until the winds die down. You may be able to make a quick fix while the eye of the storm passes, but be sure that someone else in the house is standing by to alert you when the winds start picking up again. After the storm passes, wear gloves while cleaning up and be sure to document for your insurance company.
Other threats to windows
In addition to shattering the glass, winds can devastate homes when windows fail by destabilizing the pressure and causing walls or roofs to collapse. Additionally, blown-over trees and signs become projectiles in hurricane-force winds, so considering installing impact glass is a good idea. Plus, impact glass has the added benefits of better insulation and added protection against break-ins.
Ocean Impact’s state of the art impact windows are missile-tested for strength and designed to withstand winds of up to 175 mph. If you’re not sure if your windows are ready for the next hurricane, tropical storm or windy day, contact us today for an estimate.