Wind rating – What it means and how to use it?

Share

Wind rating a building means to classify it into a category that will help us understand how much wind pressure that construction can withstand. These ratings are based on the understanding of the different wind regions and are especially important to consider in areas where high winds from tornadoes, hurricanes or microbursts are common meteorological events. At the moment, there are two different wind classifications in the United States according to the construction type:

An average of 26% of the U.S. houses are placed in areas with an expected wind speed of 110 mph or higher and should therefore be designed and built to withstand strong winds [1].

While every house in the United States is built under a regulated building code, not all states, locations neither constructions follow the same code. Manufactured homes follow the regulations set by the federal Manufactured Home Construction and Safety Standards (MHCSS) and site build homes have to adhere to the both the state and local codes from where they are being constructed.

Some state building codes, such as Florida and Virginia are considered strong, updated and capable of reducing the damage of destruction caused by strong winds [2].

Other states such as Delaware, Mississippi or Alabama, according to the Insurance Institute for Business and Home Safety (IBHS) fail to adopt the recommended building code to strengthen their constructions in front of hurricanes and strong winds [3]. By properly understanding how to use wind rating in manufactured homes or what to expect from your location building code in a site built home, you as a home owner, gain the ability to take the safest actions.

Seeing that both wind rating and building code regulations in site built construction differs from manufactured buildings, it is interesting to explore what kind of construction has a better wind rating and a higher likelihood to withstand very damaging winds. As well, it might be helpful to know that regardless of our house wind rating, there are different ways in which we can improve the wind resistance of a house by adding some upgrades and add-ons, while these actions won’t modify the wind rating of the house as defined by the builder, they will help reduce the damage.

Understanding the different wind regions in the U.S.

Wind zones are determined by the amount of wind pressure expected in the area and classified into three zones according to the federal Manufactured Home Construction and Safety Standards (MHCSS) and its HUD Code, and four zones according to the Federal Emergency Management Agency (FEMA).

While having two different classifications might sound confusing, it gets simpler when understanding that there are two different kinds of construction in the U.S.: site built and manufactured; and both stick to its own regulations and wind zones.

A site built house is a permanent construction built directly on its foundations and designed to be placed in one particular location. Manufactured homes, on the other hand, are thought and designed to be transported and are therefore built on a steel chassis that can easily be attached on wheels.

Site built homes are built under the state and local building codes of the area where they are placed. Most of these building codes are based on the International Code Council® (ICC) which takes into consideration the recommendations described by FEMA [4]. FEMA stablishes their own wind zone rating and recommendations for the ICC and all site built homes, regardless the state or location adhere to it.

Manufactured homes, on the other hand are built under the regulations of a national standardized building code defined by the MHCSS: the HUD Code [5] which has its own wind zone classification.

Wind Zones for manufactured homes according to HUD’s MHCSS

The HUD Code categorizes the territories of the United States into three wind zones:

fig. 1. HUD wind zones

  • Zone I: All the manufactured houses that during its construction haven’t been classified under a specific wind zone fall into Zone I classification. Therefore, all those homes designed previous to 1976 are considered Zone I category [6]. It wasn’t until 1976 that the HUD code was defined and enforced to all manufactured homes, before this date there weren’t any regulatory codes required for manufactured homes to follow and most of them, known as mobile homes, where built poorly, compared to today’s safety standards.  
  • Zone II: All those manufactured houses built to be placed in areas where the wind speed is expected to reach 100 mph, are designed to withstand constant wind speeds of 100 mph and peak gust winds of 120 mph for about 3 second duration [6].
  • Zone III: All those manufactured houses built to be placed in areas where the wind speed is expected to reach 110 mph, are designed to withstand constant wind speeds of 110mph and peak gust winds of 130mph of approximately 3 second length [6].

Wind Zones According to FEMA for site build homes

FEMA categorizes the territories of the United States into four wind zones:

fig. 2. FEMA wind zones

  • Zone I: Those houses built in the Zone I areas, are expected to withstand wind speeds of 130 to 140 mph.
  • Zone II: Those houses built in the Zone II areas, are expected to withstand extremely dangerous wind speeds of between 140 and 150 mph [7].
  • Zone III: Those houses built in the Zone III areas, are expected to withstand devastating wind speeds of between 150 and 160 mph [7].
  • Zone IV: Those houses built in the Zone IV areas, are expected to withstand the catastrophic damage caused by wind with speeds over 160 mph [8].

How to use wind rating in manufactured homes?

Manufactured homes are designed to be movable and their structure allows transporting it along the nation. However, the construction of the house will be built to conform to one of the three wind zones stablished by the HUD code and usually according to the desired placing location set before construction. The wind zone rating of the house has to match the wind zone rating of the location and while houses built to withstand zone III winds can be placed in zone II and zone I areas, the opposite is not allowed and a house built under a lower wind zone classification can never be installed in a higher zone area [9].

Every manufactured house built after 1976 comes with an HUD Data Plate, a sheet of paper usually placed in the inner side of a kitchen cabinet containing detailed information about the house. This document contains important construction information such as the wind zone details [10].

When building or buying a manufactured home, it is very important to contemplate the wind rating options that best fit our situation and consider building it up to zone III standards if we are planning to move to a zone III area in the near future. Because it isn’t possible to upgrade the wind rating of a manufactured home once it is built. You can definitely improve its resistance to strong winds by adding some upgrades and add-ons, but the wind zone classification will always stay as set by the manufacturer, and a zone II house can never be placed in a Zone III area [11].

For safety reasons, it is recommended as well to check the wind speed of the local area before placing your manufactured home. Some areas, regardless of the general zone classification shown in the map, might experience different and extreme weather phenomena that can seriously damage your home and jeopardize your safety.

What to expect from your state and local building code in a site built home

Updating the Building Codes up to the standards of the latest’s International Building Code (IBC) has proven very successful for states such as Florida, where by updating and implementing a State-wide Building Code, the windstorm losses were reduced up to 72% [12]. Florida took the lead during the IBHS 2021 Rating the States Hurricane Coast results by adopting the IBC building codes and therefore reducing the need for public and private disaster aid while preserving the built environment. The following map shows the performance of all the 18 states included in the report [13].

fig. 3.  IBHS 2021 Rating the States Hurricane Coast map

Those states that appeared to perform poorly in this ranking do not have a mandatory state-wide code. However, some jurisdictions within these states have implemented updates in their local codes since the past rating in 2012 and enforced improvements that have helped improve safety during strong winds [13].

If you are planning to build a house (not manufactured) in a high risk area and the state or location where your house will be placed hasn’t yet adopted the recent IBC updates, note that the state and local building codes are the minimum requirements for the construction to be approved, but you can always talk to your builder and request the adoption of the International Codes instead, to ensure the safety of your home during extreme weather situations.

What kind of construction has a better wind rating?

In those areas where winds can cause catastrophic damage, site built homes respond better than manufactured homes mainly because they are based on permanent foundations and often built with heavier materials.

Even if a manufactured home is built to be placed in a Zone III area, there is no guarantee that the winds won’t exceed the 110 mph that the house is designed to withstand and damage might occur. Moreover, even if the winds aren’t higher than the established rating for that zone, unexpected storm related damages such as the falling of a tree can cause damage to the house as well. The wind rating issued by HUD can only ensure that the risk of damage is minimized [11].

Improve the wind resistance of a house by adding some upgrades and add-ons

Unfortunately, most of the upgrades that can help strengthen the wind resistance of a house are expensive or require renovations. Therefore, planning ahead and installing them during the construction of the house could save time and money.

  • Opt for Impact Resistant Windows and Doors. During strong winds, the forces of the air aren’t the only aspect to consider. Winds tend to drag small and large objects that can easily impact your house. Impact resistant windows and doors aren’t only designed with strong building materials but are as well installed to withstand higher winds, rain and debris [14].
  • Install Shutters especially if your windows aren’t impact resistant, shutters can protect them from debris and wind [1].
  • Reinforce the Garage Door since it is one of the weak elements of the house in front of strong winds. You can use a storm ready garage door, which is constructed with heavier materials and reinforced vertically and horizontally.
  • Securing Anchorage in Manufactured Homes avoids the house to be lifted, slid and rolled off its foundation. While the HUD code requires the house to be anchored, you can improve the anchorage by using In-Line Anchors installed at 45-degree angle instead of vertical. Inspect the anchors every 5 years and replace any corroded strap only with galvanized ones to prevent corrosion.

Fig. 4.  HUD ground anchors                                 Fig. 5. Improved Anchoring

  • Reinforce Wall joints at floor and ceiling
  • Reinforce your roof by using wind resistant shingles that are made of more resistant materials such as asphalt. To reduce the possible damages and prevent water leaks after a wind storm, it is recommended to add a secondary water barrier to your roof [15].
  • Maintenance is key to protect your home from strong winds, seal the gaps, inspect and repair the exterior siding, roof and any part of the home that could cause problems during a damaging strong wind.
  • Trees while trees can be very helpful to protect the house from the wind, often damages are caused by trees falling on the house. Regular pruning is necessary to avoid broken branches but routine inspections are also key to detect any weaknesses and remove those trees showing signs of being susceptible to breakage [16], specially those close to a construction.

References

  1. Resilient Design Guide – High Wind Wood Frame Construction Edition Federal Alliance for Safe Homes (FLASH) https://flash.org/resilientdesignguide.pdf
  2. Richburg, S.C., (2021, June 1) Inconsistent building codes leave some states vulnerable to hurricane damage Insurance Institute for Business & Home Safety (IBHS)https://ibhs.org/ibhs-news-releases/inconsistent-building-codes-leave-some-states-vulnerable/
  3. Summary – Rating the States: 2021 Insurance Institute for Business & Home Safety (IBHS)https://ibhs.org/wp-content/uploads/RatingtheStatesSummary2021.pdf 
  4. (2020, November) Building Codes Save: A Nationwide Study Federal Emergency Management Agency (FEMA) https://www.fema.gov/sites/default/files/2020-11/fema_building-codes-save_study.pdf
  5. Abby R. (2020, 13 March) How are Modular Home Codes different from Manufactured Clayton Homes https://www.claytonhomes.com/studio/how-are-modular-manufactured-home-code-different/
  6. (2007, February) Understanding and Improving Performance of New Manufactured Homes During High Wind Events Federal Emergency Management Agency (FEMA) https://www.hsdl.org/?view&did=30198
  7. (2019, November) The 2018 International Building Code®: A compilation of wind resistant provisions Federal Emergency Management Agency (FEMA)  https://www.fema.gov/sites/default/files/2020-07/2018-ibc-compliation-wind-resistant-provisions.pdf
  8. Wind Ratings Stream Data Centers – Resource Library https://www.streamdatacenters.com/glossary/wind-ratings/
  9. Basic Wind Zone Map Manufactured Home Institute (MHI) https://www.manufacturedhousing.org/wind-map/
  10. Missing HUD Labels (Tags) Manufactured Housing & Standards U.S. Department of Housing and Urban Development https://www.hud.gov/program_offices/housing/rmra/mhs/mhslabels
  11. Tepfenhart, O. How much wind can a Mobile Home Withstand Upgraded Home https://upgradedhome.com/how-much-wind-can-a-mobile-home-withstand/#What_Happens_If_Winds_Exceed_The_Speed_Your_Mobile_Home_Is_Built_For
  12. FAQs for Consumers – Do stronger building codes make a difference when disaster strikes? No Code. No Confidencehttps://inspecttoprotect.org/faqs-for-consumers/
  13. Rating the States: 2021 – Hurricane Coast Insurance Institute for Business & Home Safety (IBHS)https://ibhs.org/wp-content/uploads/RatingtheStates_2021.pdf
  14. Preparing for High Winds: How to Protect a Home from Damage The Robert Decani Team https://www.newjerseyrealestatenetwork.com/home-high-winds-preparation/#Doors
  15. (2016, 7 October) Upgrading your roof? Consider these hurricane-resistant tips Renew Financial https://renewfinancial.com/resources/upgrading-your-roof-consider-these-hurricane-resistant-tips
  16. (2013, 31 January) Windy weather: How to prevent wind damage to your property’s trees DAVEY Proven Solutions for a Growing Worldhttps://www.davey.com/about/newsroom/davey-in-the-news/windy-weather-how-to-prevent-wind-damage-to-your-propertys-trees/

fig. 1. https://www.manufacturedhousing.org/wind-map/

fig. 2. https://basc.pnnl.gov/images/wind-zones-us

fig. 3. https://ibhs.org/wp-content/uploads/RatingtheStates_2021.pdf

fig. 4. & fig. 5. https://www.hsdl.org/?view&did=30198