Are modular homes Hurricane rated?

As of today, no Modular home nor any other house is hurricane rated since there is no established ranking designed to tag constructions based on their resistance to hurricanes. We can rate Hurricanes according to their intensity with the Saffir-Simpon’s Hurricane Wind Scale Not, but this rating doesn’t apply to the resistance of buildings during hurricanes.

Being able to classify a house into a visual scale showing how much storm intensity it can withstand, can easily raise safety related questions especially among those buildings placed in heavy storm prone areas. However, every permanent house being built in the United States is required to stick to their state and local building codes which are often a safety guarantee.

These codes are designed to protect the building and the people inside them and while a house build up to code is by no means comparable to a “hurricane proof” house, some states are doing a very good job establishing codes that ensure safer homes during heavy storms.

While we can ensure that modular homes are built up to the state and local code required in their area, this doesn’t necessarily entail that modular homes are safe in heavy storms, hurricanes and tornados, and in order to define their resistance we will have to consider:

• The amount of wind the structure can withstand.

• How resistant the exterior of the house is towards the impact of flying debris.

• The ability of the house to hold up in heavy rainfall and flooding.

While the intensity of hurricanes is classified according to its wind speed, wind is only one of the hazards that needs to be considered when evaluating the resistance of a house during a hurricane. Hurricanes are complex weather events and to this date, homes cannot be yet hurricane rated.

State and local building codes are often a safety guarantee, but not always

All new constructions being site built or modular, are required to stick to the minimal requirements established by their state and local building codes.

Those areas with a known recurrent impact of heavy storms are working on strengthening their codes following the directions of the Federal Emergency Management Agency (FEMA) which is working on evolving the building codes to avoid the disastrous consequences of heavy storms [1].

The fact that all constructions are built up to code, offers some sort of guarantee that the house will withstand the expected weather events of the area. This is the case of South Florida for example, which after the devastating damages caused by hurricane Andrew in 1992 has been focusing its efforts on improving the building requirements and now has the toughest building code standards of the nation [2].

However, as the Insurance Institute for Business & Home Safety (IBHS) states in their 2021 edition of Rating the States, not all states have codes up to date and in some states such as Massachusetts, Georgia or New York, the building code isn’t strong enough to reduce the critical damage of hurricanes [3].

Those houses built in states or local jurisdictions with a weak building code can still, if desired, ensure a safer construction in front of heavy storms, hurricanes and tornadoes by building their houses up to the Fortified Home ^TM construction standard, a code based on the exhaustive research conducted by the IBS, to ensure more resilient structures against severe weather [4].

Are modular homes safe in heavy storms, hurricanes and tornadoes?

Modular homes are required to stick to the same building codes as any other permanent construction, and therefore should have a similar performance when exposed to heavy storms. In those states where the building codes are tougher, modular homes will be legally required to have a better performance than those built in States with weaker codes.

However, modular construction has an advantage over site built which is the fact that it is built in a factory-like environment, a controlled environment that allows a more thorough construction process, easing the construction of complex details and joints, and ensuring an overall superior construction quality.

Moreover, modular homes seem to perform better than conventional residential framing during Hurricanes since, according to FEMA, the module to module combination provides an inherently rigid system [5].

However, even if a modular home is generally safer than a conventionally constructed house in front of hurricanes and tornados, it is not heavy storm proof by definition. As any other type of construction, its resistance to hurricanes is subject to three main factors: 

The amount of wind the structure can withstand, how resistant the exterior of the house is towards the impact of flying debris, and its ability to hold up in heavy rainfall and flooding.

The amount of wind the structure can withstand

Wind is the most obvious factor to consider when building a modular house that is resistant to heavy storms, even the tool used to measure the intensity of a hurricane is based on its wind speeds.

Hurricanes are categorized in a scale from 1 to 5 according to the Saffir-Simpson’s Hurricane Wind Scale which is based on the maximum sustained wind speeds [6]. In order to evaluate the wind resistance of a modular house we need to take into consideration:

• The shape of the building: Shapes that are symmetric, compact, round and aerodynamic perform better during strong winds [7].

• The materials used: A structure with materials that are strong and flexible is more likely to withstand strong winds. Reinforced concrete and reinforced masonry are the materials recommended by FEMA for the construction of shelters due to their good performance in front of hurricanes [8]. However, other materials such as certain kinds of wood, especially Yellow Pine lumber, [9] offer as well quite a strong resistance to the wind pressures due to its strength and flexibility.

• The connection between different elements and joints. Inadequate anchorage, connections or joints are the most common failures of structures exposed to strong winds, since weak connections can easily be displaced by the uplift pressures of the strong winds [10]. Modular homes, usually perform better in this area since being built in a factory-like environment allows a better control of these delicate details.

How resistant the exterior of the house is towards the impact of flying debris

Flying debris is one of the most damaging consequences of heavy storms and winds. The overall exterior of the house should be strong enough to resist the impact of heavy and accelerated debris, also known as missiles. Robust, heavy materials such as concrete are more likely to withstand the damaging impacts of flying debris [11].

Windows are one of the main focuses when ensuring an impact resistant exterior of the house. Protecting the windows is especially important since a broken window will let the hurricane winds inside the house, dramatically changing the pressure inside the home and increasing the risk of a catastrophic damage such as blowing off the roof [12].

Windows can be protected with hurricane shutters, though impact resisting windows will have a much better outcome if impacted by flying debris. These kind of windows will offer a greater protection than a regular glass window protected with hurricane shutters [13].

The ability of the house to hold up in heavy rainfall and flooding

The heavy rainfalls associated to storms, hurricanes and tornados can cause severe damage either due to inland flooding or to the structural damage caused by water if absorbed into the building [14].

It is recommended for houses placed in areas where flooding is common to be built on an elevated structure. Actually most of the residential buildings that are built specifically to withstand heavy storms and tagged as hurricane proof, are elevated [15].

Moreover, the materials used to build the house should be resistant to water, and resist the constant “wetting and drying” [16].

To avoid water infiltration, it is important not only to install a water resistive barrier covering the whole surface of the house including the walls, but to ensure it is a continuous load. Modular homes, since they are built in a controlled environment can offer a better sealing placement quality but need to pay special attention to the connection between modules since that will be done on site.

Homes cannot be hurricane rated yet

While it would be very helpful to be able to rate a house with one single number representing how resistant that construction is to hurricanes, this would be challenging and to this date it is not possible yet.

Wind is the most obvious factor involved in a hurricane, and since wind rating a house is a common practice it would be very helpful to equate wind rating to hurricane rating. Wind however, is not the only hazard to consider when evaluating the resistance of a house during a hurricane [17] and a hurricane rating system should take into account all these other hazards.

The Saffir-Simpon’s Hurricane Wind Scale contemplates some of these wind associated hazards such as that one caused by the impact of flying debris and offers an estimation of the potential damages a house may suffer as a result of each strength of wind intensity.

This is just an estimation and lacks to contemplate other potentially deadly hazards associated to the storm such as rainfall flooding and tornadoes. Therefore, rating a construction according to the Saffir-Simpon’s Hurricane Wind Scale would not be accurate since the associated hazards like rainfall flooding or the surge of tornadoes aren’t always directly related to the intensity of the wind, and can suppose a significant threat even during a low intensity rated hurricane [18].

Moreover, in order to offer an accurate rating, we should consider the characteristics of the location where the building is placed as well as the intensity of the storm, since coastal communities, low lying inland areas or bays often present a greater threat and suffer the most damages [14].

References:

1. Ingargiola, J. & Sheldon, A.e & Ghorbi, L. (2015, November). Building Code Changes Resulting from FEMA’s Hurricane Sandy Mitigation Assessment Team Report. 380-386. 10.1061/9780784479711.037.
2. Hurricane Resistant Requirement Graham Architectural Products https://www.grahamwindows.com/architectural-resources/hurricane-resistance/
3. (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/ 
4. Superior Protection from Hurricanes, Tornadoes, and Other Severe Weather Fortified Gold https://fortifiedhome.org/gold/
5. Modular Construction: A Safe Alternative to Stick Built  Modular Building Institute http://www.modular.org/HtmlPage.aspx?name=Modular_A_Safe_Alternative
6. Saffir-Simpson Hurricane Wind Scale National Hurricane Center and Central Pacific Hurricane Center. National Oceanic and Atmospheric Administration  https://www.nhc.noaa.gov/aboutsshws.php
7. Beautiful Architectural Concepts Designed to Resist Hurricane Force Winds Architecture Art Designshttps://www.architectureartdesigns.com/beautiful-architectural-concepts-designed-resist-hurricane-force-winds/
8. 2021, April) Save Rooms for Tornadoes and Hurricanes Federal Emergency Management Agency, FEMA P-361, Fourth Edition https://www.fema.gov/sites/default/files/documents/fema_safe-rooms-for-tornadoes-and-hurricanes_p-361.pdf
9. (2005, November 29) Tips for Building Hurricane-Proof Houses Phys Org https://phys.org/news/2005-11-hurricane-proof-houses.html
10. Marshall, T. (1993, August). Lessons learned from analysing tornado damage ResearchGate https://www.researchgate.net/publication/327020416_LESSONS_LEARNED_FROM_ANALYZING_TORNADO_DAMAGE#pf3
11. (2005, June 1) Which is the better building material? Concrete or steel? Buildings https://www.buildings.com/articles/36029/which-better-building-material-concrete-or-steel
12. Barrineau, T. (2018, 6 August) In a Hurricane, protecting doors and windows is crucial Glass.com the Industry’s Home Addresshttps://info.glass.com/protect-doors-windows-hurricane/
13. Impact Windows vs Hurricane Shutters: What is Better? Engineered Glass Systems (EGS) https://egsinternationalllc.com/impact-windows-vs-hurricane-shutters-what-is-better/
14. (1991) Primer on Natural Hazard Management in Integrated Regional Development Planning. Chapter 12 – Hurricane Hazards Department of Regional Development and Environment Executive Secretariat for Economic and Social Affairs Organization of American States, Washington D.C.
15. Feldshuh, T. (2020, May 27)Round homes and hurricanes: How circular design, emphasis on connections keep these structures standing Fox News https://www.foxnews.com/us/round-homes-hurricanes-severe-weather-storms-tropical-weather-design-build-deltec-homes
16. Florida Building Codes Panel Built https://www.panelbuilt.com/blog/hurricane-rated-construction
17. The Saffir-Simpson Hurricane Wind Scale Homeland Security Digital Library (HSDL) https://www.hsdl.org/?view&did=775313 
18. Hurricane Preparedness – Hazard National Hurricane Center and Central Pacific Hurricane Center. National Oceanic and Atmospheric Administration  https://www.nhc.noaa.gov/prepare/hazards.php