24 CFR § 3280.305 - Structural design requirements.

§ 3280.305 Structural design requirements.

(a) General. Each manufactured home shall be designed and constructed as a completely integrated structure capable of sustaining the design load requirements of this standard, and shall be capable of transmitting these loads to stabilizing devices without exceeding the allowable stresses or deflections. Roof framing shall be securely fastened to wall framing, walls to floor structure, and floor structure to chassis to secure and maintain continuity between the floor and chassis, so as to resist wind overturning, uplift, and sliding as imposed by design loads in this part. Uncompressed finished flooring greater than 1/8 inch in thickness shall not extend beneath load-bearing walls that are fastened to the floor structure.

(b) Design loads -

(1) Design dead loads. Design dead loads shall be the actual dead load supported by the structural assembly under consideration.

(2) Design live loads. The design live loads and wind and snow loads shall be as specified in this section and shall be considered to be uniformly distributed. The roof live load or snow load shall not be considered as acting simultaneously with the wind load and the roof live or snow load and floor live loads shall not be considered as resisting the overturning moment due to wind.

(3) When engineering calculations are performed, allowable unit stresses may be increased as provided in the documents referenced in § 3280.304 except as otherwise indicated in §§ 3280.304(b)(1) and 3280.306(a).

(4) Whenever the roof slope does not exceed 20 degrees, the design horizontal wind loads required by § 3280.305(c)(1) may be determined without including the vertical roof projection of the manufactured home. However, regardless of the roof slope of the manufactured home, the vertical roof projection shall be included when determining the wind loading for split level or clerestory-type roof systems.

(c) Wind, snow, and roof loads -

(1) Wind loads - design requirements.

(i) Standard wind loads (Zone I). When a manufactured home is not designed to resist the wind loads for high-wind areas (Zone II or Zone III) specified in paragraph (c)(1)(ii) of this section, the manufactured home and each of its wind-resisting parts and portions must be designed for horizontal wind loads of not less than 15 psf and a net uplift roof load of not less than 9 psf. The net uplift roof load must not be reduced by the dead load of the roof structure for the purposes of engineering design or structural load testing.

(ii) Wind loads for high wind areas (Zone II and Zone III). When designed for high wind areas (Zone II and Zone III), the manufactured home, each of its wind resisting parts (including, but not limited to, shear walls, diaphragms, ridge beams, and their fastening and anchoring systems), and its components and cladding materials (including, but not limited to, roof trusses, wall studs, exterior sheathing, roofing and siding materials, exterior glazing, and their connections and fasteners) shall be designed by a Professional Engineer or Architect to resist:

(A) The design wind loads for Exposure C specified in ANSI/ASCE 7-88, “Minimum Design Loads for Buildings and Other Structures,” for a fifty-year recurrence interval, and a design wind speed of 100 mph, as specified for Wind Zone II, or 110 mph, as specified for Wind Zone III (Basic Wind Zone Map); or

(B) The wind pressures specified in the following table:

Table of Design Wind Pressures

Element Wind zone II design wind speed 100 MPH Wind zone III design wind speed 110 MPH
Anchorage for lateral and vertical stability (See § 3280.306(a)):
Net Horizontal Drag 1 2: 3 ±39 PSF 3 ±47 PSF
Uplift 4: 5 −27 PSF −32 PSF
Main wind force resisting system:
Shearwalls, Diaphragms and their Fastening and Anchorage Systems 1 2 ±39 PSF ±47 PSF
Ridge beams and other Main Roof Support Beams (Beams supporting expanding room sections, etc.) −30 PSF −36 PSF
Components and cladding:
Roof trusses 4 in all areas; trusses shall be doubled within 3′-0′ from each end of the roof 5 −39 PSF 5 −47 PSF
Exterior roof coverings, sheathing and fastenings 4, 6, 7 in all areas except the following 5 −39 PSF 5 −47 PSF
Within 3′-0′ from each gable end (overhang at end wall) of the roof or endwall if no overhang is provided 4, 6, 7 5 −73 PSF 5 −89 PSF
Within 3′-0′ from the ridge and eave (overhang at sidewall) or sidewall if no eave is provided 4, 6, 7 5 −51 PSF 5 −62 PSF
Eaves (Overhangs at Sidewalls) 4, 6, 7 5 −51 PSF 5 −62 PSF
Gables (Overhangs at Endwalls) 4, 6, 7 5 −73 PSF 5 −89 PSF
Wall studs in sidewalls and endwalls, exterior windows and sliding glass doors (glazing and framing), exterior coverings, sheathing and fastenings 8:
Within 3′-0′ from each corner of the sidewall and endwall ±48 PSF ±58 PSF
All other areas ±38 PSF ±46 PSF

NOTES:

1 The net horizontal drag of ±39 PSF to be used in calculating Anchorage for Lateral and Vertical Stability and for the design of Main Wind Force Resisting Systems is based on a distribution of wind pressures of + 0.8 or + 24 PSF to the windward wall and −0.5 or −15 PSF to the leeward wall.

2 Horizontal drag pressures need not be applied to roof projections when the roof slope does not exceed 20 degrees.

3 + sign would mean pressures are acting towards or on the structure; − sign means pressures are acting away from the structure; ±sign means forces can act in either direction, towards or away from the structure.

4 Design values in this “Table” are only applicable to roof slopes between 10 degrees (nominal 2/12 slope) and 30 degrees.

5 The design uplift pressures are the same whether they are applied normal to the surface of the roof or to the horizontal projection of the roof.

6 Shingle roof coverings that are secured with 6 fasteners per shingle through an underlayment which is cemented to a 3/8” structural rated roof sheathing need not be evaluated for these design wind pressures.

7 Structural rated roof sheathing that is at least 3/8” in thickness, installed with the long dimension perpendicular to roof framing supports, and secured with fasteners at 4” on center within 3′-0′ of each gable end or endwall if no overhang is provided and 6” on center in all other areas, need not be evaluated for these design wind pressures.

8 Exterior coverings that are secured at 6″ o.c. to a 3/8″ structural rated sheathing that is fastened to wall framing members at 6″ on center need not be evaluated for these design wind pressures.

(iii) One-piece metal roofing capable of resisting the design wind pressures for “Components and Cladding: (Exterior roof coverings)” in the Table for Design Wind Pressures in this section is allowed to be used without structural sheathing, provided the metal roofing is tested using procedures that have been approved by HUD and that meet all requirements of §§ 3280.303(c) and (g) and 3280.401.

(2) Wind loads - zone designations. The Wind Zone and specific wind design load requirements are determined by the fastest basic wind speed (mph) within each Zone and the intended location, based on the Basic Wind Zone Map, as follows:

(i) Wind Zone I. Wind Zone I consists of those areas on the Basic Wind Zone Map that are not identified in paragraphs (c)(2)(ii) or (iii) of this section as being within Wind Zone II or III, respectively.

(ii) Wind Zone II.....100 mph. The following areas are deemed to be within Wind Zone II of the Basic Wind Zone Map:

Local governments: The following local governments listed by State (counties, unless specified otherwise):

Alabama: Baldwin and Mobile.

Florida: All counties except those identified in paragraph (c)(1)(i)(C) of this section as within Wind Zone III.

Georgia: Bryan, Camden, Chatham, Glynn, Liberty, McIntosh.

Louisiana: Parishes of Acadia, Allen, Ascension, Assumption, Calcasieu, Cameron, East Baton Rouge, East Feliciana, Evangeline, Iberia, Iberville, Jefferson Davis, LaFayette, Livingston, Pointe Coupee, St. Helena, St. James, St. John the Baptist, St. Landry, St. Martin, St. Tammany, Tangipahoa, Vermillion, Washington, West Baton Rouge, and West Feliciana.

Maine: Hancock and Washington.

Massachusetts: Barnstable, Bristol, Dukes, Nantucket, and Plymouth.

Mississippi: George, Hancock, Harrison, Jackson, Pearl River, and Stone.

North Carolina: Beaufort, Brunswick, Camden, Chowan, Columbus, Craven, Currituck, Jones, New Hanover, Onslow, Pamlico, Pasquotank, Pender, Perquimans, Tyrrell, and Washington.

South Carolina: Beaufort, Berkeley, Charleston, Colleton, Dorchester, Georgetown, Horry, Jasper, and Williamsburg.

Texas: Aransas, Brazoria, Calhoun, Cameron, Chambers, Galveston, Jefferson, Kenedy, Kleberg, Matagorda, Nueces, Orange, Refugio, San Patricio, and Willacy.

Virginia: Cities of Chesapeake, Norfolk, Portsmouth, Princess Anne, and Virginia Beach.

Wind Zone III.....110 mph. The following areas are considered to be within Wind Zone III of the Basic Wind Zone Map:

(A) States and Territories: The entire State of Hawaii, the coastal regions of Alaska (as determined by the 90 mph isotach on the ANSI/ASCE 7-88 map), and all of the U.S. Territories of American Samoa, Guam, Northern Mariana Islands, Puerto Rico, Trust Territory of the Pacific Islands, and the United States Virgin Islands.

(B) Local governments: The following local governments listed by State (counties, unless specified otherwise):

Florida: Broward, Charlotte, Collier, Dade, Franklin, Gulf, Hendry, Lee, Martin, Manatee, Monroe, Palm Beach, Pinellas, and Sarasota.

Louisiana: Parishes of Jefferson, La Fourche, Orleans, Plaquemines, St. Bernard, St. Charles, St. Mary, and Terrabonne.

North Carolina: Carteret, Dare, and Hyde.

(iv) Consideration of local requirements. For areas where wind mapping data or records or the requirements of the State or local authority indicate wind speeds in excess of those identified in this section, the Department may establish, through rulemaking, more stringent requirements for manufactured homes to be installed in such areas.

(3) Snow and roof loads.

(i) Flat, curved and pitched roofs shall be designed to resist the following live loads, applied downward on the horizontal projection as appropriate for the design zone marked on the manufactured home:

Zone (see Map in § 3280.305(c)(4)) Pounds per square foot
North Zone 40
Middle Zone 30
South Zone 20

(A) North Roof Load Zone. The following counties in each of the following states are deemed to be within the North Roof Load Zone:

Maine - Aroostook, Piscataquis, Somerset, Penobscot, Waldo, Knox, Hancock, and Washington.

Alaska - All Counties

(B) Middle Roof Load Zone. The following counties in each of the following states are deemed to be within the Middle Roof Load Zone:

States Counties
South Dakota Grant Brookings Hanson Lincoln
Codington Miner Minnehaha Yankton
Deuel Lake Hutchinson Union
Hamlin Moody Turner Clay
Kingsbury McCook
Minnesota Koochiching Stearns Renville Sibley
Itasca Swift McLeod Nicollet
Hubbard Kandiyohi Carver Blue Earth
Cass Meeker Dakota Martin
Crow Wing Wright Goodhue Watonwan
Aitkin Lac qui Parle Wabasha Brown
St. Louis Chippewa Winona Redwood
Lake Yellow Medicine Fillmore Lyon
Cook Mille Lacs Mower Lincoln
Carlton Kanabec Olmsted Pipestone
Pine Benton Dodge Murray
Wadena Isanti Rice Cottonwood
Todd Sherburne Steele Jackson
Morrison Anoka Freeborn Nobles
Douglas Chisapo Faribault Rock
Grant Washington Waseca
Stevens Hennepin Le Sueur
Pope Ramsey Scott
Iowa Hancock Mitchell Hamilton Buena Vista
Lyon Howard Webster Cherokee
Osceola Chickasaw Calhoun Plymouth
Dickinson Butler Sac Sioux
Emmet Floyd Ida O'Brien
Kossuth Cerro Gordo Humboldt Clay
Winnebago Franklin Pocahontas Wright
Worth Hardin Palo Alto
Wisconsin Douglas Oconto Pepin Lincoln
Bayfield Menominee Pierce Oneida
Ashland Langlade Dunn Polk
Iron Marathon Eau Claire Burnett
Vilas Clark Chippewa Washburn
Forest Jackson Rusk Sawyer
Florence Trempealeau Barron Price
Marinette Buffalo Taylor Door
St. Croix
Michigan Houghton Iron Presque Isle Wexford
Baraga Dickinson Charlevoix Benzie
Marquette Menominee Montmorency Grand Traverse
Alger Delta Alpena Kalkaska
Luce Schoolcraft Alcona Oscoda
Chippewa Mackinac Ogemaw Otsego
Keweenaw Cheyboygan Roscommon Leelanau
Ontonagon Emmet Missaukee Antrim
Gogebic Crawford
New York St. Lawrence Herkimer Onondaga Genesee
Franklin Lewis Madison Orleans
Clinton Oswego Cayuga Niagara
Essex Jefferson Seneca Erie
Hamilton Oneida Wayne Wyoming
Warren Fulton Ontario Monroe
Saratoga Montgomery Yates
Washington Schenectady Livingston
Massachusetts Essex
Maine Franklin Kennebec Lincoln Cumberland
Oxford Androscoggin Sagadahoc York
Montana All Counties
Idaho All Counties
Colorado All Counties
Wyoming All Counties
Utah All Counties
Vermont Franklin Orleans Caledonia Addison
Grand Isle Essex Washington Rutland
Lamoille Chittenden Orange Windsor
New Hampshire All Counties

(C) South Roof Load Zone. The states and counties that are not listed for the North Roof Load Zone in paragraph (c)(3)(i)(A) of this section, or the Middle Roof Load Zone in paragraph (c)(3)(i)(B) of this section, are deemed to be within the South Roof Load Zone.

(ii) Consideration of local requirements. For exposures in areas (mountainous or other) where recognized snow records, wind records, or the requirements of the State or local authority indicate significant differences from the loads stated in this paragraph (c)(3), the Department may establish, through rulemaking, more stringent requirements for manufactured homes to be installed in such areas. For snow loads, such requirements must be based on a roof snow load of 0.6 of the ground snow load for areas exposed to wind and a roof snow load of 0.8 of the ground snow load for sheltered areas.

(iii) Eaves and cornices shall be designed for a net uplift pressure of 2.5 times the design uplift wind pressure cited in § 3280.305(c)(1)(i) for Wind Zone I, and for the design pressures cited in § 3280.305(c)(1)(ii) for Wind Zones II and III.

(iv) Skylights must be capable of withstanding roof loads as specified in paragraphs (c)(3)(i) or (c)(3)(ii) of this section. Skylights must be listed and tested in accordance with AAMA 1600/I.S.7-00, 2003, Voluntary Specification for Skylights.

(4) Data plate requirements. The Data Plate posted in the manufactured home (see § 3280.5) shall designate the wind and roof load zones or, if designed for higher loads, the actual design external snow and wind loads for which the home has been designed. The Data Plate shall include reproductions of the Load Zone Maps shown in this paragraph (c)(4), with any related information. The Load Zone Maps shall be not less than either 3 1/2 in. by 2 1/4 in., or one-half the size illustrated in the Code of Federal Regulations.

(d) Design load deflection.

(1) When a structural assembly is subjected to total design live loads, the deflection for structural framing members shall not exceed the following (where L equals the clear span between supports or two times the length of a cantilever):

Floor - L/240
Roof and ceiling - L/180
Headers, beams, and girders (vertical load) - L/180
Walls and partitions - L/180

(2) The allowable eave or cornice deflection for uplift is to be measured at the design uplift load of 9 psf for Wind Zone I, and at the design uplift pressure cited in paragraph (c)(1)(ii) of this section for Wind Zones II and III. The allowable deflection shall be (2 × Lc)/180, where Lc is the measured horizontal eave projection from the wall.

(e) Fastening of structural systems.

(1) Roof framing must be securely fastened to wall framing, walls to floor structure, and floor structure to chassis, to secure and maintain continuity between the floor and chassis in order to resist wind overturning, uplift, and sliding, and to provide continuous load paths for these forces to the foundation or anchorage system. The number and type of fasteners used must be capable of transferring all forces between elements being joined.

(2) For Wind Zone II and Wind Zone III, roof framing members must be securely fastened at the vertical bearing points to resist design overturning, uplift, and sliding forces. When engineered connectors are not installed, roof framing members must be secured at the vertical bearing points to wall framing members (studs), and wall framing members (studs) must be secured to floor framing members, with 0.016 inch base metal, minimum steel strapping or engineered connectors, or by a combination of 0.016 inch base metal, minimum steel strapping or engineered connectors, and structural-rated wall sheathing that overlaps the roof and floor system if substantiated by structural analysis or by suitable load tests. Steel strapping or engineered connectors are to be installed at a maximum spacing of 24 inches on center in Wind Zone II, and 16 inches on center in Wind Zone III. Exception: Where substantiated by structural analysis or suitable load tests, the 0.016 inch base metal minimum steel strapping or engineered connectors may be omitted at the roof to wall and/or wall to floor connections, when structural rated sheathing that overlaps the roof and wall and/or wall and floor is capable of resisting the applicable design wind loads.

(f) Walls. The walls shall be of sufficient strength to withstand the load requirements as defined in § 3280.305(c) of this part, without exceeding the deflections as specified in § 3280.305(d). The connections between the bearing walls, floor, and roof framework members shall be fabricated in such a manner as to provide support for the material used to enclose the manufactured home and to provide for transfer of all lateral and vertical loads to the floor and chassis.

(1) Except where substantiated by engineering analysis or tests, studs shall not be notched or drilled in the middle one-third of their length.

(2) Interior walls and partitions shall be constructed with structural capacity adequate for the intended purpose and shall be capable of resisting a horizontal load of not less than five pounds per square foot. An allowable stress increase of 1.33 times the permitted published design values may be used in the design of wood framed interior partitions. Finish of walls and partitions shall be securely fastened to wall framing.

(g) Floors.

(1) Floor assemblies shall be designed in accordance with accepted engineering practice standards to support a minimum uniform live load of 40 lb/ft 2 plus the dead load of the materials. In addition (but not simultaneously), floors shall be able to support a 200-pound concentrated load on a one-inch diameter disc at the most critical location with a maximum deflection not to exceed one-eighth inch relative to floor framing. Perimeter wood joists of more than six inches depth shall be stabilized against overturning from superimposed loads as follows: at ends by solid blocking not less than two-inch thickness by full depth of joist, or by connecting to a continuous header not less than two-inch thickness and not less than the depth of the joist with connecting devices; at eight-feet maximum intermediate spacing by solid blocking or by wood cross-bridging of not less than one inch by three inches, metal cross-bridging of equal strength, or by other approved methods.

(2) Wood, wood fiber or plywood floors or subfloors in kitchens, bathrooms (including toilet compartments), laundry areas, water heater compartments, and any other areas subject to excessive moisture shall be moisture resistant or shall be made moisture resistant by sealing or by an overlay of nonabsorbent material applied with water-resistant adhesive. Use of one of the following methods would meet this requirement:

(i) Sealing the floor with a water-resistant sealer; or

(ii) Installing an overlay of a non-absorbent floor covering material applied with water-resistant adhesive; or

(iii) Direct application of a water-resistant sealer to the exposed wood floor area when covered with a non-absorbent overlay; or

(iv) The use of a non-absorbent floor covering which may be installed without a continuous application of a water-resistant adhesive or sealant when the floor covering meets the following criteria:

(A) The covering is a continuous membrane with any seams or patches seam bonded or welded to preserve the continuity of the floor covering; and

(B) The floor is protected at all penetrations in these areas by sealing with a compatible water-resistant adhesive or sealant to prevent moisture from migrating under the nonabsorbent floor covering; and

(C) The covering is fastened around the perimeter of the subfloor in accordance with the floor covering manufacturer's instructions; and,

(D) The covering is designed to be installed to prevent moisture penetration without the use of a water-resistant adhesive or sealer except as required in this paragraph (g). The vertical edges of penetrations for plumbing shall be covered with a moisture-resistant adhesive or sealant. The vertical penetrations located under the bottom plates of perimeter walls of rooms, areas, or compartments are not required to be sealed; this does not include walls or partitions within the rooms or areas.

(3) Wood panel products used as floor or subfloor materials on the exterior of the home, such as in recessed entryways, must be rated for exterior exposure and protected from moisture by sealing or applying nonabsorbent overlay with water resistant adhesive.

(4) Carpet or carpet pads shall not be installed under concealed spaces subject to excessive moisture, such as plumbing fixture spaces, floor areas under installed laundry equipment. Carpet may be installed in laundry space provided:

(i) The appliances are not provided;

(ii) The conditions of paragraph (g)(2) of this section are followed; and

(iii) Instructions are provided to remove carpet when appliances are installed.

(5) Except where substantiated by engineering analysis or tests:

(i) Notches on the ends of joists shall not exceed one-fourth the joist depth.

(ii) Holes bored in joists shall not be within 2 inches of the top or bottom of the joist, and the diameter of any such hole shall not exceed one-third the depth of the joist.

(iii) Notches in the top or bottom of the joists shall not exceed one-sixth the depth and shall not be located in the middle third of the span.

(6) Bottom board material (with or without patches) shall meet or exceed the level of 48 inch-pounds of puncture resistance as tested by the Beach Puncture Test in accordance with Standard Test Methods for Puncture and Stiffness of Paperboard, and Corrugated and Solid Fiberboard, ASTM D-781-1968 (73). The material shall be suitable for patches and the patch life shall be equivalent to the material life. Patch installation instruction shall be included in the manufactured home manufacturer's instructions.

(h) Roofs.

(1) Roofs shall be of sufficient strength to withstand the load requirements as defined in § 3280.305 (b) and (c) without exceeding the deflections specified in § 3280.305(d). The connections between roof framework members and bearing walls shall be fabricated in such a manner to provide for the transfer of design vertical and horizontal loads to the bearing walls and to resist uplift forces.

(2) Roofing membranes shall be of sufficient rigidity to prevent deflection which would permit ponding of water or separation of seams due to wind, snow, ice, erection or transportation forces.

(3) Cutting of roof framework members for passage of electrical, plumbing or mechanical systems shall not be allowed except where substantiated by engineering analysis.

(4) All roof penetrations for electrical, plumbing or mechanical systems shall be properly flashed and sealed. In addition, where a metal roof membrane is penetrated, a wood backer shall be installed. The backer plate shall be not less than 5/16 inch plywood, with exterior glues, secured to the roof framing system beneath the metal roof, and shall be of a size to assure that all screws securing the flashing are held by the backer plate.

(i) Frame construction. The frame shall be capable of transmitting all design loads to stabilizing devices without exceeding the allowable load and deflections of this section. The frame shall also be capable of withstanding the effects of transportation shock and vibration without degradation as required by subpart J.

(1) [Reserved]

(2) Protection of metal frames against corrosion. Metal frames shall be made corrosion resistant or protected against corrosion. Metal frames may be protected against corrosion by painting.

(j) Welded connections.

(1) All welds must be made in accordance with the applicable provisions of the Specification for Structural Steel Buildings, Allowable Stress Design and Plastic Design, AISC-S335, 1989; the Specification for the Design of Cold-Formed Steel Structural Members, AISI, 1996; and the Specification for the Design of Cold-Formed Stainless Steel Structural Members, SEI/ASCE 8-02, 2002.

(2) Regardless of the provisions of any reference standard contained in this subpart, deposits of weld slag or flux shall be required to be removed only from welded joints at the following locations:

(i) Drawbar and coupling mechanisms;

(ii) Main member splices, and

(iii) Spring hanger to main member connections.

(k) Attics.

(1) For roofs with slopes 7:12 or greater, the area of the attic floor that meets the ceiling-height/living-space requirements of these construction and safety standards must be designed to resist a minimum design live load of 40 pounds per square foot (psf) in accordance with paragraph (g) of this section.

(2) For roofs with slopes less than 7:12 that contain an attic area or for portions of roofs with slopes 7:12 or greater that do meet the ceiling height/living space requirements of the standards, the attic floor must be designed for a storage live load of 20 pounds per square foot (psf).

[40 FR 58752, Dec. 18, 1975. Redesignated at 44 FR 20679, Apr. 6, 1979, as amended at 44 FR 66195, Nov. 19, 1979; 52 FR 4582, Feb. 12, 1987; 58 FR 55006, Oct. 25, 1993; 59 FR 2469, Jan. 14, 1994; 59 FR 15113, 15114, Mar. 31, 1994; 62 FR 54547, Oct. 20, 1997; 70 FR 72043, Nov. 30, 2005; 71 FR 19638, Apr. 17, 2006; 78 FR 73983, Dec. 9, 2013; 80 FR 53727, Sept. 8, 2015]