How To Build A Tiny House on Wheels (THOW)

We have gathered and created some guidelines to provide a pathway for owners looking to create a permanent residence for a Tiny House On Wheels (THOW). We will guide you through the process of documenting the construction, demonstrating that it meets specific standards, and is intended for use as a permanent residence.

Tiny House On Wheels (THOW)

Definition: A tiny house on wheels (THOW) is a structure which is intended as a full time residence or year-round rental property and meets these five conditions:

1. Built on a trailer that was registered with the owner’s local DMV.
2. Ability to tow via bumper hitch, frame-towing hitch, or fifth-wheel connection. It cannot, nor designed to, move under its down power.
3. Is no larger than allowed by applicable state law. (The typical THOW is no more than 8’6″ wide, 30′ long, and 13’6″ high. Larger tiny houses may require special a commercial driver’s license and/or special permits when being towed.) Please note:
   1.  Some states are more restrictive than others. Here’s a handy but unofficial summary of size limitations. Please check with your local DMV for the laws in your state.
   2.  Roof height is from bottom of tires to the top of the highest exterior point on the house, including any protrusions. The roof height may be taller when stationary, as long as it is collapsible for towing of the THOW. Chimney piping may need to be removed for travel and then reinstalled to meet clearance requirements for use.
   3.  If slide-outs are included, the plans should be reviewed and approved by a structural engineer licensed in the state in which the THOW is built.
4. Has at least 70 square feet of first floor interior living space, and no more than 400 square feet (excludes any lofts).
5. includes basic functional areas that support normal daily routines (such as cooking, sleeping, and toiletry).

Disclaimer: these guidelines are provided for the tiny house builder who wants to record how his or her home was constructed. These guidelines are NOT recognized by any government authority and following them does not convey any particular benefits or special privileges. It is intended as a first step in establishing standards that might later be adopted by agencies when creating unique, new zoning or code regulations for tiny houses.

Purpose

The purpose of these guidelines is to support the construction of safe and sturdy tiny houses on wheels (THOWs). Well established standards exist for recreational vehicles (RVs), residential homes on foundations, and manufactured homes (formerly known as mobile homes). Yet none is fully applicable to the THOW. In addition, construction of a THOW is often a Do-It-Yourself project, but professional associations are generally not accessible to DIYers.

Independent Inspection

Since inspection for a DIY tiny house on wheels is not available through professional associations (like the Recreational Vehicle Industry Association), you may wish to explore an alternative program:

1. Pacific West Associates can certify that your tiny house on wheels meets the same standards as a traditional RV, either NFPA 1192 or ANSI 119.5
2. National Organization of Alternative Housing (NOAH) can certify that your tiny house on wheels meets the unique standards that it created for tiny house construction.

Having your tiny house inspected as you build, and certified when you’re done that the house was built to specific standards, can open doors to additional places to live and can help protect the resale value of your tiny house.

These guidelines don’t apply to tiny houses on foundations. Residential building codes establish minimum standards for houses on foundations. See the Emerald Village Tiny House Prototype from Tent City Urbanism for information on how to meet IRC while building a tiny house on a foundation.

Why is a THOW unique and not some other type of vehicle or house? See below.

Process

We recommend that you create a documentation package as you build your tiny house, including the following items:

1. Detailed structural plans illustrating the location of studs, joists, rafters, and engineered connectors (hurricane clips, tension ties, etc.). Plans should clearly address how the structure is secured to the trailer, and how the floors, walls, and roof are framed and sheathed. Plans should also include an illustration of a floor, wall and roof section, showing the building members, insulation, vapor barrier, moisture barrier, sheathing, siding, and roofing.
2. Detailed diagram of the electrical plan.
3. Photographs of the framing, roof, insulation, rough plumbing, and rough electrical.
4. Complete, detailed bill of materials. Where salvage or donated materials are used, the builder should provide a full description of the materials. It is recommended that receipts be saved along with the bill of materials, as the DMV may request these when you register your THOW.
5. A statement describing your construction methods along with the names and addresses of any subcontractors you may have hired.

Guidelines

THOWs should meet the following construction guidelines:

1. Meet RVIA NFPA 1192 Standards (also known as ANSI 119.2) with the following exceptions:
   1. The definition of Recreational Vehicle does not apply and is replaced with the definition of a Tiny House On Wheels (as stated above).
      Wherever “RV” or “recreational vehicle” appear, the acronym “THOW” or phrase “tiny house on wheels” is to be substituted.
   2. A secondary means of escape (egress window) is not required to be labeled, and there is no requirement as to the type or color of its handle.
   3. Warning labels are not required. However, it is recommended that NFPA 1192 requirements for warning labels be met for any THOW that is intended for sale.
   4. Similarly, an owner’s manual is not required but is recommended for any THOW that is intended for sale or rent.
   5. Composting and incinerating toilets are acceptable, and when present, must be installed according to the manufacturer’s specifications (which may be out of compliance with NFPA 1192 Chapter 7 which requires a toilet to be connected to a drainage system).
2. Be built for earthquake Zone 4 and able to withstand winds of 130 mph, as specified in the ASCE/SEI 7 Minimum Design Loads For Buildings and Other Structures.
3. Have adequate moisture barriers, vapor barriers, insulation, fans and vents to minimize condensation. These requirements go beyond what is specified in the IRC.
4. Be secured to a trailer that is adequate for the weight of the tiny house, its furnishings and occupants. The trailer must meet Department of Transportation (DOT) guidelines of the state in which the THOW is registered. The frame must be securely fastened to the trailer as specified below in the framing guidelines.
5. It is recommended that the THOW weigh less than 10,000 pounds. Above this weight, some states have special driver’s licensing regulations.
6. Use quality materials.
   1. New structural components (steel, lumber, plywood/OSB, ties, and fasteners) should meet the International Building Code, Residential Code. However, salvage materials may be used if they are of equivalent strength and durability as new materials that are specified in the code.
   2. Fasteners must be corrosion resistant at structural tie-points and where the house attaches to the trailer frame or foundation. This is done so that a future water leak, or the actions of chemicals in treated lumber, do not damage structurally important fasteners. The two most common types of corrosion-resistant fasteners are hot-dip galvanized nails and screws and coated exterior screws.
   3. For engineered straps, hurricane clips, tension ties, joist hangers, and header hangers, use the fasteners recommended by the manufacturer. Only use the nails and screws designed for the engineered system. “Tico” nails (10d 1.5-inch) are the most common. SD series screws can be used for anchoring straps and tie-downs.
   4. For the interior “envelope” (enclosed space) of the house, materials (plastics, glues, insulation, paints, and finishes) must be labeled by the manufacturer as safe for interior use or known to be chemically safe for interior use.
7. Framing
   1. Welded Steel frames: Steel frames must be secured to the trailer by welding to the trailer frame.
   2. Wood frames: Wood frames must meet the guidelines for earthquake and high-wind loading in the International Residential Code for One- and Two-Family Dwellings, section R602. Proper bridging supports and frame supports are required.
      1. Frame walls with graded 2×4 lumber and either 16d hot-galvanized ring shank nails or structural screws (extra strong, thin, sharp screws that meet stringent engineering standards. Made by GRK, Spax and FastenMaster), maximum 24″ on center (recommended: 16″ on center ).
      2. Use appropriate headers and frames for windows, doors, and other openings. While building code allows as little as 16% of a wall to be braced (e.g., a 16 ft wall only needs 2′ 7″ of bracing), THOWs are subject to sustained wind loads and minor “seismic” loads. To add windows without increasing the need for bracing, you may use small windows that do not span more than one stud bay (a 14.5″ rough opening for traditional framing).

         If open spaces make up more than 25% of a wall, crossbeams may be required for additional support. Consult a structural engineer. (Note: structural engineers may charge $100 or more per hour, and review of plans can take four or more hours. If you’re low on funds, use a standard tiny house design).

      3. Double top plates are recommended. For THOWs with a standard peaked roof, use 2×6 lumber for ridge beam and rafters. Use at least one 2×4 collar tie at least 1/3 the way in from either end wall, and fastened with a rated angle bracket or hurricane strap to the top plates of the side walls. The collar tie can become part of your sleeping loft.
      4. Sheath walls with minimum 3/8″ CDX plywood or OSB (1/2″ recommended), fastened with ring shank nails or structural screws, 6″ on-center along the edges of the panel, and 12″ on-center elsewhere. Use of adhesive on the fastener locations is recommended.
      5. Sheath roof with minimum 1/2″ CDX plywood or OSB (5/8″ recommended), fastened with ring shank nails or structural screws, 6″ on-center along the edges of the panel, and 12″ on-center elsewhere.
      6. Use adhesive on all sheathing.
      7. Tie the roof to the walls with hurricane straps, and 10d nails.
      8. Tie the walls to the foundation/trailer frame with one of these methods:
         • Hurricane tie-down brackets and minimum 3/8″ galvanized bolts.
         • A threaded rod and turnbuckles from trailer frame to top of wall plate, then hurricane roof ties to tie studs, plates, and roof trusses.
         • Another method that is detailed in the THOW plans and that has been reviewed and approved by a structural engineer.
      9. For floor joists, 2×4 lumber is acceptable; 2×8 can provide greater R value.
      10. When building on a trailer frame, the steel frame and crossmembers may BE your floor frame and structure. You may place the subfloor directly on trailer frame, fastening every 12″ with #12 coated or galvanized self tapping metal screws. Keep in mind, this method is not in the International Residential Code. If your trailer does not have sufficient crossmembers, you may build a separate floor structure (a box floor) and bolt it to the trailer rails.
   3. SIPs: If Structured Insulated Panels (SIPs) are used, they must be installed according to the manufacturer’s guidelines, and secured to the trailer in a similar manner as a wood frame.
   4. Windows & other exterior glass must be tempered, laminated, or secured with shutters during tow.
   5. Roofs: Minimum roof slope (measured in rise over 12″ run) must meet roofing material requirements. Vardo roofs are permitted.
      The roof pitch of a tiny house in a region that does not normally have snowfall must be at least .5/12 (a half inch inch risefor every foot of run).
      1. In regions where snowfall is expected, the roof must meet standards set by the American Society of Civil Engineers. If you are unfamiliar with these standards, consult a structural engineer. (Note: structural engineers charge about $100 per hour and review of plans can take four or more hours. If you’re low on funds, use a standard tiny house design).
8. Vapor and Moisture Barriers and Venting
   1. The THOW must be properly sealed with flashing, vapor barriers and moisture barriers:
      1. The undercarriage must have a barrier to prevent rain splash and ground moisture from penetrating.
      2. A rain screen or other weather barrier is recommended to keep the wall assembly dry.
   2. The roof must be vented. Exception: if closed cell spray foam insulation is used, a roof vent is not required.
   3. An exhaust fan must be present in the kitchen area or bathroom. A limiter is helpful, as well as a humidistate and timer. Adherence to ventilation standards in ASHRAE 62.2 is recommended.
   4. It is recommended that an infiltration test be conducted through use of a blower door test or equivalent. Identification of air leaks is critical for energy performance. An envelope leakage ratio meeting the International Energy Conservation Code is recommended.
   5. Energy recovery ventilators are recommended (but not required) in tight homes to protect indoor air quality. In humid climates, a dehumidifier may be also be needed.
9. Insulation
   1. Roof and floor insulation must be at least R19.
   2. Wall insulation must be at least R13 (recommended: R15).
   3. The THOW must have insulation around wheel wells and between the trailer and the floor joists, or, in the case of the trailer frame being the floor structure, a layer of at minimum 1/2″ insulation between the subfloor and the trailer frame is recommended to prevent thermal bridging.
10. Water, Sewer, Gas, and Electrical Systems
    1. Tiny house appliances and systems should be installed with extra caulking to withstand road vibration or be properly secured during travel.
    2. Pipes must be fastened in a manner that enables them to remain securely connected during towing.
    3. Gas connections made with threaded fittings must be made with high flexibility and high vibration resistant pipe thread compound. Inspect all gas connections for leaks prior to using system after each trailer move.
    4. Any appliance used in, on, or attached to, the house must be UL approved, or considered safe for residential use. For the interior envelope (enclosed space) of the house, use appliances that are UL approved for interior use. An example of what NOT to attach to or use in a house would be a typical propane camp stove.
    5. All electrical and gas systems must be provided with a way to disconnect. These systems should be turned off when the house is moved.
    6. Heavy objects that are part of the house may need to be fastened to the structure (beyond what is normally used in a conventional house.) This includes: wood stoves, bathtubs, appliances, propane tanks, water tanks, storage batteries, permanently placed furniture, etc. If they cannot be fastened, they should be removed when the house is moved.
    7. Drinking water should be supplied by food grade hoses, not standard garden hoses.
    8. Sinks, showers, and toilets must be either plumbed into holding tanks that can be emptied at an appropriate RV or marine pump out station, or plumbed into a sanitary sewer or septic system. Exception: Incinerating and composting toilets are acceptable.
    9. Plumbing must be vented through either the roof or a side wall (just below the roof line with an RV wall vent cap), in accordance with NFPA 1192, Chapter 7.6. However, unvented sinks are acceptable with HepVo traps.
11. Heating Systems
    1. Electric heating systems must be approved (UL listed) and installed in accordance with the manufacturers instructions and local codes.
    2. Wood stoves: Wood stoves must be EPA approved and use EPA approved piping. A CO (carbon monoxide) detector is required in the general living area and each bedrooom.
       1. There must be an exterior air inlet at or under the woodstove. This should remain completely open whenever the woodstove is (or has recently been) used.
       2. Ventilation fans should be rated at less than 60 cfm or a limiter must be used. Fans should be turned off and a window opened if the wood stove is smoking.
    3. Propane heaters: When a propane heater is present, propane gas detectors with electrical propane shut-off valves must be installed.
    4. Kerosene heaters are not permitted.
12. Fire safety: THOWs must be built to NFPA 1192 Chapter 6 standards that specify fire safety requirements, including egress (doors and windows that enable you to get out of the THOW in case of fire).
    1. If a loft is present, a stable means of getting to and from the loft is required but a ladder is acceptable; stairs are not required.
    2. A smoke detector is required in the general living area and each bedroom. The smoke detectors may be battery operated or wired.
    3. If the THOW has gas or wood burning appliances, a CO (carbon monoxide) detector is required in the general living area and each bedrooom. A combination propane-CO detector is acceptable. This is stated above under Heating Systems, but repeated here to include all gas or wood burning appliances, such as cook stoves and water heaters.
    4. A fire extinguisher must be readily accessible.

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REMEMBER to document your materials and methods as you progress in your build.
Once your tiny house is complete, you won’t want to have to open up the walls to get a picture of your wiring!

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Glossary

Basic functional area: a space allocated for performing an essential daily activity.

1. Area for sleeping: a separate room is not required; a mat on the floor is sufficient.
2. Area for preparing meals: a kitchen is not required. Shelves for storing food and a counter or tabletop for food preparation is sufficient. Appliances such as a refrigerator or stove are not required.
3. Area for basic toiletry: a toilet must be present as well as a facility for washing hands. This can be as simple as a water jug that drains into a basin.
4. Bedroom: a room separate from the main living area, with a door that closes, and a sleeping mat, futon, or mattress. A closet may or may not be present. Note: a bedroom is not required in a THOW. However, if it is present, it must have an egress window.
5. Egress window: a window with an opening of at least 17″ x 24″ to allow for exit in case of fire. A skylight can be an egress window, if the opening is large enough.
6. Loft: a platform 5 feet or more above the main floor and with dimensions of at least 3′ wide by 6′ long by 2′ 8″ high (at the highest point). Smaller lofts are considered storage areas rather than lofts. They must not be used for sleeping; they are not required to have an egress window.

Why is a tiny house not some other type of vehicle or house?

RV: Legally, a tiny house on wheels is an RV. However, a THOW differs from a traditional RV in that an RV is a recreational vehicle intended for non-permanent living, whereas a THOW is built as a residence. In addition, while RVs are typically built to the NFPA standard 1192 and have certification to prove that (either by the RVIA or an independent service), a THOW may be owner-built and may or may not be certified.

Park Model RV: Park models are RVs that are either wider than 8.5 feet in the transport mode or are greater than 320 sq ft in the set up mode. Park Model RVs, like regular RVs, must still be no bigger than 400 sq ft in the set up mode. While sometimes referred to as a “park model mobile home,” a park model is a type of RV and is intended for non-permanent living. According to the RVIA, “Park model RVs are actually titled and registered just like any other RV. Due
to their design, small size and use as recreation, vacation and seasonal units, PMRVs are explicitly excluded from being
considered or used as a manufactured home under the codes and regulations of the U.S. Department of Housing and
Urban Development (HUD) specifically because they are a type of recreation vehicle (Title 24 § 3282.8(g)).” Park Model RVs are built to the ANSI 119.5 standard.

Manufactured Home: A THOW is not a manufactured home because it is not built in a factory and it is often smaller than 320 square feet.
A manufactured home (formerly known as a mobile home) is built to the Manufactured Home Construction and Safety Standards (HUD Code) and displays a red certification label on the exterior of each transportable section. Manufactured homes are built in the controlled environment of a manufacturing plant and are transported in one or more sections on a permanent chassis. National manufactured home standards specify that manufactured homes must be at least 8 foot wide, or 40 feet long, for a footprint of at least 320 square feet.

References

1. Recreational Vehicle Industry Association – RVIA NFPA 1192 Standards To view online, click on “free access to the 2015 edition of NFPA 1192” ANSI 119.5 for Park Models RVs. Scroll down the link to find the standard and the handbook.
2. International Residential Code for One- and Two-Family Dwellings: 2015 IRC code book.
3. American Society of Civil Engineers ASCE/SEI 7 Minimum Design Loads For Buildings and Other Structures
4. Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, ASHRAE 62.2
5. Manufactured Housing Institute
6. PART 3280 Manufactured Home Construction and Safety Standards see especially definitions, including the definition of a manufactured home.
7. HUD Office of Manufactured Housing Programs