Codes Corner: A larger perspective on tiny house plumbing
Advancements allow for all the bells and whistles while still maintaining a small footprint.
When you hear the phrase “tiny house,” you may imagine a tepee tent house, an igloo or maybe a treehouse. But, unlike these structures, tiny houses are constructed similar to larger homes with basic necessities such as a water supply and drain system. Advancements in material, craftsmanship and plumbing have allowed tiny houses to resemble larger homes with all the bells and whistles while still maintaining a small footprint.
A common question that comes to mind is: What are the code requirements for tiny houses? The Uniform Plumbing Code (UPC) and the National Standard Plumbing Code (NSPC) address the health and safety provisions for larger houses. Like larger houses, tiny houses also must meet the necessary plumbing requirements found in the codes. However, there are distinct provisions that are particular to only tiny houses; the 2021 NSPC addresses such provisions in Appendix L.
The 2021 NSPC defines tiny houses as a structure that, where erected, is 400 square feet or smaller. Tiny houses are commonly built on fixed foundations or on wheels. The appropriate local zoning codes will indicate where such tiny houses are permitted. Some may think there are major plumbing design differences for a tiny home, requiring intricate detail, but the plumbing systems are like a standard home. One main difference is that much less material is needed and most tiny house plumbing systems are made up of only one kitchen sink, one lavatory, one shower and one water closet.
SIMILARITIES BETWEEN TINY HOUSES AND LARGER HOUSES
Access to potable water is required for drinking, cleaning and hygiene. Where a fixed potable water supply and meter are available, the water supply and distribution would be installed in accordance with the plumbing code. Sizing the supply and distribution piping will be dictated by the fixtures in the house and the corresponding water supply fixture units (WSFU).
Where no public water is accessible, alternate methods may be implemented where permitted and regulated by the local jurisdictions. The UPC and NSPC are great resources and contain provisions for alternate water sources such as private water supply systems and storage water tanks.
The drain, waste and venting piping system will be sized by using the plumbing code tables for Drainage Fixture Units (DFU) for all the tiny house fixtures. Just as with a larger home, the drainage is required to be sloped not less than 1/4-inch per foot and can deviate where appropriate per the plumbing code. Fixture traps are also required as they are needed to prevent sewer gases from entering the living space.
Water heating for tiny houses is also very similar to a standard plumbing system, only in a smaller capacity. The plumbing code contains water heater sizing provisions based on the first-hour rating method. Water heaters can be a storage tank type or instantaneous type. These water heaters run on natural gas, liquid propane gas or electricity. The available sources and whether the tiny house will be mobile will dictate the type to consider. Where solar thermal systems are being considered, keep in mind that an auxiliary backup water heater may be required, as weather and storage limitations may not always be reliable in providing hot water.
Determining the location and construction of a tiny home is largely dependent upon how waste disposal from water closets is handled. The option to connect directly to a fixed drainage system where available would be encouraged, as it would allow for self-cleansing water closets that would leave the premise as soon as you flush. However, this is not always an option for more remote or off-the-grid conditions. Composting is a popular alternative and the NSPC contains detailed provisions on the composting system components, materials, design and maintenance.
Macerating or sewage pump toilets may be an option where pumps are necessary to push waste uphill to reach a drainage system. Some models are designed to receive drainage from not only a water closet but also lavatories and bathtubs/showers within the same room. These systems liquefy waste and then pump it to the designated drainage system. These systems typically allow the waste to be pumped through a smaller diameter pipe. Such systems contain mechanical pumps and moving parts essential for their functionality. These do work well, but permanent, long-term installations may require maintenance.
What about freezing conditions? Tiny houses built on a lifted frame may contain exposed supply lines, drain-pipes, traps, vents and other piping or tubes subject to freezing. Insulation is the first resort against freezing. You may want to consider sealing and insulating the perimeter of the house. Hose bibbs should be frost-proof, with integral backflow prevention capabilities or as required by the local jurisdiction. The UPC guides the end-user to bury the supply piping not less than 12 inches below the local frost line for protection. The NSPC and UPC require specific minimum vent terminal sizing, above, below and through the roof to prevent these vent pipes from freezing.
For example, the NSPC says, “Where the Authority Having Jurisdiction requires protection against frost closure, vent terminals less than 3-inch pipe size shall be increased at least one pipe size to not less than 3-inch size. Where an increase is necessary, the increase in size shall be made inside the building, at least one foot below a roof or ceiling that is thermally insulated and in an area not subject to freezing temperatures.”
SPECIFIC TO TINY HOUSES
So far, we have discussed general plumbing provisions that pertain to all plumbing systems, regardless of size. The 2021 NSPC contains a new appendix (Appendix L) for tiny houses. The appendix separates the provisions between single tiny houses (Part I) and tiny house communities (Part II) as there are distinct requirements for “tiny house communities” that are not required when a lot contains a single tiny house. Section L.2.1 of the NSPC defines a tiny house community as follows: “Tiny House, Community: A structure(s), where erected, is 400 square feet (37 m2) or less, and of not less than two structures in the same lot.”
The new appendix contains specific provisions for tiny houses such as requiring a kitchen with no fewer than one sink and a restroom with no fewer than one water closet, one lavatory and one water-tight bath, shower or combination bath/shower. Further, the new Appendix refers the reader to the appropriate code sections for ease of use of the code for compliance.
The appendix also contains dimensional provisions for bathrooms, such as a minimum ceiling height no shorter than 6 feet, 8 inches (2032 mm) from the floor when measured at the center, front area of the fixtures. Ceiling heights above fixtures should not interfere with the fixture’s intended purpose.
Showers or combination bath/shower require a ceiling height no shorter than 6 feet-8 inches (2032 mm) where measured from the shower drain.
Because of spacing limitations, the provisions in the NSPC appendix give designers the minimum spacing requirements in tiny house restrooms with the intent that the space is usable, and the fixtures are placed with enough space to utilize safely.
There are many choices and possibilities when designing and building a tiny house. The UPC and NSPC are valuable documents giving the person appropriate guidelines for plumbing systems. Since the number one focus is health and safety of the public, these respected codes provide a wealth of information that will assist in completing a well-built tiny house plumbing system for years to come.