Turn the tap and nothing happens. Off-grid, there is no water utility to call, only the system you designed into the floor plan: where the tanks sit, how the roof drains to them, where filtration lives. Water is not a fixture choice; it is an architectural decision.
The ten layouts below integrate the whole water cycle into the plan: gravity-fed loft tanks, under-floor cisterns, first-flush diverters, a dedicated filtration closet, and dual potable and non-potable plumbing. You will get the weight math, pipe-run discipline, and freeze-protection details that keep water reliable instead of a recurring crisis.
Water Is a Floor Plan Problem
An off-grid tiny house does not have a water utility; it has a water system, and the system lives in the floor plan. Where the tanks sit, how the roof drains to them, where the filtration lives, and how the plumbing routes determine whether the house has reliable water or a recurring crisis. These ten layouts integrate rainwater catchment, storage, filtration, and distribution into the architecture from the first sketch.
1. The Gravity-Fed Loft Tank (Roof-to-Tank Simplicity)
The simplest system: roof gutters feed a 300-gallon tank in the loft (or on a raised platform at 8+ feet), and gravity pressurizes the house (0.43 PSI per foot of elevation; 8 feet gives 3.4 PSI, enough for a low-flow shower with a booster pump for the rest). The floor plan puts the tank in a dedicated loft bay (engineered for 2,500 pounds), with the bath directly below it (shortest pipe run, least heat loss).
Weight: water weighs 8.34 pounds per gallon. A 300-gallon tank weighs 2,500 pounds; the platform needs engineered joists (2×10 minimum, 12″ OC) or a steel frame. Never guess at water weight.
2. The Under-Floor Cistern (1,000 Gallons Below)
Put the storage under the house: a 1,000-gallon cistern in the crawlspace or basement, fed by gutters, feeding a pressure pump and pressure tank. The floor plan is unaffected (the cistern is invisible), the water stays cool and dark (no algae), and 1,000 gallons is a month of conservative use. The crawlspace needs 4+ feet of clearance for tank installation and maintenance access.
3. The Wet Wall Manifold (Centralized Plumbing Core)
Cluster all fixtures on one 12-foot wet wall with a PEX manifold at its center: kitchen sink, bath vanity, shower, toilet, and laundry all within 6 feet of the manifold. Total hot-water pipe run under 30 feet (less waste, faster hot water); the manifold’s individual shutoffs let any fixture be serviced without killing the whole house. The floor plan’s discipline (one wet wall) makes the plumbing trivially simple.
4. The First-Flush Diverter Plan (Roof Washer Integration)
Design the gutter system with first-flush diverters: the first 10 gallons of each rain (which wash dust, pollen, and bird droppings off the roof) divert to a separate pipe, then clean water flows to the cistern. The floor plan implication is exterior (the diverter needs a drain-away location), but the water quality improvement is dramatic. The diverter resets automatically; the owner does nothing.
Sizing: divert 10 gallons per 1,000 square feet of roof per storm. A 200-square-foot tiny house roof needs a 2-gallon diverter; small roof, small diverter.
5. The Filtration Closet (3×4 Treatment Room)
Dedicate a 3×4 closet to water treatment: sediment filter (20 micron), carbon filter (5 micron), UV sterilizer, and the pressure tank, in sequence along one wall. The closet is inside the thermal envelope (filters and UV cannot freeze), with a floor drain (filter changes spill water) and a light. The floor plan treats water treatment as a room, not an afterthought; maintenance takes 15 minutes, not an excavation.
6. The Dual-Plumbing Plan (Potable + Non-Potable)
Run two water systems: potable (filtered rainwater) to the kitchen sink and bath vanity, and non-potable (unfiltered rainwater) to the toilet, shower, laundry, and garden. The dual plumbing halves the filtration load (only 5-10 gallons per day need full treatment) and extends filter life 3x. The floor plan marks every non-potable outlet (different handle color); the plumbing diagram is posted inside the filtration closet.
7. The Composting Toilet Bypass (No Blackwater)
Eliminate blackwater entirely with a composting or incinerating toilet: the plumbing plan handles only graywater (sinks, shower, laundry), which is simpler, smaller, and legal in far more jurisdictions. The bath layout gives the composting toilet its ventilation (a 4-inch vent through the roof, 12V fan) and access (a 2×3 service zone). Water use drops 30%; the septic question disappears.
8. The Hot Water Core (On-Demand at the Fixtures)
Put point-of-use on-demand water heaters at the shower and kitchen sink (electric or propane, 3-6 kW each) instead of one central tank: zero wait for hot water, zero standby loss, zero long pipe runs cooling between uses. The floor plan gives each heater a 2×2 utility zone with ventilation (propane) or a 30-amp circuit (electric). Total energy use drops because nothing is kept hot all day.
Sizing: a low-flow shower (1.5 GPM) needs 18,000 BTU/hr (5.3 kW) for a 70°F rise. Size the heater for the flow rate, not the house.
9. The Seasonal Overflow (Wet-Season Management)
Design for too much water, not too little: an overflow from the cistern to a rain garden or infiltration swale, sized for the 100-year storm, with the floor plan grading the site away from the house. The gutter system includes overflow scuppers (so a clogged downspout does not back water into the eaves). Wet-season planning prevents the ironic disaster of water damage in a rainwater house.
10. The Community Well Plan (Shared Water Infrastructure)
For tiny house communities: one shared well or large catchment system (10,000+ gallons) serving all houses through a buried loop, with each house connecting via a metered lateral. The individual floor plans need only a pressure tank and a water heater; the community handles collection, treatment, and storage. Per-house cost drops 60% versus individual systems, and a professional maintains the shared infrastructure.
Rainwater Math Reference
- Collection: 1 inch of rain on 200 sq ft of roof = 125 gallons (at 90% efficiency).
- Usage: conservative tiny house use = 20-30 gallons per person per day.
- Storage rule: size the cistern for 30 days of use (600-900 gallons per person).
- Roof material: metal or tile only; asphalt shingles shed petroleum and granules.
- Gutter sizing: 5-inch K-style handles a 200 sq ft roof in most climates; 6-inch for monsoon zones.
Freeze Protection for Water Systems
Every component outside the thermal envelope needs freeze protection: bury supply lines below the frost line (or heat-tape them), insulate the cistern (or bury it), and put the filtration closet inside the house (never in an unheated shed). The floor plan’s thermal boundary is the water system’s thermal boundary; design them as one.
Final Considerations
An off-grid water system is plumbing, filtration, and storage designed as a single machine, and the floor plan is the machine’s chassis. Put the tanks where the structure can hold them, the treatment where hands can reach it, and the pipes where they stay warm. Water is life; the layout is the life-support system.