Water is the constraint everything else depends on. You can go three weeks without food. You go three days without water — less in heat, less under physical exertion, less if you are already compromised. In a grid-down scenario where municipal water has failed and stored water is running low, the ability to produce potable water from what is available in the ground and air around you is not a useful skill. It is the skill.
A solar still uses the sun’s energy to evaporate moisture from soil, vegetation, or contaminated water, then condenses that vapor on a cooler surface and collects the condensate as clean water. The process is simple physics — evaporation and condensation — and it has been used for emergency water production since at least the 1950s when the US military standardized it in survival manuals. It requires no fuel, no manufactured filter media, no electricity, and no moving parts. It requires a hole, a sheet of plastic, a container, and sun.
What a solar still is not: a high-volume water production system. A single ground still in good conditions produces ½ to 1 liter of water per day. That is survival quantity — enough to keep one person minimally hydrated at rest in cool conditions. It is not enough for cooking, hygiene, or any physical activity beyond sitting still. A solar still is an emergency last resort and a supplementary source, not a replacement for stored water or a gravity filtration system fed by rain collection. Know what it is and know when it is the right tool.
This post covers three builds: the ground pit still (the most basic, requires only a hole and plastic), the transpiration still (uses living vegetation as the moisture source without digging), and the improvised distillation still (produces higher volume from a contaminated water source using a rocket stove or fire). Each has a different application and a different yield.
HOW A SOLAR STILL WORKS
Solar radiation heats the soil and any moisture or vegetation inside the still. That heat drives evaporation — water molecules leave the surface and enter the air inside the sealed plastic enclosure. The underside of the plastic sheet, being slightly cooler than the air inside (it is exposed to ambient temperature on its upper surface), causes the water vapor to condense into droplets. Those droplets run down the slope of the plastic toward the lowest point — the center, where a weight has been placed to create a cone — and drip into the collection container below.
The process is self-contained. It runs as long as there is sun and moisture in the system. It produces water that is effectively distilled — evaporation and condensation leaves behind contaminants, salts, biological organisms, and most chemical pollutants. This is the primary advantage of a solar still over filtration: it does not filter contaminants, it leaves them behind entirely. Salt water, urine, contaminated surface water, and water from toxic soil can all be processed through a solar still and the condensate collected is potable.
Yield factors: Soil moisture content, ambient temperature, solar intensity, plastic clarity and tightness of seal, and the presence of added vegetation or water inside the still all affect yield. A still dug into moist soil in direct sun in a hot climate produces significantly more than a still dug into dry soil in a cool climate. Know your conditions and plan accordingly.
BUILD 1 — GROUND PIT STILL
The standard. Requires only plastic sheeting, a container, and a hole.
Materials:
- Clear polyethylene sheeting — 6×6 feet minimum, 6-mil thickness. Clear is essential — translucent or opaque plastic blocks the solar radiation that drives the process. Cost: $5-10 for a sheet this size at any hardware store. In a scenario where plastic is not available, any clear material that can be sealed over a pit functions — a car windshield, a glass panel, salvaged greenhouse glazing.
- A container for collection — any non-toxic container that fits in the pit. A cooking pot, a wide-mouth jar, a cut plastic bottle.
- A weight for the plastic center — a clean rock, a full water bottle, any object of moderate weight.
- Optional: a length of flexible tubing (aquarium tubing, surgical tubing) to drink from the collection container without disassembling the still. This dramatically increases practicality — opening the still to remove the container breaks the seal and loses the humid air inside, reducing efficiency of the next day’s production.
- A shovel or digging tool.
Site selection: Choose a site in full sun for the maximum possible daily hours — south-facing slopes or open ground without shadow from trees, structures, or terrain features. Dig in moist soil if possible — low areas, stream banks (not the stream itself), areas where vegetation is greener and denser than surrounding ground. Avoid sandy, very dry, or rocky soil — yield will be negligible. In an absolute emergency with no soil moisture, the still can be charged with urine, contaminated water, or even cactus flesh to provide a moisture source.
Digging: Dig a bowl-shaped pit approximately 3 feet in diameter and 2 feet deep. Bowl shape — sloping sides, rounded bottom — maximizes soil surface area exposed inside the still. A flat-bottomed pit with vertical sides works but is less efficient.
If you have optional vegetation available — green leaves, grass, cacti, succulent plants — layer them in the bottom of the pit. This significantly increases moisture available for evaporation and improves yield by 50-100% over bare soil in dry conditions.
Place the collection container in the center of the pit bottom. If using tubing, run it from the container up and out over the rim of the pit, long enough to reach your mouth while you sit beside the still. Weigh down the free end of the tube outside the pit so it does not slip in.
Covering: Lay the clear plastic sheet over the pit, centering it. Seal the edges by piling soil, rocks, or sand around the entire perimeter — the seal must be airtight. Any gap around the edges allows humid air to escape and reduces yield significantly.
Place the weight in the center of the plastic, directly over the collection container. The weight creates a low point — a cone — that directs condensate to drip into the container. The lowest point of the plastic should be 12-18 inches above the collection container opening, directly centered over it. Too close and the plastic touches the container and condensate runs down the outside. Too far and droplets fall with enough force to splash out of a shallow container.
Operation: The still begins producing within 30-60 minutes of sun exposure. Do not open it to check — you lose accumulated humid air every time the seal is broken. If you installed a drinking tube, sip from it as needed throughout the day. Disassemble at the end of the day to collect what the container holds, then reassemble immediately if you want overnight production (which is minimal — the still produces almost nothing without solar input, but retaining humid air inside improves morning startup).
Yield expectation: In moist soil with good sun and added vegetation: 1-1.5 liters per day. In dry soil without added moisture: 0.25-0.5 liters per day. In charged conditions (added contaminated water or urine in the pit): 1-2 liters per day from the added moisture source.
Extending production: Add fresh vegetation or additional moisture source daily. Move the still every 2-3 days as surrounding soil dries out from the harvesting effect of the still itself. A series of three stills rotated produces 3x the yield of a single still and allows each site to partially recover between uses.
BUILD 2 — TRANSPIRATION BAG STILL
No digging required. Uses living vegetation as the moisture source.
A transpiration still requires no excavation and no specific site — it uses the water that living plants continuously exhale through their leaves (transpiration) as the moisture source. It is buildable in under 5 minutes from a clear plastic bag and a branch. Yield is lower than a ground pit still but the build time and site independence make it valuable as a rapid emergency water source.
Materials:
- Clear plastic bag — a large clear trash bag, a clear storage bag, or any clear plastic that can be sealed around vegetation. Minimum 2-gallon capacity. Clear only.
- A tie, rubber band, or cord to seal the bag around the branch.
- A small rock or weight.
Site selection: Any living tree or large shrub in full sun. Deciduous trees in leaf are the highest producers — broad-leaf species like maple, oak, cottonwood, and willow produce more than conifers. Avoid toxic species — oleander, yew, and any plant you cannot positively identify as non-toxic. The water produced is distilled condensate from the plant’s own moisture, but you will be handling the bag and there is some contact risk from handling toxic vegetation.
Build: Select a branch with dense leaf coverage — a branch tip with 20-30 leaves is more productive than a bare branch. Do not cut the branch — it must remain attached and living to continue transpiring.
Enclose the leafy end of the branch in the clear plastic bag. Slide a small rock into the lowest corner of the bag before sealing — this creates a low point where condensate will pool. Seal the bag around the branch as airtight as possible using the tie or cord. The bag should be taut enough that it does not lie flat against the leaves — air space inside allows vapor to accumulate and condense.
Orient the low corner of the bag (where the rock creates a collection point) to face down and toward the sun. Secure the sealed end to the branch above to prevent the bag from slipping.
Operation: The bag will begin fogging with humidity within 30 minutes of sun exposure. Condensate collects at the low corner where the rock sits. Yield is visible within 1-2 hours. Collect by untying the bag carefully, tilting the collection corner down, and pouring or drinking directly.
Replace the bag on the same branch immediately after collection — the branch is still transpiring. Move to a fresh branch when the current one shows wilting — a stressed branch transpires less.
Yield expectation: 0.25-0.75 liters per bag per day depending on species, leaf density, temperature, and sun intensity. Set multiple bags simultaneously — a dozen bags on a productive tree in good conditions can yield 3-6 liters per day with no digging and minimal setup.
The limitation: Transpiration stills depend entirely on living vegetation and sun. In winter, on deciduous trees without leaves, yield drops to nearly nothing. In low-sun conditions, yield drops proportionally. This is a warm-season, sun-dependent method.
BUILD 3 — IMPROVISED DISTILLATION STILL
Higher volume from a contaminated water source. Requires heat.
When a contaminated water source is available — a pond, a puddle, a stream with biological or chemical contamination, salt water — an improvised distillation still produces significantly higher volume than either ground pit or transpiration method by actively boiling the contaminated source and condensing the steam. This requires a heat source (rocket stove, campfire, or conventional range) but produces 2-4 liters per hour in good conditions — enough for a household’s basic needs from a single setup.
Materials:
- A large pot with a lid — any metal pot, 2-gallon or larger.
- A length of metal or heat-resistant tubing to carry steam from the pot — copper tubing is ideal (available at hardware stores, salvageable from plumbing), rigid aluminum tubing works, a metal funnel connected to flexible tubing works in improvised situations. Length: 3-5 feet.
- A collection container for distillate.
- A cooling method for the condensing tube — cold water bath, wet cloth wrapped around the tube, or simply sufficient tube length in cool air.
- A heat source — rocket stove, fire, camp stove.
- A drill or punch to make a hole in the pot lid (or use a pot with a vent hole, or use a pressure canner’s vent port).
The principle: Boiling water produces steam. Steam is pure water vapor — contaminants remain in the boiling pot. If that steam is captured and routed through a cooled tube, it condenses back to liquid water in the tube and drips out the far end as clean distillate. This is distillation — the same process used to purify alcohol, produce essential oils, and desalinate water — applied with improvised materials.
Build: Drill or punch a hole in the pot lid sized to fit your tubing — the tubing passes through this hole and the joint is sealed with wrapped aluminum foil, high-temperature silicone, or dough pressed around the fitting if nothing else is available. The seal does not need to be perfect but should be close — steam that escapes the connection rather than entering the tube is water that does not get collected.
Angle the tube so it exits the lid at a downward slope — steam rises and will enter the tube; the downward slope allows condensate to run toward the collection end under gravity rather than pooling in the tube.
Route the tube through a cooling bath if possible — a bucket of cold water with the tube coiled inside several times dramatically increases condensation and yield. Without a cooling bath, extend the tube length — 4-5 feet of tubing in cool air condenses adequately for a slow but functional yield.
The free end of the tube points down into the collection container. No seal is needed at this end.
Operation: Fill the pot 2/3 full of the contaminated source water — salt water, pond water, collected rainwater with biological contamination, or any water source you cannot verify as safe. Bring to a rolling boil on your heat source. Distillate begins dripping from the collection end of the tube within a few minutes of boiling.
Maintain boiling. Add source water to the pot as needed — do not let the pot boil dry. Discard the concentrated residue in the pot when the source water is exhausted — it contains all the contaminants removed from the distillate.
Yield expectation: 2-4 liters per hour from a 2-gallon pot at a rolling boil, depending on tube length and cooling efficiency. This is the only method in this post capable of producing household-scale water quantities from a contaminated source.
What distillation does and does not remove: Distillation removes biological contaminants (bacteria, viruses, protozoa), salts, heavy metals, and most chemical contaminants. It does not remove contaminants with boiling points lower than water — certain volatile organic compounds (VOCs) including some industrial solvents will carry over with the steam. For agricultural or household water sources with biological contamination, distillation is fully effective. For heavily industrially contaminated sources, combine distillation with activated charcoal filtration of the distillate for maximum safety.
COMBINED SYSTEM — SOLAR AND DISTILLATION
In a prolonged scenario, the correct water system uses all available methods simultaneously:
Stored water — 30-90 day supply, used first and replaced first. Rain collection — the barrel system (see Rain Barrel System in DIY Schematics) feeding a gravity filter (see Gravity Water Filter Build) for the bulk of ongoing water needs. Solar stills — supplementary production from soil moisture and vegetation, particularly useful when rain collection is not possible. Distillation still — high-volume processing of available contaminated surface water when other sources are insufficient, or when the water source is saline or chemically compromised beyond what filtration addresses.
No single method is sufficient on its own for a household’s ongoing water needs. The system approach — using each method for what it does best — is what produces reliable water security.
SITE-SPECIFIC CONSIDERATIONS
Arid and desert climates: Ground pit stills produce minimal yield from dry soil. Charge every still with cactus flesh, green vegetation, or any available moisture source. Transpiration stills are more productive than ground stills if any living vegetation is present. Distillation from any available water source is the highest priority in extreme aridity.
Humid and temperate climates: Ground pit stills produce good yields from moist soil, particularly in low areas. Transpiration stills are highly productive during the growing season. Rain collection (see Rain Barrel System) is the primary water strategy — solar stills are backup.
Cold climates and winter: Solar still productivity drops significantly in winter — shorter days, lower sun angle, frozen ground. Snow and ice melt on the rocket stove or any heat source is the more practical collection method — 1 gallon of packed snow yields approximately 1 quart of water. Solar stills are a warm-season method in cold climates.
Coastal and saltwater access: The distillation still is the primary water production method — converting available salt water to fresh. One gallon of salt water processed through the distillation still yields approximately 0.75 gallons of fresh distillate, with the salt remaining in the pot.
WARNINGS
Yield is not volume. A solar still produces survival quantities, not living quantities. If a ground still is your only water source, you are in a serious situation. Use it to buy time while establishing a higher-volume source.
Clear plastic only. Translucent, milky, or colored plastic blocks solar radiation. The still may look assembled and functional and produce nothing because the plastic is wrong. Clear 6-mil polyethylene is the correct material and it is cheap enough to store several sheets in your emergency supplies.
Do not rely on untested methods in an emergency. Build a ground still now, in your yard, and measure what it produces in your specific soil conditions and climate. The yield figures in this post are averages across a wide range of conditions — your conditions may be better or significantly worse. Know your numbers before you need them.
The distillation still and toxic sources. Distillation is not a universal solution for chemically contaminated water. If your water source comes from an area with known industrial contamination, fracking activity, or superfund-adjacent land, distillation alone may not be sufficient. When in doubt, distill and then filter through activated charcoal, and use that water for sanitation rather than drinking if any doubt remains.
FINAL THOUGHTS
A solar still is not a comfortable answer to a water problem. It is an uncomfortable answer to a water crisis — enough to keep one person alive while a better solution is being established. Build the ground pit still because the skill and the supplies cost almost nothing and the knowledge costs nothing at all. Store several sheets of clear 6-mil polyethylene in your emergency supplies — they weigh almost nothing and serve half a dozen uses beyond water collection. Know the transpiration bag technique because it requires nothing more than a clear bag and a tree and produces water in 30 minutes.
Then build the rain collection system, the gravity filter, and the water storage that make the solar still the backup it should be rather than the primary source it should never have to be.
Water first. Always water first.
For rain collection that feeds a gravity filter, see Rain Barrel System in DIY Schematics. For filtering collected water to potable quality, see Gravity Water Filter Build. For the rocket stove that powers the distillation still, see Rocket Stove Build. For water storage targets and treatment, see The Storage Blueprint in the Field Rations Archive.