Building in Mexico · off-grid water
Off-grid water in Mexico: how I run a house on rainwater
Everyone planning rainwater worries about catching enough. On my plot the opposite is true — in a hard downpour I can catch more than I can use in a week, and then nothing falls for months. Off-grid water isn’t really a collection problem. It’s a storage and timing problem.
How it works
My land came with no water connection. So the house runs on rain: caught off the roofs, filtered simply, stored in a buried cistern, pumped up to a roof tank, and fed back down by gravity — with the dirty water filtered again through a system I built by hand. This is what that actually takes, and what living on it is really like.
What this covers
This is based on one self-managed build in San Cristóbal de las Casas, Chiapas — not a sanitation design, a drinking-water safety standard or a legal guide to water rights. It’s the off-grid half of the water story: the companion to my water system for a house in Mexico, which covers getting water into and through the house — cistern build, pump, roof tank, gravity feed and the CPVC-vs-TuboPlus pipe choice. This page is about where the water comes from and where it goes afterwards.
My off-grid water loop at a glance
Tin roof → simple stone filter → 10,000 L cistern → pump → roof tank → house → wastewater filter → soakaway
The honest version
I’m not an off-grid purist — this is about resilience
Let me be honest about what this is and isn’t. I’m not living a zero-impact dream. I have flush toilets — I broke the off-grid rule about composting — though I fitted the smallest tank I could find and dropped a 600 ml bottle inside it, which brings each flush down to about 3.1 litres. I run a pump on grid electricity. This isn’t ideology; it’s a house with no water connection, made to work on what falls on the roof.
I spent years on international-development work in rural Guatemala — at one point as an innovation specialist, where I tested a rainwater-storage bag from Africa that sat under a roof like a small tent. Supplying my own family needed something far more robust than that bag, and an off-grid water system is what I settled on.
The real constraint
The real problem with rainwater isn’t catching it
If you’re planning rainwater harvesting in Mexico, the first thing to learn is that catching the rain is the easy part. Before I built anything, I thought rainwater was mostly about collection — point the roof at a barrel and you’re done. Three things turned out to be the real challenge, and only one of them is easy.
The three challenges of rainwater
| The challenge | Why it’s harder than it looks | What I did |
|---|---|---|
| Getting the water off the roof | Roofs usually aren’t pitched evenly across a flat plane, so runoff doesn’t all arrive where you want it. | The easy one — gutters and routing solve it. Plan it into the roof rather than bolting it on later. |
| Filtering fast enough | Torrential rain arrives all at once; a fine filter clogs or overflows and you lose the very downpour you wanted to catch. | Deliberately under-filter at the catchment — keep flow high, clean the water later in the chain (below). |
| Storing it through the dry months | This is the real constraint. Rain comes in a few intense months, then little for the rest of the year. | Size the cistern for the drought, not the downpour. Storage is everything. |
The third row is the whole game. My tin roof — the one that supplies the house — is about 50 m² (5 × 10 m), and on the heaviest days I can catch up to 5,000 litres from it in a single day. That takes a real tropical storm, though: a millimetre of rain on a square metre is a litre, so 5,000 litres off 50 m² means around 100 mm of rain in a day — which only happens a handful of times a year. After the rains end around mid-November, you’re living on what you stored through October and November, with only a handful of light, infrequent showers until the next wet season arrives in May or June. That long dry stretch is exactly why the first real decision is storage.
Storage
Start with storage — it’s the one thing you can’t redo
Almost everything in an off-grid water system can be adjusted later. The cistern can’t. It’s poured concrete in the ground, and you live with the size you chose.
Mine is a concrete-lined cisterna holding about 10,000 litres. It works — it has carried a family of four through the dry seasons. But if I were pouring it again I’d go to 12,000–15,000 litres, because the storage is what bridges the gap between the months it pours and the months it doesn’t. Once it’s set, enlarging it means breaking concrete. The build itself — what it cost, how it’s lined — sits in the water system guide alongside the pump and roof tank; here the point is simpler: err bigger. Storage is the cheapest insurance against a long dry season, and the hardest thing to add afterwards.
A rough way to sanity-check the size
Work backwards from a dry spell, not from rainfall
Frugal, we can run on about 1,000 litres a week as a household of four — two quick showers each and reduced-flush toilets, before any clothes washing. Wash clothes regularly and it climbs nearer 2,000. Hold your own figure against the longest dry stretch you need to cover and you get a feel for the storage you need between meaningful rains. It’s back-of-an-envelope arithmetic from one house, not a design calculation — your roof area, climate and habits will move it a lot — but it’s the right question to ask before you pour.
Filtering
The mistake is filtering rainwater too finely, too soon
This is the counterintuitive part: at the point of catchment I filter less, not more. A fine filter is exactly the wrong thing when 5,000 litres wants through in an afternoon — it clogs, backs up, and the rain you were trying to catch runs off the edge.
So my first-stage filter is deliberately crude: a 50-litre drum with large stones (4–6 inches) at the bottom and smaller ones (around 2 inches) above. No fine media, nothing that chokes the flow. It takes out the leaves and grit and lets the volume through. The cleaning then happens further along the chain, where it doesn’t have to keep up with a downpour:
My filtering chain — crude first, finer later
- Plastic drum on the ground — takes out debris, keeps the flow rate high
- The inline filter that comes with the Rotoplas roof tank — a second pass as water is pumped up
- Cheap shower filters (from Amazon Mexico) at the point of use
- A kitchen filter for drinking water
Catch fast and dirty. Clean slowly and often.
The quality surprised me. The water leaves no hard-water scale in the showers, sinks or toilets, and to my taste it’s cleaner than the town supply — I’d rather drink it than rely entirely on refilled garrafones. But I’ll be straight: taste isn’t a safety test. I drink mine — mostly as tea, so it’s boiled — and I’ve had no problems, but that’s my experience in my house, not advice. Roof runoff can carry bacteria and whatever’s on your roof. If you’re drinking rainwater, filter and test it properly, and treat the well-permit and water-quality side as something to confirm with someone qualified for your own site.
Roof routing
Which roof’s water goes where
Not all rainwater is equal, and the roof it comes off decides what it’s good for. Water from my lámina (tin) roof is noticeably cleaner than runoff from the concrete roof — so I route them to different jobs rather than mixing them.
Water by roof — each source to its job
| Source | How clean | What I use it for |
|---|---|---|
| Tin (lámina) roof | Cleanest runoff | Feeds the cistern → pumped to the roof tank → kitchen and bathrooms |
| Concrete roof | Dirtier, more porous | Routed to a separate Rotoplas tank for washing clothes |
| French drain | Ground runoff — debris | Buried barrel in the garden for plants and animals |
This is one more reason the roof decision and the water decision are tied together — the material you put overhead changes the water you can collect under it. I get into the wider trade-off in tin roof vs concrete roof in Mexico. It’s also why, if your land has no connection, the roof and the catchment should be planned together from the start, not stitched together afterwards — the same point I make about buying land in Mexico with no water on it.
Overflow
Planning for too much water
The flip side of catching 5,000 litres in a day is that, in the wet season, you have far more than you can store or use — and water you don’t direct deliberately ends up where you don’t want it. The tin roof fills the main cistern; my concrete roof and the French drain can deliver roughly as much again, but the tanks they feed are small — a 1,000- and a 500-litre Rotoplas — so on a big storm day most of that second stream just overflows. A diversion pipe sends the excess out to the street once the tanks are full. So part of an off-grid setup is routing the overflow on purpose — and accepting you’ll lose the rest.
My approach is low-tech and cheap. I run PVC tubing with taps at the junctions so I can switch the flow different ways — diverting it into a buried garden barrel, for instance, to keep water for the plants and animals. On a slope, French drains do the same job for ground runoff: they feed the same storage rather than letting the water cut channels across your land.
Wastewater
The hard part I underestimated: bathroom wastewater
I assumed the water leaving the bathrooms would be the easy part. I’d read about filtration systems, watched videos, and thought it would be a case of digging, placing tanks, adding filter material and moving on.
It wasn’t. It became one of the biggest manual jobs I took on during the whole build.
Rainwater gets most of the attention because it comes off the roof, but the other half of an off-grid water system is what happens after water leaves the house. In my case the bathroom wastewater goes through a three-stage settling and filtration system I built from ordinary hardware-store materials — partly because I couldn’t get many of the materials I saw recommended online. I’m setting it out here as a cautionary example, not a design to copy.
The first two stages are buried plastic water-storage tanks. The first simply catches and settles the solids. The second I turned into an upflow filter: water is piped in at the very bottom, so the only way out is back up through a bed of filtration stones, held off the tank floor on a raised platform I improvised from upturned buckets. The one thing I’d change is the mesh in that platform — I used steel because it was what I could find locally, but it will rust, and plastic or stainless would last.
The final stage is a plastic barrel from the market, packed with larger 1–2 inch stone, as a last filter before the water soaks away into the ground — into a pit I backfilled with stone and rubble left over from the construction.
The first lesson was about the tanks themselves. I went cheap and used standard plastic water-storage tanks rather than reinforced tanks made for burial — and an empty tank in a wet hole does not stay where you put it. To weigh them down while I worked I part-filled them with water; half full felt like more than enough. Then the first heavy downpour of the year filled the excavation overnight, and the next morning I found one tank floating in it, lifted clean out of position. Clearing that up meant bailing the flooded hole by hand, bucket by bucket — the most miserable, grinding stretch of the whole build.
What it taught me is to treat a buried plastic tank as a buoyancy and pressure problem, not just a container. Half-filling mine wasn’t enough — once the hole flooded, the tank still had enough buoyancy to lift out of position, and an empty one could be crushed by saturated soil instead. So if I were doing it again I’d fill it right to the top before backfilling and keep it full until the earth was packed tight around it.
The second lesson was access. I was improvising as I built, and I hadn’t thought through how I’d do maintenance until the tanks were already in the ground. That became a real problem, because the wastewater pipes from the house ran much deeper than the level of the garden. So I didn’t just need deep holes for the tanks — I needed a way to create an access cavity above them, below garden level.
In the end I had the builder make a small roof-like concrete structure over the tanks, with two removable concrete slabs for access. Because the whole thing was going to be buried under the garden, I placed large used truck tyres over the access slabs — free from a tyre shop — so I wouldn’t have to dig up the garden every time I needed to get in. Then I put big plant pots inside the tyres, with stones around them, which hid the whole structure completely.
It works, but it was a workaround. If I were doing it again, I’d design the wastewater access before digging anything, and I’d avoid sending the drainage so deep into the garden in the first place. That one decision added cost, labour and difficulty to everything that followed.
Two honest caveats, because this is the part where getting it wrong has consequences. First, if I were building it again I’d probably have a mason build the stages in concrete rather than repurposed plastic tanks — more capacity, especially for solids, and a more durable system overall. Second, I badly underestimated the labour. It took months of digging, moving earth and finding the right rocks for the filtration, and then having a builder create that buried roof around the tanks and pipes. This was not a weekend job.
Important caution
Wastewater is a health and regulatory matter
I’m describing what I built for my own house, not a sanitation design. Bathroom wastewater, greywater and blackwater can be regulated, and mistakes can create health problems. Treat this as one person’s improvised system and confirm the design, distances, maintenance and legal requirements with someone qualified for your own site.
What I’d change
What I’d do differently
I’d pour a bigger cistern — 12,000 to 15,000 litres — because storage is what carries you through the dry season and it’s the one thing you can’t enlarge later. I’d plan the catchment and the roof together rather than treating collection as something to add once the house was up. And I’d accept sooner that the easy-sounding part (catching rain) is trivial next to the parts nobody pictures: storing it, routing the excess, and dealing with what leaves the house.
Low-water living
What low-water living actually feels like
People imagine off-grid water means hardship. Mostly it means attention — and there isn’t a single number, there’s a range. Where you land in it comes down mostly to two choices: how you wash clothes, and what kind of toilet you have.
Water use per week — where you land in the range
| Scenario (household of four) | Roughly per week | What it assumes |
|---|---|---|
| Frugal | ~1,000 litres | Two quick showers each, reduced-flush toilets (~3.1 L), no clothes washing |
| With laundry | ~2,000 litres | The same household, but regularly washing clothes — the big swing |
| Compost toilets | ~1,000 litres | Toilet savings traded for more frequent showers, still near the frugal figure |
The biggest swing is laundry — washing clothes is genuinely water-intensive, and it’s most of the gap between those columns. Toilets are the other lever: mine are conventional flush cut to about 3.1 litres, but compost toilets would buy back enough to shower more often and still sit near the frugal end. Either way, one thing worth knowing: washing dishes can use as much water as a shower depending on your taps and how you do it, so the savings often hide in the ordinary jobs rather than the obvious ones.
What you gain in exchange is independence and, honestly, better water — soft, scale-free, and not trucked in. What you give up is the ability to forget about it. You watch the cistern through the dry months the way other people watch a fuel gauge. After a few years it’s just a rhythm, not a chore.
The rain is the easy part. Everything that decides whether you have water in April happens before the first drop falls.
FAQ
Common questions
Can you really run a house on rainwater in Mexico?
On my plot in Chiapas, yes — with no mains connection, rainwater off the roofs is my main supply, stored in a 10,000-litre cistern and pumped to a roof tank. Whether it works for you comes down to your rainfall, roof area, storage size and how long your dry season runs. The limiting factor is almost always storage, not how much rain you can catch.
How much water does a house use per week in Mexico?
For my household of four in Chiapas, how much rainwater we actually need runs from about 1,000 litres a week when we’re frugal — two quick showers each and reduced-flush toilets, before any clothes washing — up to around 2,000 once we’re regularly washing clothes, which is by far the most water-intensive job. Compost toilets would let you shower more often and still sit near the lower figure. Your own number depends on showers, laundry and toilet type more than anything else.
How big a cistern do you need to live on rainwater?
Mine holds about 10,000 litres and has carried a family of four (using roughly 1,000–2,000 litres a week, depending on laundry) through the dry seasons. If I were pouring it again I’d go to 12,000–15,000 litres for more margin. Size it against your longest dry stretch, not against a heavy day of rain — and remember it’s the one part you can’t easily enlarge afterwards.
How do you filter rainwater for a house?
I filter crude-first, fine-later: a stone-filled drum at the catchment to keep the flow rate high in heavy rain, then the inline filter on the Rotoplas roof tank, then cheap shower filters and a kitchen filter for drinking. A fine filter at the catchment just clogs in a downpour. For drinking water specifically, filter and test it properly — clean-tasting isn’t the same as tested-safe.
Is rainwater off a tin roof cleaner than off concrete?
In my experience, clearly yes. Runoff from my tin (lámina) roof is cleaner than off the concrete roof, so I feed the cistern (and the house) from the tin roof and send concrete-roof water to a separate tank for washing clothes. The roof material and the water you can collect are linked decisions.
Do I need a permit for an off-grid water system?
Rainwater catchment is one thing; drilling a deep well (pozo profundo) is another — as a foreign resident I understood I needed to confirm permission before drilling, and water is regulated. Wastewater handling is also subject to rules. I’d treat all of this as something to confirm with a lawyer or the relevant authority for your own site rather than assume — in Mexico, water is regulated federally through CONAGUA. I’m describing my setup, not the law.
Written by Luke Burchell
I self-managed the construction of my own house in San Cristóbal de las Casas, Chiapas, which runs off-grid on rainwater — catchment, a 10,000-litre cistern, gravity feed, and a wastewater system I built by hand. Before that I spent years on rural-development work in Guatemala. Everything here is from that experience: what I built, what I’d change, and what I’d plan earlier.