Follow along as we investigate a couple considering buying land and building a home in El Salvador. Before they buy, we’re putting the property through a series of real-world tests, from what’s beneath the ground to whether the land can actually support the home they imagine. Will they ultimately buy it? The file’s still open.
The last investigation looked down.
This one started by looking up.
Because there was another potential water source nobody had investigated yet.
The roof.
Could the roof become the well?
The math starts simple. One millimetre of rain falling across one square metre of roof produces one litre of water. That doesn’t sound like much, until the roof gets bigger and the rain starts falling.
The final roof dimensions haven’t been determined yet, so for the calculation, the investigation used a representative roof of 150 square metres.
September is one of the wettest months of the year here. MARN’s 1991–2020 climate data for this part of El Salvador records an average of 424.2 millimetres of rain that month.
Put a 150-square-metre roof underneath that, and the number becomes more than 63,000 litres of water.
Falling on one ordinary-sized roof.
In a single month.
And that’s before counting the rest of the rainy season.
Suddenly the roof wasn’t just something keeping the rain out. It was potentially collecting an enormous amount of water. Maybe the buyer didn’t need to find all of the property’s water underground. Maybe some of it had been falling on the house all along.
SO… PROBLEM SOLVED?
For a moment, it looked that way.
More than 63,000 litres in a single month is not a small number. And MARN’s climate data records roughly 2.16 metres of rain here over an average year.
Across that same 150-square-metre roof, that’s somewhere in the neighbourhood of 324,000 litres of rain annually.
On paper, the roof looked less like a backup water source and more like an answer.
Except annual rainfall was the wrong number to be reassured by.
THEN THE RAIN STOPS
Spread month by month, the picture changes considerably.
September: 424.2 millimetres.
October: 389.1.
November: 74.3.
December: 7.4.
January: 1.5.
February: 1.6.
The roof hadn’t stopped working.
The sky had.
The problem wasn’t how much rain fell over an entire year. It was when it fell. During the wet season, the roof could receive tens of thousands of litres in a single month. Then the dry season arrived and, for months at a time, it could collect next to nothing.
The investigation was no longer asking whether the roof could collect enough water.
It was asking something harder.
Could the property keep enough of it in reserve until the rain came back?
HOW LONG CAN THEY LAST?
Answering that required another number: how much water would the house actually use?
The buyers are a couple. But designing the calculation around two people would make the test unusually favorable. The kind of assumption that flatters a conclusion rather than testing it.
So the investigation made it harder.
It modelled the house for a family of four.
For the daily figure, it used 150 litres per person per day, a residential design figure drawn from Salvadoran sanitary guidance.
Four people at 150 litres each comes to 600 litres a day.
Multiplied across the dry months, that adds up quickly.
Thirty days: 18,000 litres.
Ninety days: 54,000 litres.
Four months: 72,000 litres.
Five months: 90,000 litres.
And every morning, whether it rained or not, that water had to come from somewhere.
The 63,000 litres that had looked enormous falling on the roof in September didn’t look quite so enormous anymore. At that rate of use, a family of four could go through roughly that much water in just over three months.
Collecting the water wasn’t enough.
They had to keep it.
THE TANK GETS… BIG
A family of four could demand as much as 90,000 litres over five dry months.
That doesn’t mean the property needs a 90,000-litre cistern. Some rain still falls during the transition into the dry season, and the actual storage requirement depends on collection losses, month-to-month rainfall and how much water is already sitting in reserve when the dry season begins.
But it establishes the scale of the problem.
This isn’t a rain barrel.
It’s tens of thousands of litres of stored water.
And it has to go somewhere.
On this property, that matters. The land is steep, and flat, usable ground is valuable. And storing tens of thousands of litres of rainwater means finding space for a very large, protected cistern.
Keeping that water out of direct sunlight matters too. Light entering stored water can encourage algae growth. That’s another reason an exposed tank isn’t necessarily the obvious answer.
But there’s one large piece of flat ground the house would have to create anyway.
The terrace.
What if the cistern didn’t sit beside the house?
What if it sat underneath it?
A long, low cistern incorporated beneath a terrace or deck could potentially keep the stored water enclosed and out of the sun, while using space the house already needs rather than consuming more of the property’s limited flat ground.
What if the water wasn’t stored beside the house?
What if the terrace sat on top of it?
A long, low cistern incorporated beneath a terrace or deck could potentially use space the house already needs rather than consuming more of the property’s limited flat ground.
Whether that works structurally, and exactly how large it could be, is a question for the eventual engineer, not this investigation.
But the property’s layout points toward an intriguing possibility.
The water tank might not have to be added to the house.
It could become part of it.
AND THEN IT RAINS AGAIN
Solving the dry-season problem creates another one.
Eventually the cistern fills.
The rain keeps falling.
The roof keeps collecting.
Now what?
On a steep volcanic hillside, thousands of litres of overflow can’t simply be dumped wherever gravity decides to take them. Water released in the wrong place could become an erosion or slope-management problem.
Which means the catchment investigation has just collided with another question this property hasn’t answered yet.
Drainage.
The overflow path would have to be engineered on purpose, not discovered during the first serious storm.
There’s one more reality check.
Catching rain doesn’t automatically make it drinking water. A household system would still need first-flush diversion, filtration and, depending on how the water will be used, treatment.
But does every litre this house uses actually need to be drinking-quality water?
Toilets don’t care.
Neither does irrigation.
Rainwater doesn’t necessarily have to replace every other water source on the property to matter.
It only has to reduce how much the house demands from them.
Rainwater is no longer just a backup idea.
On this property, it could become part of the water system itself.
THE RAINWATER — BUILDABILITY: PASS
A pass doesn’t mean the buyer can forget about the water connection, a possible future well, or drainage.
It means the roof isn’t just shelter.
It’s a potential water source.
Investigation 02 spent its time beneath the mountain, trying to determine whether usable groundwater might be moving somewhere below.
And after all of that, one of the property’s biggest potential water sources may have been sitting above its head the entire time.
The roof.
INVESTIGATION 03 · THE RAINWATER — COMPLETE
5 QUESTIONS REMAIN
FIELD NOTES: After this investigation was published, Build El Salvador took the question to people with experience building and storing water in the country:
Has anyone actually built a rainwater catchment system in El Salvador and how did they store the water?
The answers quickly showed that the tank itself may be as important as the rain falling into it.
A Salvadoran explained that there isn’t a single answer even to the most basic question: should the cistern be buried or sit at ground level?
That, he said, depends on the space available and the budget.
Sizing is equally site-specific. He said capacity depends on the catchment area, location, climate and average rainfall.
That closely matched what this investigation had begun to uncover.
The problem wasn’t simply whether enough rain falls in El Salvador.
It was how much could be collected from a particular roof, how long it needed to last and where all that stored water could physically go.
Then the responses from people who had lived with water storage systems added another dimension.
One homeowner described a cistern of at least 1,000 gallons built underground beneath the family home.
It wasn’t a rainwater system. The cistern was filled with municipal water, kept full as a reserve and pumped into the house.
But one detail was particularly relevant to this investigation.
The cistern came first.
The homeowner’s mother had the cistern constructed underground and then built the house over it.
Years later, the homeowner remembered that reserve being particularly valuable during the war.
That experience doesn’t establish whether a cistern can be built beneath the terrace on the property being investigated here, that remains an engineering question.
But it does show that incorporating water storage beneath a structure isn’t merely a theoretical idea.
Another homeowner, Roberto, had taken almost the opposite approach.
His rainwater tank sits above ground.
There was a reason for that too: elevation allows the stored water to be used by gravity, without relying entirely on a pump.
Roberto described the water quality as very good. But there was an important unanswered question: he hadn’t yet tested the system through the dry season.
And that may be the most important test of all.
The investigation had already shown that the wet months can produce extraordinary amounts of water. The difficult part isn’t necessarily filling a tank.
It’s making that water last until the rain returns.
Another suggestion took storage in an entirely different direction: if enough land is available, rainwater could be stored in a cistern or an artificial lagoon.
For this particular property, an open lagoon isn’t the direction being considered. But the suggestion illustrates just how much the available land can shape the water system.
Taken together, the responses changed the question again.
A cistern isn’t simply a container that gets added after the house is designed.
Its size can depend on the roof and rainfall. Its position can determine whether a pump is needed. Burying it can preserve usable ground. And, in at least one Salvadoran home, the water storage was planned first and the house was built over it.
For a steep property where flat land is valuable, that last experience is particularly interesting.
The investigation had already arrived at the possibility of putting a long, low cistern beneath the proposed terrace.
The Field Notes didn’t prove that solution will work.
But they did suggest something perhaps more useful:
Water storage may need to be designed as part of the house, not something that gets added to it later.
That lesson sounds familiar.
In the previous investigation, a homeowner discovered a potential groundwater source only after construction had begun, when the house had already claimed the spot where a well might have gone.
This time, the lesson may be the inverse.
Before deciding where the house goes, decide where the water goes.
This is Investigation 03 of CAN I BUILD HERE?, a Build El Salvador series following the real-world due diligence behind a potential land purchase in El Salvador. Interested in sponsoring this investigation or a future installment? Become a Build El Salvador sponsor