Managing water around a house in Costa Rica

Costa Rica's national weather institute puts average annual rainfall on this coast at about 3,700 to 3,900 millimetres. For a roof of 200 square metres, that is roughly three quarters of a million litres of water a year to be collected and sent somewhere harmless. Most water damage to houses here is a failure somewhere along that route.

From the roof to the ground

Gutters and downpipes are the first stage, and they are cleared before the rains and repeatedly during them. What matters as much is where each downpipe ends. Water dropped at the foot of a wall soaks the foundation, splashes soil onto the paint and finds its way under the slab. It should be carried away in a pipe or a lined channel to a point from which it keeps flowing away.

The ground around the house

The ground next to a building should slope away from it. The model building code used in the United States gives a useful benchmark: a fall of at least six inches over the first ten feet, about fifteen centimetres over three metres, with paved surfaces sloping away as well. Gardens settle and planting beds get built up over the years, so this is worth checking with a level and not by eye.

Channels, grates and the culverts under driveways need the same attention as gutters. A culvert blocked with branches sends a stream across the driveway and takes the surface with it.

Slopes and retaining walls

Many houses on this coast sit on or below a cut slope. Water is what moves slopes: saturated soil is heavier and weaker, and water trapped behind a retaining wall pushes on it. The drain holes at the base of a retaining wall are there to let that water out, and they have to be kept open.

The United States Geological Survey lists the warning signs of ground movement. They include new cracks or bulges in the ground or paving, soil pulling away from foundations, leaning trees, walls or fences, doors and windows that start to stick, and springs or wet ground where it was dry before. Any of these in the wet season is a reason to call an engineer promptly, and to keep people away from the slope until one has seen it.

The septic system and the rain

Houses here treat their own wastewater in a septic tank and a drain field. The Environmental Protection Agency in the United States advises inspecting a household system at least every three years and pumping the tank every three to five. Its guidance on drain fields is all about water: keep roof drains and other rainwater away from the area, because excess water slows or stops treatment, and never park or drive on it.

In saturated ground a drain field cannot absorb what the house sends it. Slow drains and smells after days of rain are the sign. The answer is to use less water until the ground drains, not to pump the tank: the EPA warns against pumping in saturated conditions, because an empty tank can float and break its pipes. Pumping belongs in the dry season.

Only human waste and toilet paper belong in a septic system. In a rental house that needs saying on a sign, because wipes and cooking grease are common causes of a blockage.

After a big storm

A walk around the property after heavy rain shows more than any dry-season inspection: where water is pooling, where it has cut a channel, which gutter overflowed, whether the drains in the walls are running. Photographs taken from the same spots each time make slow changes visible.

Drainage and slope problems are specific to each site. This is a guide to what to watch for, not a substitute for an engineer's assessment.