Building in the rainy season in Costa Rica: dates by region and how to plan your build

You can build a house in Costa Rica during the rainy season, and most projects do, because a typical custom home spends 5 to 8 months on site and the Pacific rainy season runs from about May to October. What changes is the order of work: earthworks, foundations and the roof go best in drier months, while interior work, finishes and permits fit well into the wet ones. The region matters too, since the Caribbean has no defined dry season at all (IMN).

This guide covers when the rainy season runs in each region according to the Instituto Meteorológico Nacional (IMN), what heavy rain does to earthworks, concrete, roofs, drainage and access roads, how to plan a build timeline around it, and what to check on site when it rains.

When is the rainy season in Costa Rica?

Costa Rica has two climate patterns, one for the Pacific side and one for the Caribbean side, and they behave very differently (IMN, climate of the climatic regions).

• Pacific side and Central Valley: the dry season runs from December to March, with April as a transition month. The rainy season runs from May to October, with November as a transition month. March is the driest and hottest month, and September and October are the wettest.
• The veranillo: in July and August the Pacific side usually gets a relative drop in rain, called the veranillo or canícula, when the trade winds strengthen.
• Time of day: on the Pacific side, rain falls mostly in the afternoon and early evening. On the Caribbean side it is most likely at night and in the morning.
• Caribbean side: there is no well-defined dry season. Even the least rainy months get roughly 100 to 200 mm of rain. The relatively drier periods are February to March and September to October, and the wettest month is December.

The tourism board's climate page describes the same pattern: a May to October rainy season on the Pacific side and no clearly defined dry season on the Caribbean side (Visit Costa Rica, ICT). In the IMN start forecasts below, the southern Pacific gets its rains first and Guanacaste gets them last.

Rainy season start dates vary every year

The IMN publishes a forecast for the start of the rains each year, and the dates move by several weeks from one year to the next. Recent forecasts show the spread:

Sources: IMN forecasts reported by El Observador (April 2023 and March 2024) and Delfino (March 2025).

The end of the rains moves in the same way. In 2023 the IMN placed the shift to dry season conditions between November 3 and 15 in the Pacífico Norte, November 6 to 23 in the Pacífico Central, November 23 to 27 in the Pacífico Sur and December 3 to 7 in the Zona Norte. The IMN also said the idea of an end to the rainy season does not apply to the Caribbean, even in a normal year (El Observador, November 2023). In 2024 the forecast for the Pacífico Norte, Pacífico Central and Central Valley was November 25 to December 1 (El Observador, November 2024).

How much does it rain where you plan to build?

Annual rainfall differs a lot between regions, and so does the length of the dry season. The IMN's climatic regionalization study gives these ranges for its subregions:

Source: IMN, Regionalización climática de Costa Rica.

The takeaway for a build: Guanacaste, around Playa Flamingo and Tamarindo, has one of the longest dry seasons in the country, 3 to 5 months by IMN subregion. The southern Pacific around Uvita and Dominical can get nearly twice as much rain in a year and has a much shorter dry window. On the Caribbean side there is no dry window to plan around, so the site has to be set up to work in the rain.

What the rainy season does to a construction site

Earthworks, cuts and fills

Earthworks are the phase most affected by rain. Saturated soil is hard to excavate and compact, trucks and machines get stuck, and open cuts can slide. Costa Rica's emergency commission (CNE) has pointed to badly made cuts, poorly compacted fill and poor water management as common causes of landslides, and has said that 60% of the country has high vulnerability to landslides (CNE, reported by El Observador, September 2022).

The seasonal pattern is clear in the CNE figures for 2022: 8 landslide events in March, 43 in April, 127 in May and 209 in June, when the heavier rains set in. Even after the rains end, the ground can stay soft. Howler notes that in some locations the soil may still be too soft for excavation when the dry season begins in November, so it is often better to wait until the dry season is clearly established.

On a hillside lot this matters even more. Our guide to building on a slope in Costa Rica covers retaining walls and cut and fill design, and a soil study tells the engineer how the ground behaves when wet.

Concrete pours

Concrete can be placed in the rainy season, but rain on fresh concrete causes real damage if the crew is not ready for it. Heavy rain can wash cement out of the surface layer, which leads to scaling, dusting and fine cracking, and extra water upsets the water to cement ratio and weakens the mix (Converge, April 2023). Converge's guidance is to:

• Check the forecast and reschedule a pour if heavy rain is expected and the surface cannot be protected.
• Cover the work if rain is expected soon after finishing.
• Push rainwater off the edge of the slab once the rain passes, without working it into the surface.
• Never pour into forms or excavations that are already holding water, and never add dry cement to soak up rainwater.
• Treat rain right after the pour as the biggest risk. The mix starts to stiffen at around 4 to 8 hours, rain after about 8 hours is unlikely to cause significant damage, and after 24 hours rain should not be a problem.

On the Pacific side, where rain usually comes in the afternoon (IMN), the simple fix is to start pours early in the morning so finishing is done before the clouds build. Have plastic sheeting on site before every pour, not on order.

Roofs and drying in

The most useful milestone in a rainy season build is getting the roof on and the openings closed, often called drying in. Once the structure is covered, plastering, electrical, plumbing, tile and carpentry can carry on regardless of the weather. Until then, rain soaks walls and slabs, damages stored materials and stops most trades.

That is why a good schedule aims to finish the structure and roof before the wettest months, September and October on the Pacific side (IMN). Roof design matters for the life of the house too: generous overhangs, enough slope, gutters sized for tropical downpours and downspouts that discharge away from the foundation.

Site drainage and erosion

Water management is part of the work from the first day. A site that sheds water badly will flood footings, wash out graded pads and send mud onto neighbours and roads. Plan temporary drainage before the rains: swales or channels around the excavation, sediment control at the low edge of the lot, covered stockpiles and a stable, gravelled area for deliveries. Permanent storm drainage, including culverts, channels and the discharge point, belongs in the plans, not on the to-do list after the house is built. If the house uses a septic system, the drain field also needs protection from runoff.

Access roads and deliveries

Many building lots are reached by unpaved roads, and those roads can become impassable in heavy rain. Howler advises counting the creeks, streams and hills between a property and essential services, and checking whether your vehicle can manage them in the wet. It also notes that some river valleys in Nicoya and Guanacaste flood after even short storms, and that the southern Pacific gets flash floods. For a build, that means concrete trucks, block deliveries and crews. If the road fails, the site stops. Plan to stock materials ahead of the wettest weeks and budget for road maintenance where the access is long or steep.

How to plan a build timeline around the rainy season

Because the on site phase of a custom home usually runs 5 to 8 months, almost every Pacific build overlaps part of a rainy season. The goal is to put the right work in the right months. Our guide on how long it takes to build a house in Costa Rica covers the full sequence from design to handover.

In Guanacaste, a common approach is to finish design and permits during one rainy season so the excavator can start when the ground firms up, then push to close the structure before the next heavy rains. On the southern Pacific and Caribbean sides, where dry windows are short or missing, it is safer to assume rain throughout and plan covers, drainage and access from the start.

Two other calendar points come up often. Howler recommends avoiding a start right before the year end holidays or Semana Santa, because crews are thinner, and it warns that most projects cannot be finished within a single season, dry or rainy. Building contingency into the schedule is as important as building it into the budget, where 10 to 15% is a sensible allowance. Our construction contract guide covers which delays, such as extreme weather, should extend the deadline in your contract.

What to check on site in the rainy season

Before you buy the lot

The rainy season is the best time to see how a lot really behaves. Howler suggests watching whether rain runs off cleanly or collects on the site, checking that culverts, channels and gutters are clear, and treating land that slopes toward the building area as a red flag. Look for erosion on the hillside above the lot, ask neighbours about flooding and visit after a heavy storm. A topographic survey shows where water will go before anyone designs the house.

During construction

• After every big storm: check excavations and cuts for slumping, standing water in footings and washouts around graded pads.
• Before every pour: confirm forms and trenches are dry, plastic is on site and the forecast has been checked.
• Rebar and materials: steel should be off the mud and cement bags stored dry and off the ground.
• After the roof goes on: look for leaks at valleys, skylights, roof penetrations and wall to roof joints during the first heavy rains, while the contractor is still on site.
• Drainage: make sure downspouts and site drains discharge where the plans say, away from the foundation and the septic field.
• Retaining walls: check that weep holes and drains behind walls are flowing, not blocked.
• Warning signs: the CNE lists cracks in the ground or walls, doors and windows that start to stick, and leaning trees or power lines as signs of ground movement (El Observador, September 2022).

If you are building from abroad, ask for photos or video after major storms, not just on a fixed schedule.

How Decentra Homes helps

Decentra Homes is a design-build custom home builder in Costa Rica, founded in 2020, with design and construction CFIA-licensed under SANYC Ingeniería (CFIA registration CC-10880) and a showroom in Altos de Flamingo, Guanacaste, by appointment. We build first in concrete, and also in hybrid and steel frame systems, and have completed 50+ residential and commercial projects. Typical builds run 5 to 8 months on site. Minimum construction budget $350,000 USD, before land (new builds). If you are weighing a lot or a start date, our land and construction consulting is $200 per hour with a CFIA-licensed engineer and architect with 15 years of experience. Message us on WhatsApp +506 8733-2930 or through our contact page.