Masika and vuli: a builder's rain calendar for Tanzania

Which regions get masika and vuli, and which get msimu?

The north and the coast get two rainy seasons; the south, centre and west get one. That is how the Tanzania Meteorological Authority (TMA) divides the country, and it decides your building programme before anything else about the weather does. A roofer in Dar es Salaam and a roofer in Mbeya work to different calendars: the first has a gap in January and February that the second does not.

Two places cross the line. TMA puts northern Morogoro in the two-season area and southern Morogoro in the one-season area. Its seasonal outlooks also count the north of Kigoma, meaning Kakonko and Kibondo districts, as two-season, along with Geita and Mafia island. Find your region in the table before you fix a start date.

What can you build in each part of the year?

Exposed work belongs in June to September, which TMA calls dry months for the entire country. Dry does not mean rainless: on gridded data for 1981 to 2010 in the University of Cape Town's climate profile, Dar es Salaam still gets less than 100 mm across those four months, not nothing, so keep the covers on site.

January and February are the second window, in two-season areas only. TMA calls them relatively dry, not dry. They also come in the hot season, which TMA runs from October to February or March. In Dar es Salaam the mean daily maximum in those months is 32.8°C, the highest of any season. Several sheets stop at +35°C, and Mapefloor I 300 SL ME applies that to the substrate as well as the air. A slab in full sun runs hotter than the air, so measure it.

How heavy does the rain get?

Heavy enough that a bad day matters more than the seasonal average. TMA treats 50 mm or more in 24 hours as severe weather, and in 2025 it recorded 119 such events, 20 of them above 100 mm. April had the most (33), then March (32). In Dar es Salaam, the same climate profile found that daily rain intensity in March to May rose by nearly 6 mm per day per decade, while the number of rain days fell by almost 4 per decade. So masika can mean fewer wet days, each one worse.

Seasons also vary from year to year. Masika 2025 reached 125.6% of average nationally, while Vuli 2025 reached 88.9%.

For this season, read TMA and nobody else. Its Vuli 2026 outlook, issued on 18 August 2026, expects a strong El Nino to affect the season. On the northern coast it expected rain from the first or second week of October, ending in January 2027, although vuli normally ends in December. If that holds, January there will not be the usual drier window. TMA's advice to builders: plan around the forecast, move plant and materials out of valleys before the rain, and buy early and store the materials that spoil when damp, such as cement and steel bars.

When should you waterproof a roof?

Finish before the rains, and leave room for the drying the data sheet asks for. In two-season areas that means by the end of September; in one-season areas, before November, or earlier if TMA's outlook for your region says so.

Moisture is the catch. YHColorWall-5300, a liquid membrane for exterior walls and roofs, asks for substrate moisture of 4% or less and gives 72 hours at +30°C before the applied product is ready for use. A slab soaked yesterday will not meet that today. Mapelastic Easy, for terraces and balconies, must be protected from rain for the first 24 hours. YHCoat-1000, which lists flat roofs, says to protect fresh material from rain, stops at +35°C, and gives no traffickable finish, so it needs a levelling screed. X-Shield FlexPruf lists roof waterproofing among its uses.

One product to keep off roofs: Antipluviol is a water-repellent for walls, and its sheet rules out terraces and flat roofs. Linked product pages carry the data sheet and a coverage calculator.

When should you pour and cure concrete?

Pour when you can keep the slab covered and wet afterwards, which for large exposed slabs means June to September. Rain on a fresh surface adds water to the top of the concrete, where the finish and the next trade's bond depend on it. In the hot months the problem reverses: sun and wind take water out too early. TMA also warns of soil erosion and flood damage to work in progress.

Curing belongs on the programme; take the period from your engineer's specification. Idrocrete KR 1000, a crystallising admixture, only gives guaranteed results in concrete with a water/cement ratio below 0.5 and adequate wet curing. Keraflex Maxi S1 and Mapelastic Easy both exclude concrete that is not sufficiently cured, and Mapefloor CPU+/MF excludes concrete younger than 7 days.

Store bagged goods off the floor and under cover. Mapeset and Mapegrout T60 ME both warn that shelf life may be reduced at high temperature or high humidity.

When should you tile a terrace?

Start early enough that the whole build-up finishes before the rains, and count backwards from them. A terrace is a cured slab, a membrane and a tile bed, each with its own wait.

Using Mapelastic Easy and Keraflex Maxi S1 as the example: wait at least 5 days after the membrane before laying tiles, then the adhesive sheet gives 24 hours before grouting floor joints and foot traffic, and 14 days to ready for service. That is 5 + 14 = 19 days from the last coat of membrane to a terrace in service, before counting the tiling days. The Mapelastic Easy sheet notes that its 5 days can change with the weather. Cover freshly laid tiles if rain threatens.

Keraflex Maxi S1 applies between +5°C and +40°C with an open time above 30 minutes, but a hot slab in wind can skin the adhesive sooner. Lift a tile now and then and check the back is fully covered. For a final sealer, UltraCare Stain Protector W Plus should not go on surfaces in direct sun or if rain is forecast within the next few hours.

Can you lay a resin floor in humid months?

Sometimes, but the calendar will not tell you when; a hygrometer will. Mapefloor FC 200 ME says air relative humidity should not be greater than 75%. A published study of Dar es Salaam gives monthly relative humidity of 77 to 86% over the year. So the monthly figures sit above that limit all year, and the dry season does not solve it.

Humidity moves through the day. The same study's simulation of four Dar es Salaam districts in February ran from 55% to 83%, highest at 10:00 and lowest at 16:00. That is a model, not a promise, but it explains why you measure on the day, in the room, before mixing. Check the dew point too: a slab cooler than the air can collect condensation.

Then test the floor. FC 200 ME and Primer SN exclude substrates above 4% moisture or with capillary rising damp. The Mapefloor I 300 SL ME sheet prints no air humidity figure, but rules out damp substrates. Book the moisture test before you book the crew.

These dates describe the normal pattern. For any forecast, follow TMA's current outlook for your region.