Planning Around the Rainy Season on Your Build Schedule

Why the rainy season catches construction teams off guard every year

Thailand's rainy season officially begins in May and runs through October, with the heaviest rains typically falling between July and September and one or two tropical storms expected to sweep through the North and Northeast in that window. For most Thai SME contractors, this is not news — and yet the same teams are caught short by weather delays year after year. The reason is rarely a lack of awareness that rain is coming; it is that the construction schedule was written as if rain would not arrive. A plan that places concrete pours, bricklaying, and external plastering back-to-back through August without buffer is not a conservative plan — it is an optimistic one that breaks the first wet week. Naming the rainy season explicitly inside the Gantt, before the season arrives, is the whole difference between a resilient schedule and one that collapses on the first heavy rain.

Which tasks can continue through rain — and which must stop?

Not all construction work stops when it rains. Internal tasks — electrical rough-in, tiling, joinery, plumbing inside a weathertight structure, and ceiling fit-out — can continue even during heavy rain, provided the building envelope is closed. Outdoor and exposed-concrete tasks are a different matter: concrete placement, bricklaying with mortar, external plastering, earthworks, and any finishing coat applied to an open surface are all genuinely weather-dependent. The practical step at the planning stage is to go through the Gantt and mark each task as rain-sensitive or rain-tolerant, then cluster the rain-sensitive tasks toward the drier months — November through April — wherever the build sequence allows. When the sequence is fixed, those tasks need explicit buffer time built in around them.

How much weather buffer should you actually add to a wet-season schedule?

A practical rule of thumb for construction in central Thailand is to plan for two to three lost working days per week during peak wet-season months — typically July through September — for tasks that are fully exposed to weather. On a project running through those three months, that is fifteen to twenty-five working days a buffer-free schedule does not account for. Many contractors add a single two-week float at the project end and call it rain cover, but this is too blunt: it does not protect the individual task sequences that actually drive the timeline. A more effective approach is to add two to four days of buffer after each rain-sensitive phase, so a late-July concrete pour does not cascade directly into a mid-September plastering run with no slack between them.

A worked example: an eight-month Bangkok townhouse build through the wet months

Consider a contractor managing an eight-month townhouse build on the outskirts of Bangkok, starting in April. The structural slab pour is planned for mid-July — peak wet season — followed immediately by bricklaying in week three and external plastering in week six. A single week of rain in the slab window pushes bricklaying a week, which pushes window frame installation, which pushes external plastering into October when rain is still running. By handover the project has slipped seven weeks. The same project replanned with a four-day buffer after the slab pour and a five-day buffer before external plastering finishes three weeks late rather than seven, because each buffer absorbs a hit rather than passing it downstream to every dependent task that follows.

What to do when rain actually lands mid-project

When a heavy-rain week arrives mid-project, the instinct is to wait and see. The better move is to act on day one: open the Gantt, drag the affected tasks forward, and look at what cascades downstream. Most of the time there are indoor tasks that can be pulled earlier to fill the gap — electrical rough-in, tiling, interior joinery — which is exactly what the Gantt is built to show. The on-site progress update feature is especially valuable here: when the foreman logs that a task stopped due to weather, that entry becomes a timestamped record against the specific task in the plan. When the owner asks three weeks later why the bricklaying is behind, there is a documented answer rather than a reconstruction from memory.

How to document rain delays so you can defend your schedule when it matters

In Thailand, construction contracts almost always include a penalty clause for late completion, and weather is only a valid defence if it is documented. A message in a LINE group saying work stopped because of rain is not documentation in any dispute — it is a chat message that will have scrolled away by morning. What does hold up: a timestamped entry against the affected task in the plan, a site photo showing conditions on the day, and a short note recording what stopped and why. SiteBoard's on-site progress updates let the foreman attach a photo and a note to any task update in under a minute, so the rain record lives inside the same system as the schedule — not buried in a phone gallery where it cannot be retrieved six weeks later.

Is it worth replanning the whole schedule for rain at the project start?

For any project that runs through or into the wet season, a thirty-minute planning pass at the outset pays back many times over. Open the Gantt, identify every task that is rain-sensitive, check which months they fall in, and add buffer to the exposed sequences. You are not designing for a once-in-a-decade storm; you are designing for a typical Thai rainy season, which every contractor who has worked through an August in Bangkok or Chiang Mai knows is not an unusual event. Projects that build weather buffer in at the start almost always finish closer to the original handover date, because each rain event is absorbed within the plan rather than accumulating into a gap that only becomes visible at the very end.

Does a two-week end-of-project buffer really cover rain delays?

Rarely. A two-week buffer at the project end is a safety net for the final task — it does not protect tasks sitting in the middle of the schedule. If the structural concrete slips in July, that slip cascades through every dependent task that follows. By the time the end buffer is reached, it is often already spent before the final month has begun. Distributed buffer — small amounts of float added after each weather-sensitive phase — is almost always more effective than a single end float. The Gantt makes this easy to understand: drag the delayed task forward and every dependent task shifts with it. You can see exactly where the cascade lands, and where a small amount of buffer would have caught it.

When should you tell the owner that rain is causing a delay?

As soon as it is clear the delay will affect the handover date — not after the fact. Owners who receive bad news at the end are far harder to manage than owners who receive it early with a clear recovery plan. A contractor who sends an update through the owner-sharing link during the wet season, with the foreman's photo and a note explaining which tasks were rained out and what the revised sequence looks like, manages the situation professionally. The owner sees the original plan, the current progress, and the revised dates in one place. That transparency turns a weather delay from a trust problem into evidence that the contractor has the project under control.

Can you recover time lost to rain without pushing the handover date?

Sometimes. The clearest path is parallel acceleration: identify tasks that are not on the critical path — work the crew could start before a delayed task unblocks them — and run them concurrently while the recovery happens. A rained-out exterior task is an opportunity to advance interior fit-out that was not scheduled until later. The other option is to add crew temporarily: a second bricklaying team for two weeks after the rain event can often recover what the wet week cost, at a price typically far lower than a late-handover penalty clause. The critical step is seeing the gap on the Gantt while it forms, not after the full downstream sequence has already shifted — which is exactly the visibility that daily progress logging provides.