Can I render today? The PlasterFlow weather verdict

The PlasterFlow weather card showing good rendering conditions, the hourly strip and the humidity curing window against the 85% limit

The card, on a good morning

The verdict

Three answers, and the reason for the one you got

The card gives you Good conditions, Take care or Not ideal — and always says why: “too cold — below 5°C”, “frost risk overnight”, “humid 88% — slow cure”. A verdict without a reason is just a colour.

It's worked out on the limits the manufacturers actually publish, not on whether it feels nice out:

What it checks	The rule it uses
Temperature	Above 5°C and below 30°C. It judges “too hot” on the temperature now, not the afternoon peak, so the banner matches the number you're looking at.
Frost	The overnight low, not the daytime high — because frost within the curing window is what wrecks a fresh coat, and that happens after you've gone home.
Humidity	Under 85%. Silicone and acrylic finishes cure slowly and a damp still day is the classic way to end up with patchy colour.
Rain	Forecast rainfall in millimetres — see below. Rain washes fresh render off the wall.

It says so on the card: weather guide only — your call on the day. You know your materials and your site. The app's job is to stop you driving 40 miles to find out.

The rain problem

A percentage chance of rain is not a measure of rain

This is the detail that took the longest to get right, and it's the one that matters most on green render.

4% chance — and 0.4mm of rain

In the same forecast response, the chance of rain and the amount of rain are two separate numbers, and they don't have to agree. A real case: one town showed 6% and 0.00mm while a town 40 miles away showed 4% and 0.40mm. Read the percentage and you'd have called both of them dry. One of them wasn't.

So the verdict runs on the millimetres. Any measurable rainfall in the forecast window pushes the risk up regardless of what the percentage says — a tenth of a millimetre is enough to mark fresh render, so there's no “small enough to ignore” threshold.

Two forecasts, worst case wins

As well as the general forecast it pulls the Met Office's own 2km UK model — the fine-grained one built for this country's weather — and takes the worse of the two, hour by hour. When they disagree about a shower, you hear about the shower.

Live rain radar

Flip the card to Radar and watch the actual rainfall moving across the map. It also checks the radar over the job directly, so “it's raining now” is something the app can know rather than forecast.

A warning before it lands

On the phone you can be told roughly 40 minutes before rain reaches you — enough to get the sheets on or stop mixing.

A 7am brief, if you want it

An optional morning notification: today's job, today's verdict, what needs chasing. Off by default — it's a tool, not a nag.

The curing window

“Last coat by 17:00” — the answer to the four o'clock question

The one you ask yourself on the scaffold: is there time for one more, or am I coming back tomorrow?

Humidity, hour by hour

A strip of the next 24 hours, colour-coded under 75%, close to the limit and over 85%, with the 85% line drawn across the graph. You can see the damp coming in at nine o'clock tonight.

A last-coat time

From the humidity curve and the temperature it works out the last useful coat of the day — and tells you plainly whether conditions hold for the rest of the day or fall away.

All-weather systems get their own rules

If the job you're on uses an all-weather mix, the temperature and humidity limits relax to match its datasheet — down to −5°C and humidity to 99% — while rain still counts, because rain always counts. The card says “all-weather” so you know which rules you're being judged by.

Anchored to the real observation

The curve starts from the temperature and humidity being reported now, so the graph and the big number on the card never disagree with each other.

Where it gets the location

Your phone's location on the app, or the area you've saved if you'd rather not share GPS. Either way it's the weather at the job, not at the nearest city.

The week ahead too

Each of the next seven days carries its own verdict, so booking a render week is a decision you can make on Thursday rather than hope about on Monday.

Next: jobs & money → Back to the whole walkthrough

“Can I render today?” answered properly