Build and finish: when to stain a deck

How the page picks a day for stain or paint, and what it cannot see from here.

What this lens tells you

Pick Build and finish at the top of the forecast page and the one sentence changes. The page checks today and the next three days against one rule and names the first day that passes. The sentence starts "Stain the deck" and the day, then says "dry for 24 hours either side", the average temperature of the working hours and a word about humidity. If no day passes, it says "Hold off on the stain" and gives a reason.

Use it when you have something outdoors to stain, seal or paint and some say over the day. It reads the weather. It has not read the can, and it has not seen your wood.

Open the forecast with Build and finish switched on.

The rule

These are our numbers, not a standard. A day passes when every row is true. Working hours are 9 AM to 5 PM, by the clock of the place in the forecast.

A wet hour means the same thing here as on the rest of the page. The forecast says rain, drizzle, snow, a wintry mix or storms, and the chance is at least 40 percent or the expected amount is at least half a millimetre. If it gives one of those words and no chance, the word is enough. If the forecast names none of those, it takes a chance of at least 60 percent and at least 0.3 millimetre (about 0.01 inch). In the United States there is one step before either rule: an hour the Weather Service gives a chance under 25 percent is read as cloud whatever word it uses, so it falls under the second rule.

How long after rain can I stain my deck?

This lens waits a day. The 24 hours before 9 AM have to be dry by the test above. One wet hour in that stretch and the day fails.

For today, that stretch is mostly yesterday. Hours already past come from Open-Meteo. That is a model's account of what fell, not a rain gauge in your yard. The page holds only the last 26 hours, so by the afternoon the look-back from 9 AM that morning is short of a full 24 hours. If we have nothing from Open-Meteo, the page has almost nothing from yesterday to check, and today can pass this test with no evidence behind it. The forecast page says "Some data is late" when that happens.

A dry day is our rule of thumb and nothing more. The lens does not know how long it rained before that, how much shade the deck gets, or how wet the boards are now. A moisture meter can tell you that. If the can asks for longer, the can wins.

Can I stain my deck if it rains the next day?

The lens checks the 24 hours that start at 9 AM on the day: the working hours and the night that follows. One wet hour in that stretch fails the day. So does half a millimetre in total.

A slight chance is not a wet hour. An hour with a chance under 40 percent and less than half a millimetre expected does not count, so a day can pass with a shower still possible. The week card on the forecast page shows each day's chance of rain.

It stops looking at 9 AM the next morning. Rain later that day will not change the answer. How long a finish needs before it can take rain is printed on the can, so hold that number against the forecast yourself.

Can I stain a deck in high humidity?

Not by this rule. The lens averages the humidity over the working hours and fails the day at 80 percent or more. Under 60 percent the sentence says "low humidity". From 60 up to 80 it says "workable humidity". It says "workable humidity" as well when the forecast has no humidity and the check was skipped, so that phrase is not always a reading.

It is an average, so a damp first hour can sit inside a day that passes. The lens does not check the dew point. If the label on your can sets a humidity limit, follow the label.

What is the best temperature to stain a deck?

We do not name a best one. The lens has a range: every hour from 9 AM to 5 PM no colder than 50°F (10°C) and no warmer than about 90°F (32°C). One hour outside the range fails the day.

That is the air temperature in the forecast. The page cannot see sun on the boards, so it does not know how warm the surface is. It does not check the temperature of the night after, either. Your finish has its own range on the label, and that is the one to follow.

Can I paint outside today?

The lens has one test for every finish. Its sentence talks about stain, and the same test is all we have for paint. If today passes, the sentence says today and the chart marks 9 AM to 5 PM as the "Finish window". If today fails and a later day passes, it names that day.

Today stays in the running until 5 PM, and the lens judges the whole working day, not the hours you have left. Paint has a label too. Read it first, then check the forecast with this lens on.

When it says hold off

If no day passes, the sentence is "Hold off on the stain" and one reason. The lens tries the reasons in order. Rain, if every day it checked fails the dry test before or after. Too cold or too hot, if every day has a working hour outside the temperature range. Too windy, if every day is. If none of those is true of every day, it says "Humidity is too high". That covers the mixed case too, where one day is wet and the next is windy, so read it as "no day passed".

It chooses between too cold and too hot by the first day's average over the working hours. Under 50°F (10°C) it says too cold. Anything else it calls too hot. So days that fail on a cold morning hour with a mild average get called too hot. That is a flaw in the rule, so check the temperatures yourself.

If the lens has no day it can judge, the reason is "We have too little forecast to say".

Under "Why we say that" the page gives the rule in one line and the average humidity for the first day it checked.

The moisture figure

When a day passes, "Why we say that" lists what the lens used: the average temperature of the working hours, the strongest wind and the average humidity. It ends with a moisture figure, the percentage that wood "settles near" in air like that.

The figure is the moisture a board settles to in the air, worked out from the average temperature and humidity of the working hours with the Hailwood-Horrobin equation as printed in the USDA Wood Handbook. It is shown as a whole percent. With no humidity in the forecast there is no figure. It is not a reading of your deck. Your boards may not be there yet. A moisture meter can tell you, and we cannot.

What to wear and carry

The "What to wear" card has its own rules for this lens. It dresses you for now and the hours just ahead, not for the day the sentence names. Where it says "feels like" below, that is the average over the next six hours.

• Always on the list. Apron and work gloves, safety glasses, and a respirator for finishes.
• Wind. When the wind in the next 12 hours reaches 12 mph, with gusts counted at three quarters, the glasses come with a note that the wind will move the dust. When the wind now is under 6 mph, the respirator note says calm air lets fumes linger. Otherwise it says moving air thins the fumes and does not remove them.
• Cold. When it feels like 40°F or below, the card adds a warm hat and layers, and says finishes cure slowly in the cold.
• Footing. Above that, it lists boots with grip. When this hour's humidity is 80 percent or more, the reason it gives is dew on the deck until mid-morning. That is a guess from the humidity, and the card makes it at any time of day. The page cannot see dew.
• Pack. A moisture meter and a drop cloth, every time. A tarp for the stack if rain or snow is falling or due within 8 hours. If not, water and shade for the can when it feels like 85°F or more, and "Rags that are not your good ones" below that.

What the page does not know

• Your finish. The label on the can is the authority for any stain, paint or sealer: its temperature range, its time before rain, its time between coats. Where the can and this page disagree, follow the can.
• Your wood. The moisture figure is what a board would settle to in that air. It is not a reading of your deck. A moisture meter is the thing that knows.
• Your yard. A forecast cannot see shade, sun on the boards or dew.
• The night after. Temperature, wind and humidity are checked from 9 AM to 5 PM only. Rain is the only thing checked overnight.
• Gusts. The wind check uses the hourly wind speed and nothing else.
• The future. A forecast is a forecast. The methodology page says where the data comes from and where it can be wrong.

Official warnings come from the National Weather Service, not from us. When it marks a warning as immediate, the warning becomes the headline in every lens, this one included. The lens does not read any other alert, so a day can pass our rule while a warning, a watch or an advisory is in force. The forecast page shows the alert above the sentence. Read it first, and follow it and your local officials before anything here. Outside the United States the page shows no official alerts, so check your own country's weather service.

Ask it from an assistant

An assistant can ask the same service about painting and wood finishing weather. Its limits are kept separately. They are close to this lens and not identical. The page for agents says how to connect.

The other lenses

• Run and ride: the best hour for a run or a ride in the next day or so.
• Commute: rain and ice at both ends of the working day.
• Hunt and fish: pressure, light and plain storm warnings.
• Home and yard: freezes, mowing windows and when to skip the sprinklers.
• Methodology: how the sentence gets made, and where it can be wrong.

If you find something wrong, tell us: help@thelumberoutletgroup.com.