Should I Stain the Underside of My Deck or Leave as It Is?

Staining is vital for wooden outdoor objects such as desks, picnic tables, wood fences, and of course, your decking area.

In my opinion, anything that will be seen by anyone should be stained just so it looks better.

To stain your deck is pretty simple, as it is similar to painting.

You just need to follow the instructions on your wood stain, and you can do so.

But the main part we are focusing on here in this article is whether you need to stain the underside of a deck or not.

This is the question I have come across many times.

The short answer is NO; it’s not essential to stain or paint your deck’s underside.

The reason why I say so is that the underside of the deck is naturally protected from the elements like direct sunlight by the top side of the deck.

This is particularly true if the deck is constructed using pressure-treated wood that keeps the insects out.

However, there may be other reasons to stain the underside of the deck, especially if it can be seen or if there is a unique situation that might expose the underside to the elements.

We will go through them below one by one…

Benefits of Staining Underside Deck

Some of the benefits of staining the underside deck boards are:

a) Visible:

If the deck is part of a balcony or second story that will be seen, you can have it stained for both aesthetic and practical reasons.

One layer of stain will provide good protection from the elements for the underside of the deck.

b) Moisture Damage:

Normally you will leave the decks’ underside (and stain only the exposed side facing up) because it’s not exposed to direct sunlight and harmful UV rays that cause discoloration and other damages.

But what about the water damage and excess moisture?

The underside of your deck floor is not protected from them if you do not stain them.

So, it’s good to stain all sides of deck boards.

c) Prevent Mold & Mildew:

The biggest advantage to staining the underside of the deck is to prevent mold and mildew from sticking to the surface.

This is especially true if the climate is primarily hot and humid or the wood is exposed most of the year to moisture.

If leaves or other debris tends to collect under the deck, it may increase the humidity levels and provide a home for insects, fungi, and other pests to start going after the wood itself.

You can clean out the underside of the deck regularly to prevent this from happening, and a layer of stain will provide extra protection.

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How to Stain Underside of Your Deck?

If you decide to stain your deck’s underside, you will first need to clean the area properly.

Try a deck wash that uses a formulated cleaner designed to remove dirt, debris, and other contaminants while also getting rid of loose particles.

Plus, any fungi that have managed to take hold will also be removed.

Step 1. Get Plenty of Stain

You’ll want to have enough stain to cover the entire underside, so be sure to mix the stain you purchase together.

This will let you know if you have enough, and it will smooth out any color variations that may exist between the stains.

Step 2. Splash Test

To ensure the stain will stick, splash a little water on different areas of the wood.

If the water beads or the wood does not absorb the water in 15 minutes, then it will not take to the stain either.

Using a wood restorer or cleaner will help open up the wood to accept the stain.

Step 3. Apply the Stain

Now you are ready to apply either a water-based or oil-based stain.

If you want a shiny, deep finish, then oil-based is the way to go.

A water-based stain provides more of a flat finish, so choose the one that works best for your needs.

Apply the stain evenly across the underside of the deck and wait 24 hours for it to dry properly.

Once it has dried, you should check for any open spots to ensure it is covered.

After that, you should only need to check the underside of the deck annually to see if the coating is still present.

You may want to check more often if debris has built upon the underside.

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Do You Need to Seal Your Decking Area?

Some people may want to seal the wood they just stained because, logically, it makes sense.

However, I’m here to tell you that you probably don’t have to.

If you can tell on the wood stain you used, most stains actually have a sealant built-in.

This sealant will protect your precious wood from moisture, the natural enemy of wood.

But more importantly, a sealant will tend to provide an extra layer of protection for the wood in case of scratches or other impacts.

Using a sealant on a stained piece of wood doesn’t combine the power of the two but rather will result in an undesirable appearance.

However, if you didn’t stain your wood, you can and should apply sealant.

Deck sealers usually provide a clear and transparent coating to protect the wood.

It’s ideal for those who need the most natural wood look while offering the best protection.

If you are working with pressure-treated wood, you will want to have a sealant that is specifically designed to be UV repellent.

This will avoid wood turning gray faster due to harmful UV rays.

You can apply a deck sealer very similar to paint or stain, just brush it on, and that’s all you really have to do.

A little fact is that most outdoor projects such as decks actually are required by building codes to have pressure-treated wood as it is simply better at surviving weather conditions.

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What to Do if it Rains after Staining Your Deck?

After you have successfully applied the stain under your deck boards, you have not won the war.

There are a few things you should consider before and after you stain your pressure-treated wood.

As stain seeps into the wood and becomes part of it, it is super important for it to dry properly.

The biggest concern for most staining projects is the weather, as the rain could ruin a stain job even on the bottom deck.

Staining in good weather is the best thing you can do. We briefly mentioned it before, but stain loves a nice cool and dry place when it dries.

If it is too hot or wet, you will get splotchy spots, and it won’t look as good.

We recommend staining on a day when there is a light overcast, but ensure there is no chance of any rain coming for at least two days.

The overcast will prevent direct sunlight from ruining the drying process.

And as we mentioned, if it rains, the stain will be completely ruined.

If an overcast day isn’t an option for you, you can wait for the sun to go behind your house or near the end of the day where the sun won’t be beating on the wood for a long time.

The Results of Rain & Fixing Rain Damage

We mentioned a few times that rain or water would ruin a stain, but how exactly does it work?

Well, stain takes multiple hours to set and then up to two days to completely dry.

If water gets on the freshly stained pressure-treated wood deck, the water will seep into the wood, like how the stain did.

But as there is only room for one of the two liquids, the stain will be pushed out on top, leaving different areas where there is too little stain in one part and too much stain in others.

In my opinion, the whole point of staining is to get a nice look on top of the wood, and getting the wood wet in that stage completely ruins it.

There isn’t an easy way to fix the stain after it is ruined by rain.

In fact, the only way to completely fix it is to sand the stain off the wood and completely re-stain it after it dries, of course.

If this isn’t too appealing to you (I hate sanding!), you can try to add additional stains and try to blend away the blotches.

However, it won’t be perfect if you don’t do it correctly the first time!

Final Thoughts

While the boards of your deck are not being pelted with rain on the underside, they can constantly get wet and absorb moisture that can cause great damage over time. 

It’s therefore good to stain and protect your deck from all sides, although it’s not a requisite.

With that said, if you are planning to stain only the boards underneath your deck floor, the process can be very laborious, time-consuming as well as expensive.

So, if you haven’t stained the bottom of your deck ever, it’s good to leave it as it is and repaint only the exposed sides of the deck boards that are prone to sun damage.