Deck restoration: pressure washing and a waterproof coating

Deck restoration on a deck like this comes down to two things: get the grey, mildewed wood clean, then put a waterproof coating back on before the bare boards take on more water.

What did the deck look like before and after?

The photos show a raised deck running along a red brick wall with white-framed windows, and the long, straight wooden staircase that climbs to it from the yard.

Before, the deck boards were grey all over, with darker blotches and streaks where water and dirt had sat. Leaves and bits of debris were caught in the gaps. The railings and spindles had faded to the same dull grey.

The staircase was in worse shape than the deck. Several treads had a greenish film across them.

The after shot is taken from nearly the same spot. The boards, railings, spindles and stairs are an even honey-cedar colour, and you can see the grain and knots through the finish. The blotches and the green film are gone. It’s a sunny day in that photo, which warms the colour a little.

The stair treads look a shade darker than the deck floor. Part of that is the light, and part is likely the wood, since weathered boards absorb more finish.

Why does a deck go grey and grow mildew?

A deck goes grey once its finish wears off and sunlight starts breaking down the bare wood. The University of Missouri Extension describes the grey residue this leaves on the surface. Rain then soaks straight into the boards.

The USDA Forest Products Laboratory’s guide to finishes for wood decks lists what follows on a deck that isn’t maintained: checks and cracks, raised grain and mildew, which raise the risk of decay and insect attack. Shaded stair treads like these dry slowly after rain, so that’s where green growth tends to take hold.

What does a deck restoration involve?

On this deck it meant pressure washing and a waterproof coating. A job like this usually goes in this order:

1. Furniture, planters and mats come off the deck and stairs so every board can be reached.
2. The brick wall, windows and plants below the deck are covered against spray and drips.
3. The deck and stairs are pressure washed to lift dirt, mildew and loose wood fibres, the cleaning the Missouri Extension calls for before staining. The pressure and nozzle are set for wood, which is far softer than interlock. The Forest Products Laboratory notes that power washing can strip wood from the surface, especially on soft wood like western red cedar.
4. The wood dries right through. Wood Preservation Canada warns that too much moisture can stop a finish soaking in properly.
5. The coating goes on a few boards at a time along their full length, keeping a wet edge and stopping only at a natural break like the stairs. That’s how Benjamin Moore’s data sheet for its Arborcoat semi-solid deck and siding stain says to keep lap marks down. The railings, spindles and stairs get the same treatment.

The published numbers for this kind of job:

Scroll sideways to see the whole table.

When should you not stain or seal a deck?

Don’t coat a deck while the wood is still damp, since a finish can’t soak into wet boards. Benjamin Moore also says not to apply Arborcoat when the air or the wood is below 4.4°C, which in Ontario rules out a lot of late fall days. Our deck staining guide covers timing and stain choice in more detail.

Why wasn’t the deck rebuilt or painted?

There was no sign of rot or lifting boards, so washing and recoating dealt with the real problem: bare wood taking on water. Soft or popping boards would have needed deck repair first.

On a deck floor, a finish that soaks in does better than paint. The Forest Products Laboratory recommends penetrating finishes for decks. Paint and solid-colour stains form a film that wears quickly where people walk, can trap moisture, and fails by peeling.

A penetrating finish goes into the wood instead of sitting on top, so it doesn’t peel and is easier to redo. It also lets the grain show, as it does in the after photo.

How do you keep a restored deck looking good?

Mostly by keeping it dry and clean, and recoating before the wood goes grey again.

• Sweep it often. Leaves and seeds hold water against the wood, and the Western Red Cedar Lumber Association points out that a deck that stays damp doesn’t last as long as one that dries out.
• Watch the stairs. Shady treads stay wet longest, so brush them off and clear anything growing on them early.
• Move planters, mats and furniture now and then so the wood under them can dry.
• Do a water test each spring on the busiest boards. If the water beads up, the coating is still working. If it soaks straight in, the wood is ready for another coat. Wood Preservation Canada uses the same drop test. The floor and treads usually need it before the railings, since they take the foot traffic.
• Go easy if you wash it yourself. A garden hose and a soft brush handle most routine dirt. Our guide on pressure washing decks without damage explains how to avoid furring the wood.

For typical price ranges, see how much deck staining costs.

If your deck looks like the before photo, text a picture of the boards, stairs and railings to (437) 684-2535, or use the form below for a free quote. We do deck staining and waterproofing across Markham and nearby.