Why Proper Deck Drainage Matters (And How to Get It Right)

The most common reason decks fail prematurely isn't bad materials or poor framing—it's water getting trapped underneath and rotting the substructure from below.

You can't see it happening. But by the time you notice, the damage is often serious and expensive to fix.

This guide covers everything you need to know about deck drainage.

Why Deck Drainage is Critical

When water sits under a deck, it:

• Rots timber joists and bearers – The hidden parts that hold the deck up
• Encourages mould and fungal growth – Creates dark, damp conditions
• Promotes termite infestations – Termites love wet timber
• Causes rust – Steel bolts and fasteners corrode
• Destabilises the deck – Posts settle unevenly, causing cracks and unsafe movement

All of this happens where you can't see it. By the time you notice sagging boards or soft timber, the damage is extensive.

How Water Gets Under Your Deck

1. Rain running off the decking surface – Water flows between boards and pools below
2. Water splashing from the ground – Especially in coastal areas or heavy rain
3. Condensation – Moisture forms underneath the deck on humid days
4. Water from nearby plants or gardens – Irrigation and plant root zones
5. Poor site grading – The ground slopes toward the deck instead of away

The Right Way to Design Deck Drainage

1. Ensure Proper Slope

Decking boards should slope slightly away from buildings and downhill at least 1:100 (1cm drop per 1m length). This ensures water runs off quickly rather than pooling.

2. Create Space Under the Deck

There must be clearance between the deck and ground—at least 150mm (6 inches), ideally 200-300mm.

This allows:

• Air circulation (drying)
• Water to drain away
• Space for repairs
• Access for pest inspections

3. Install a Moisture Barrier (When Needed)

In high-moisture areas, we often install:

• Landscape fabric – Stops soil moisture wicking up but allows some airflow
• Gravel base – Promotes drainage and prevents mud splash
• Perforated plastic sheeting – For very wet sites (less common)

The key is not using solid plastic or concrete, which traps moisture below and defeats the purpose.

4. Design the Underside for Water Flow

• Joists and bearers should be sized to span properly with minimal deflection
• Support posts need concrete footings below the frost line (stops heaving)
• No timber should touch soil directly (use galvanised metal feet)
• Gutters or drip edges above can direct water away from critical areas

5. Grade the Ground Away

The ground around and under the deck should slope away from the deck base. Aim for at least 50mm drop over 1m distance.

If your site is naturally low, consider:

• Raising the deck platform
• Adding fill and re-grading
• Installing sub-surface drainage (French drain) in extreme cases

Common Drainage Mistakes We See

Mistake 1: Building Too Close to the Ground We often find decks with less than 100mm clearance. That's not enough for water to escape or air to circulate.

Mistake 2: No Slope on the Decking A perfectly flat deck collects water. Always slope 1:100 minimum.

Mistake 3: Sealing the Underside Some people paint or seal joists to "protect" them. This traps moisture inside the timber instead of letting it breathe.

Mistake 4: Poor Ground Drainage Building on clay or in a dip means water naturally collects underneath. You need proper grading or drainage solutions.

Mistake 5: Ignoring Coastal Salt Spray In Northern Beaches areas, salt spray accelerates corrosion. Stainless steel fasteners and galvanised posts are non-negotiable.

What to Do Right Now

If you have an existing deck:

1. Check clearance – Lie flat and look underneath. Is there at least 150mm of daylight?
2. Look for water pooling – After rain, is there standing water?
3. Inspect for rot – Poke the timber with a knife. Does it feel soft or spongy?
4. Grade the surrounding ground – Does water flow away from or toward the deck?

If you spot issues, address them now. Small fixes (better grading, removing soil buildup) are cheap. Replacing rotted joists costs thousands.

For a New Deck

When we build decks at MJ Leckie Carpentry, drainage design is built in from the start:

• Proper clearance above ground
• Adequate slope on the decking
• Gravel base and landscape fabric underneath
• Galvanised or stainless fixings
• Posts on concrete footings (never buried in soil)
• Ground sloped away from the structure

It costs a bit more upfront, but it's insurance against $10,000+ repairs in 5 years.

The Bottom Line

Good drainage isn't glamorous, but it's the difference between a deck that lasts 30 years and one that falls apart in 10.

If you're building a new deck or have concerns about an existing one, this is worth getting professional input on. Get a free site measure and drainage assessment. We'll identify any risks and recommend fixes.