What Is Joist Tape?

Joist tape (also called decking tape or flashing tape) is a self-adhesive waterproof tape applied to the top of deck joists and framing before installing decking boards.

Its main purpose is simple:

👉 Create a waterproof barrier to protect the timber substructure

Most joist tapes are made from butyl rubber or bitumen-based materials, designed to seal the most vulnerable parts of your deck — especially where screws penetrate the wood.

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Why Joist Tape Matters (Especially in Australia)

Many homeowners focus on decking boards — but in reality:

👉 Your joists determine how long your deck lasts

Without protection:

• Water sits on top of joists

• Moisture enters through screw holes

• Timber starts to rot from the top down

This is one of the most common failure points in decks.

Joist tape helps by:

✔ Preventing water penetration
✔ Sealing around fasteners
✔ Reducing rot and decay
✔ Extending deck lifespan significantly

In climates like Adelaide's (hot sun, occasional rain, coastal air), this protection becomes even more important.

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Do You Really Need Joist Tape?

Technically:

👉 No — but you absolutely should use it

Skipping joist tape can lead to:

• Premature structural failure

• Costly repairs

• Reduced the lifespan of the decking

Even high-quality composite decking (like PureSolid) is installed on timber joists, which remain vulnerable to moisture.

👉 Think of joist tape as cheap insurance for your entire deck.

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How Much Joist Tape Do You Need?

This is where most people get it wrong.

Simple Rule:

👉 You need tape for every joist line

Step-by-Step Calculation

1. Measure Total Joist Length

Add up all joists in your frame.

Example:

• Deck width: 4m

• Joist spacing: 400mm

• Number of joists: ~11

👉 11 joists × 4m = 44 linear meters

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2. Add Extra for Key Areas

You also need tape for:

• Double joists (joins)

• Rim joists

• Ledger board

• Stair stringers (if any)

👉 Add 10–15% extra

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3. Final Formula

✔ Example:
44m × 1.1 = 48–50 meters of joist tape

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Quick Reference Table

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What Width Joist Tape Should You Use?

Joist tape typically comes in:

• 50mm (2") – standard joists

• 65–75mm – wider joists or extra coverage

👉 Rule:

✔ Tape should fully cover the joist top
✔ Slightly wrap edges for better water runoff

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Where Should You Apply Joist Tape?

Apply tape to:

✔ All joists
✔ Double joists (VERY important)
✔ Ledger board (against house)
✔ Beams and rim joists

Joist tape protects the areas most exposed to water and fasteners — the highest-risk zones.

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How Joist Tape Works (Simple Explanation)

When it rains:

• Water flows between decking boards

• Lands on joists below

• Without tape → soaks into timber

• With tape → runs off safely

It also:

👉 Self-seals around screws, preventing water entry at penetration points

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Why It’s Critical for Composite Decking

Here’s something many people don’t realise:

👉 Composite decking = MORE water reaches the joists

Why?

• Composite boards don’t absorb water like timber

• Water drains straight through gaps

• Joists take the full exposure

That means:

👉 Joist protection is even more important with composite decking

This is why professional installers almost always use joist tape with systems like PureSolid decking.

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Common Mistakes to Avoid

❌ Not Using Enough Tape

Only taping some joists defeats the purpose.

❌ Applying on Wet Timber

Tape won’t stick properly → fails early.

❌ Not Covering Edges

Water still seeps in from the sides.

❌ Skipping Double Joists

These areas trap the most water.

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Pro Tips (From Installers)

✔ Always apply on clean, dry timber
✔ Use a roller or firm pressure
✔ Overlap tape at joins
✔ Install immediately before decking

These small details make a big difference in lifespan.

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FAQ

Does joist tape really work?

Yes — it creates a waterproof barrier and significantly reduces rot when installed properly.

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How long does joist tape last?

High-quality butyl tape can last 20–50 years, matching the life of composite decking.

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Can I add joist tape later?

Not effectively — it must be installed before decking boards go on.

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Is joist tape worth it?

Absolutely — it’s a low-cost upgrade that can double the life of your subframe.

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Final Thoughts

Joist tape is one of the most underrated parts of decking installation.

It’s:

• Cheap

• Easy to install

• Extremely effective

But most importantly:

👉 It protects the part of your deck you can’t see — but depends on the most.

When building a long-lasting outdoor space with PureSolid composite decking, don’t just invest in the boards—protect the structure underneath.