How to protect joists - tape vs exterior waterproof paint?

[snipwhip]

Hi, I'm tossing up between joist tape and uv stable exterior waterproofer for the top of my joists like https://www.bunnings.com.au/crommelin-1l-exterior-grade-brushable-waterproofer-1l_p0961573?srsltid=A...

Or bitumen membrane like https://www.bunnings.com.au/gripset-betta-1l-waterproofing-membrane-bitumen-rubber_p0960165. Are there advantages or disadvantages to using the paints? I've seen some posts where the bitumen waterproofing is not uv stable and only good for underground application but the external one linked above is uv stable. Joist taping will cost more than $100 while paint will be $30 hence I would prefer paint if it's good for the job.  Thank you

[MitchellMc]

When protecting deck joists, both joist tape and exterior waterproof paints can help extend the timber’s lifespan @snipwhip, but they perform differently. Joist tape is designed specifically for decking applications and provides a physical moisture barrier on the top of the joist where water tends to pool. It also seals around screw penetrations, preventing water from entering through fastener holes, which is one of the main causes of rot. It’s UV-stable and highly durable, but can be more expensive, especially if you’re covering a large area. On the other hand, exterior waterproof paints can offer decent protection when properly applied, forming a flexible waterproof film that prevents moisture absorption. However, these won’t self-seal around screws like joist tape does, meaning it provides surface protection rather than full moisture isolation. If you’re after a budget-friendly solution, the exterior waterproofer is a good compromise. However, if longevity and total protection are your priorities, joist tape remains the superior choice despite the higher cost.

Please let me know if you have any questions.

Mitchell

[snipwhip]

Thank you