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Hi @jeffl_au,
I'd tend to agree with @RSS's comments that the joist tape is specifically designed for that purpose and would be better suited. The Crommelin 15L Blackseal Heavy Duty Bitumen Waterproofing Membrane is only suitable for below-ground use and in non-UV exposed applications. I'm a bit concerned that there will still be some UV exposure through the gaps in your decking boards.
Please let us know if you have further questions.
Mitchell
You could paint it with the Crommelin product @jeffl_au.
The best thing to do is to ensure that there will be adequate drainage under the deck. Water needs to fall through it, hit an open drainage material and then drain away. If water pools under the deck and there is no airflow, then humidity rises, and decay begins. You should consider a wider-than-normal gap on your decking boards to allow some airflow. I'd suggest at least 5mm. Your timber is treated to prevent decay in above-ground use; the humidity in the enclosed structure is what you need to prevent.
If you haven't already, check out this guide on How to build a deck.
Mitchell
@jeffl_auremoval of that top 50mm of soil would make a big difference, as the water is probably soaking in to that top 50mm and then running down the slope. Removing the top 50mm, will also remove most of the grass/weeds and prevent them from growing back, especially with a bit of gravel. The gravel will prevent the soil underneath from being washed away and you won't see the water sitting/flowing.
For best result, don't put down the gravel until the posts are in. Make sure there are no low areas next to the post, where you have compacted the soil down next. You could use compacted soil to create a slope around the post, it doesn't have to be concrete as shown in picture.
50mm is enough for the water to seep in, as @RSS has mentioned. It would always be best to have drainage to collect the water, but at a minimum, gravel is needed so pools of water don't splash back up and onto the underside of your decking and joists.
Mitchell
