The foam boards themselves shouldn't let water through, but the joints will.
It's £60 ISH, for piece of mind and might save you having to rip it all out. Belt n braces.
Pay attention to the detailing around the tray itself, lots of posts on here from people with leaking trays.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local