I’d replace the rotten hardboard for a start then get some roofing adhesive and glue down a water proof membrane. Should stop the leak.
I should point out I have no experience in this.
Flexacryl and some 'scrim' is good - the scrim provides a scaffold for the Flexacryl to bond to. The thin fleece material that you wrap plants to protect them from frost would work.
@superbagneres started
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I’d replace the rotten hardboard for a start then get some roofing adhesive and glue down a water proof membrane. Should stop the leak.
I should point out I have no experience in this.