Underfelt is essential for use with EPDM roof covers and with pond liners. Prevents the main cover or liner being punctured or damaged by sharp stones, roots etc.
for EPDM roof covers, essential when laying on concrete or over old felt, and recommended for all roofs if possible
for flexible pond liners, highly recommended for all new jobs
Order Underfelt by the square metre to match your roof cover or pond liner. It's not actually "made to size" but the exact quantity you need will be supplied and it's very easily trimmed to size with scissors.
In really rough circumstances use two layers of Underfelt