When installing underfloor heating in aluminium plates you will need to purchase the aluminium plates separately.
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These aluminium plates are 390mm wide and 1,000mm long (1m)
To work out how many plates you will need measure the length of your joists. Aluminium plates will not fit up to the wall so you can bend the pipe round. Then times the length by the joist spaces. A ruff way to work this out is to times the sqm of the room by 2.2.

Underfloor Heating laid in Aluminium Plates