Ceiling service void size for domestic sprinkler system

[MattD]

We're in Wales, meaning all new builds must have a sprinkler system. We're designing a two-bed timber framed cottage with one-and-a-half floors (bedrooms in the attic space). Sprinklers are required in bedrooms, meaning we will need to allow for the sprinkler pipes in the ceiling service void. Since the bedrooms are in the attic (roof space), service void space is limited.

 

I was under the impression that a ceiling service void is typically 25-75mm: 25 if you only plan on running cables, and 50 or even 75 if you plan on running water pipes too. Just to be on the safe side, I thought I'd contact the sprinkler company we're thinking of using to ask them how much space they need. They said 120mm (as a minimum!) Does this sound right? Can anyone else who has built for a domestic sprinkler system confirm if this much space is really necessary? If we make our ceiling service void that large we would effectively need to box in the entire timber frame, which would be a real shame. I'm not entirely convinced that the sprinkler company really understood what I was asking about, but thought it worth asking here in case anyone else has had experience with this. They said the sprinkler heads require 120mm.