Service Void size for down lights

[Andrew]

Our build has mostly vaulted ceilings upstairs and I'm just trying to finalise the spec. with the timber frame supplier. Our roof build up includes mineral wool between the rafters and then 50mm of PIR on the inside across the rafters, the vapour control layer and then 35x45mm battens to form a service void which will then take the plasterboard on top.

 

I recall reading here about difficulties installing down lights due to the service void being too shallow (unfortunately I can't find the post again) and this got me thinking as to what is a suitable size for the service void. Looking at low profile down lights, they seem to need 60-70mm of clearance and even taking into account 12.5mm of that depth is going to be through the plasterboard, I don't think the 35mm we currently have specified is going to be enough.

 

I wondered what others had done for the sloping ceiling service void. I'm thinking 2 x 25mm deep battens may be enough but would love to hear any real-world experience in this area. 

 

 

[Jeremy Harris]

We have low profile down lights in our kitchen.  They are barely any thicker than the plasterboard.  I have a spare here in front of me and the maximum depth from the rear face of the front flange to the top of the spring that retains the light in place is a fraction under 20mm, so they project about 5mm behind a boarded and skimmed ceiling, perhaps 8mm behind a taped and filled ceiling.

[lizzie]

Mine are 70mm.  They are tiny little lights. I have downlights of some sort everywhere. We didnt have a big enough void so we had to cross batten to get our depth, reduced our ceiling height marginally but has made no difference.

[Andrew]

Thanks for the responses.

 

I think the main difference in depth comes down to wether the downlight is fire rated or not - all of the really slimline ones seem not to be fire rated. Same goes for dimmable - I haven't been able to find a 20mm or so deep one which is dimmable. 

 

With regards to the fire rating - I can't find a definition of when a downlight needs to be fire rated. I've had a read through of the building regs and I'm none the wiser.

 

Is there an easy to understand set of rules around when a downlight needs to be fire rated? The ones I'm concerned with are upstairs rooms with only the roof above. 

[Temp]

TLC Electrical have a range of downlighters of different types

 

 https://www.tlc-direct.co.uk/Main_Index/Lighting_Menu_Index/Lighting_Downlights_Index/index.html 

 

The void depth is typically given somewhere on each page but I would also check the makers data in case of errors. 

 

 

 

[phatboy]

Something like this:

https://www.ledhut.co.uk/spot-lights/ceiling-lights/18-watt-led-slim-downlight-100lm-w-tpb-4000k.html

 

Fire rating is only usually needed when there is a separate unit above, I think.  So basically in flats, not in houses (I think)