minimum distance air gap between roof and plaster board

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hi folks hoping some can tell what is the minimum air/breather gap you need between the roof tiles/membrane on the inside of the roof and the finished plaster board.

The roof trusses are 4 inch thick could I get away with putting 2 or 3 inches of kingspan and leaving a 1 or 2 inch air gap

hope this is clear

cheers

jon
 
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Leave a 2" air gap if your felt is the old-fasioned bitumen-based stuff.
Make sure you have sufficient ventilation at both eaves and ridge.

If you have the modern breather felt, your gap can be reduced to 1".
 
Leave a 2" air gap if your felt is the old-fasioned bitumen-based stuff.
Make sure you have sufficient ventilation at both eaves and ridge.

If you have the modern breather felt, your gap can be reduced to 1".

Does it make any difference if the modern breather membrane placed under the slates is situated on top of continuous (i.e. under the tiles / membrane the roof would look like it is covered with solid wooden plank nailed to the rafters) wooden sarking? Or would 1" be sufficient?
 
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50mm minimum with both high and low level ventilation in your case jiminy.

Thanks, that very helpful. 50mm should still leave enough room still for a decent bit of insulation. Though, we currently don't have any vents in the soffits or ridges, so I guess I'm going to have to put quite a few in...
 
A tile vent in between each rafter.

That's a lot of tile vents. While there are plenty of older building sarked the same way and that have loft conversions in our area, I've seen none with that number of tile vents!!! And presumably for a tile vent to be any use, the sarking will need cut to allow air to communicate internally, because the slates themselves are nailed directly into the wood sarking?
 
In order for the ventilation to work it needs to be open at the top and bottom of the slope. So you either have a continuous ridge vent system that has a vent between the ridge tile and the first tile on the slope or you have a vent in a tile between each rafter. Otherwise how can air flow magically through the ridge board or through each rafter?

If the OP or you think its excessive or unnecessary so be it.

It wasn't a comment criticising the necessity of the the ventilation, it was a question questioning the method of achieving it, because presumably the loft converted houses round where I am do have adequate ventilation, and the fact that aesthetically imagining what a tile vent placed a 40 centres would like like would automatically lead me to think of trying an alternative mechanism to achieve the same ventilation. I realise you need top and bottom ventilation, we were already thinking of ridge venting, and I thought you were suggesting that a tile vent would be needed in place of soffit vents, the latter which are more inconspicuous and which I'm hoping to go for is it can provide sufficent ventilation.
 
Last conversion we done was around 2yrs ago and was tile vents every 1200mm at low level with continuous ridge ventilation.
Over fascia vents are also an option if your eave detail allows.

Continuous ridge vents will require you to cut 50mm either side from top of sarkin boards to allow airflow.

Insulation boards are only taken as far down so to meet up with insulation in walls. Behind the walls is vented space allowing airflow above insulation.
 
Continuous ridge vents will require you to cut 50mm either side from top of sarkin boards to allow airflow.

Thanks, but I hope the position of the top slate will allow for that, and I've a fear it won't!

Insulation boards are only taken as far down so to meet up with insulation in walls. Behind the walls is vented space allowing airflow above insulation.

Yes, was thinking that would be sensible:
- soffit or fascia vents
- running into the eave air space that then communicates with cavity wall below
- running into air space behind loft converted walls (along full length of one side of the roof)
- running into >50mm space behind kingspan insulation of loft converted roof within each centre between rafters
- running all the way up the roof to full length ridge vents with 50mm of sarking cut back at the top

At the moment, there is currently no ventilation in our roof, no soffit vents and no ridge vents and no tile vents. The only evidence I've seen of condensation was the presence of an occasional bead of water present on the very tips of the slate nails poking through the sarking which occurred during some extended very wet weather. Never enough to drop onto the loft floor and only on a couple of nails. At first I thought it must be coming through the roof, but the wood was completely dry to touch, and realised it was because of the coolness of the nail on which the moisture was condensing. Probably not a good sign though.
 

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