On 15/03/16 15:04, The Natural Philosopher wrote:
On 15/03/16 13:52, Martin Bonner wrote:
On Tuesday, 15 March 2016 14:45:04 UTC+1, The Natural Philosopher wrote:
On 15/03/16 13:13, David wrote:
If I insulate, there will be a ventilation gap beneath the tiles and
membrane from eaves to ridge and back to eaves, but no ridge vents
(unless I pay someone to go up on the roof).

How necessary are ridge vents?

Not really necessary at all

Well, not unless you want to prevent condensation on the timber rafters.
(Or, to put it slightly less elliptically, "Actually quite necessary").

Where does the water come from then?

(Or, to put it slightly less elliptically, "Actually quite unnecessary").


Damp air.

My shed had no humans living in it. Nor does it have a source of water
vapour other than the air.

However, if I leave it all winter, during the worst months it will have
condensation running down the windows, mould growing on odd bits of ply
and tools will go rusty.

That was last year - before I starting running a dehumidifier in it.

Warm air gets in, cold makes it condense, without adequate ventilation
the water remains liquid and makes things damp.

I believe the same thing is likely to happen in a roof unless you have
an adequate air path.

The BCO advised me to ensure I had a through air path under my flat roof
decking as I was sealing the inside up with celotex. He even advised
mushroom vents either side of teh chimney as that was blocking the
natural air path from the north soffit vents to the south soffit vents
(being continuous airpath inside).