If a natural slate roof is unfelted, does this provide any ventilation to
the underside? I'd like to jam some celotex between the rafters, but won't
be bothered if I need to install specific ventilation at eaves and ridge.
The roof covering will probably be replaced in the next five years, when
felt and specific ventilation can be added. I just want to still have the
rafters in one piece when this happens.

The proposed layers would be:

----- --------------- ---- -- SLATES
------------ -------------
+---+ no felt +---+
| | 50 mm | |
| | air gap | |------ RAFTERS
| | | |
###| |###########| |##
###+---+###########+---+## -- 25mm celotex
##########################
########################## -- 25mm celotext x layed
************************** -- 12.5mm foil backed p/b

The loft area will be used for light storage. Several pipes, including
rising cold main pass through, so I'd prefer a warm loft situation.

Christian.

On Mon, 1 Dec 2003 11:12:18 -0000, Christian McArdle wrote:

If a natural slate roof is unfelted, does this provide any
ventilation to the underside?

Ample, you ought to see the Tyvec flapping about under our slate
roofs. Once you re-new the covering and fit a sarking layer then you
need to worry about ventilation.

With what you propose I'd be more worried about driven rain/snow
getting in and running down the back of the celotex. Where is that
water going to go?

--
Cheers
Dave. pam is missing e-mail

Ample, you ought to see the Tyvec flapping about under our slate
roofs. Once you re-new the covering and fit a sarking layer then you
need to worry about ventilation.

Excellent, I'd hoped so. I seem to recall some make of felt that is
breathable that can be used instead of the standard stuff. Am I imagining
it, or does the stuff exist, and does it mean that when I replace the roof,
I don't need to install ventilation? It is an Edwardian property and I'd
rather have a totally invisible installation.

With what you propose I'd be more worried about driven rain/snow
getting in and running down the back of the celotex. Where is that
water going to go?

I'll think about it when I install it. In any case, the water have just
landed on the lath and plaster below, so whatever I decide will probably be
an improvement. A bit of guttering sending it to an overflow will probably
do the trick.

Christian.

"Christian McArdle" wrote in message .net...
If a natural slate roof is unfelted, does this provide any ventilation to
the underside?

Lots. And water as well.

I'd like to jam some celotex between the rafters, but won't
be bothered if I need to install specific ventilation at eaves and ridge.
The roof covering will probably be replaced in the next five years, when
felt and specific ventilation can be added. I just want to still have the
rafters in one piece when this happens.

If the insulation dries out quickly OK, but if its so dense that it
dosent, I wouldnt. Unlined slate roofs do let in water, then the air
comes along and dries them off again. They are by no means waterproof.


Regards, NT

If the insulation dries out quickly OK, but if its so dense that it
dosent, I wouldnt.

I thought that celotex was closed cell, so couldn't get soaked? Am I
mistaken?

Also, would it break down over long periods? I have seen no sign of moisture
ingress through the roof during these last weeks of rain, so it wouldn't be
a deluge of water, just spots, at the most. I could always use foil backed
stuff facing upwards. This could even reflect excess summer heat radiated
from the tiles back up. (The insulation would be sufficient that no useful
solar gain in winter would ever occur).

Christian.

Christian McArdle wrote:

If a natural slate roof is unfelted, does this provide any ventilation to
the underside? I'd like to jam some celotex between the rafters, but won't
be bothered if I need to install specific ventilation at eaves and ridge.
The roof covering will probably be replaced in the next five years, when
felt and specific ventilation can be added. I just want to still have the
rafters in one piece when this happens.

The proposed layers would be:

----- --------------- ---- -- SLATES
------------ -------------
+---+ no felt +---+
| | 50 mm | |
| | air gap | |------ RAFTERS
| | | |
###| |###########| |##
###+---+###########+---+## -- 25mm celotex
##########################
########################## -- 25mm celotext x layed
************************** -- 12.5mm foil backed p/b

The loft area will be used for light storage. Several pipes, including
rising cold main pass through, so I'd prefer a warm loft situation.

Christian.



Looks pretty good to me. Yes, there is enormous ventilation in unfelted
slate rooves. In fact they are ****ing icy in winter - been there, done
that, rented flat, no central heating, no loft insulation either....I
stuck my head through the loft hatch and although there were naff all
eaves vents snow was drifting through cracks...I didn't stay more than
one winter there.

If you are concenred, drill for eaves vents as well - then when you have
tp re-slate, you can put paper underneath and not have to mess with teh
eaves.

Make sure you foil seal VERY well as well. Deven micro draughts make a
HUGE difference. I am stopping up all mine at the moment.

Make sure you foil seal VERY well as well. Deven micro draughts make
a HUGE difference. I am stopping up all mine at the moment.

What's a Devon microdraft?

BTW, there will be two layers of 25mm celotex cross layed, to eliminate cold
bridging and cracks.

Christian.