Tanner-'op wrote:
The Natural Philosopher wrote:
Tanner-'op wrote:
George (dicegeorge) wrote:
There are lots of slipped slates on my roof, with rusty nails,
the local builder recommends reslating the whole roof
rather than patching it again and again.
A good decision.

At the moment there is no felt, he advises felt,
is there any reason not to use felt?
There is every reason to *USE* felt. The main one is that if a
slate slips or breaks then the felt (if properly fitted) will
prevent water from entering the roof space until the slate can be
replaced.
The purpoe is to avoid low pressure over the slates due to wind
ripping them off: the felt later is not for waterproofing, it is for
windproofing.

You cannot avoid "low pressure over the roof" - and it is usually the low
pressure that lifts the slates off on the leeward side of the roof (this
creates a vacuum and 'sucks' the slates upwards and snapping them) and not
on the windward side. Direct wind pressure usually just pushes the slates
down onto each other/

Exactly,. By having the felt, you also guarantee that the air inside the
loft at a higher pressure cant travel to wards that low pressure and
push te slates off. I.e. the felt allows a low pressure area in BITH
sides f the slates.





The felt is for 'windproofing' ?? Not sure what mean here as the felt has
no wind proofong qualities as such in this situation - it is there simply as
stop-gap against water penetration.

No, it is not. It is there to prevent the above. It does very little to
prevent water ingress which is why we dont have fully felted rooves, and
we do have many tiled rooves without felty.



As a matter of interest, you need a certain amount of airflow under the
edges of the felt/slates/tiles/eaves/cavites to prevent interstitial
condensation forming on the underside of the felt in the attic. Stopping
all that airflow can cause trouble - as people have found to their cost
when insulating the cavities and loft space without leaving room for an
airflow.

I know that: that's the big danger point. It allows the inside pressure
to be at atmospheric when a low pressure zone exists outside: thats why
the felt is needed, to stop that blowing back out except via the eaves..


By and large there are too many holes in it where nail;s get driven
through to rely on it for waterproofing. I know then when my roof was
felted up before tiling, there were loads of leaks when it rained,
****ing all over the timbers.

Then the felt was poorly fixed or nails have 'missed' the rafters - I have
seen roofs around 20 tiles missing and no water in the attic!

So have I, when its eithetr not raining or its all soaked into the
insulation/


or is there an even better option?
A close boarded roof with felt and slate (Welsh slate being the
best) but the cost will frighten you! :-)


Tanner-'op