New Roof Advise

It would help if you could post as best you can detailed pictures of the inside of your loft room and the appearance outside.

 

3/4 inch laths are totally adequate for a slate roof,there is no need to upgrade them and disturb your roof Windows.If you are replacing front and back roofs use all the sound original ones on the front and the Spanish ones on the back.

 

Breathable membrane is a must too

 

Did you insulate the roof when you did the loft conversion?

 

You may have a condensation issue rather than leaks if you fired a load of insulation in the roof construction and didn't add a VCL or breathable felt. If your roof was watertight and now all of a sudden "leaking everywhere" that would be my strong suspicion.

Has anyone got up and identified where the water is getting in?

It would take a load of your Welsh slates to be cracked or missing, lead in bad order to get all over water ingress.

 

I'm not aware of any regulation to use 1 inch laths,what about where you join into your neighbours slate laths.Your rafters on a house that age will probably be around 400 centres,a new build with trusses could have 600 centres,this is where bigger laths are needed,not in your case.If you have condensation it will appear in cold dry weather,not only when it rains.

 

St Asaph Cathedral,Wales,it's slate roof had been there nearly 400 years & was still ok, so plenty of life still in your slates.

A lot to be said for counter battening your roof, will allow ventilation & prevent the condensation issues.

 

This shows counter battening better than I can explain it, as you can see in photo, you have a ventilation gap to allow air flow,allow moisture to escape.

 

They have been doing it that way up in Scotland for decades,there would also be a sarking board under the felt/membrane.

 

Some 3/4" sarking, the proper way for slate,a uniquely scottish pratice, seems to be sadly out of fashion some parts of England, seen a slate roof being done other month, no sarking just a breathable membrane, then slates.

 

Usually 18x150mm softwood boards, but there are modern alternatives now, like those from Steico.

https://www.ecomerchant.co.uk/steico-special-dry-wood-fibre-insulation-sheathing-board.html

 

What is the benefit of fixing lathes to sarking?
We have sarking followed by membrane on our new roof (horsehair on the old bits) and the slates nailed directly into the sarking. Does make it fairly hazardous when in the loft as you have to dodge the slate nails when close to the roof which I guess would be less of an issue if nails go into the lathes.