I too have an old house with slates, some leaks, but mostly patched with tape a few years ago.
i've been following discussions on this group for almost a year, learning lots...
It has been shown to me that the nails in my roof are rusty, so patching the slipped slates is only temporary, they've all got to come off and go on again.
i'm too unskilled and clumsy and scared of falling (again) to do it myself, i will however do insulation, ceilings etc, and am clearing away the old lathe and plaster and dirt and ancient unused pipes and wires to make it easier for them.
sarking/ felt of a modern kind is recommended.
some recommend the scottish method of putting boards on the rafters, then battens, then slates, but i think this probably an unnecessary expense for herefordshire- i have a lot of roof to get done.
Maybe boarding is a good idea for london where insulated roofspace is valuable
some say the nails should be aluminium, not copper.
the spray-on foam people guarantee their job for 25 years, but will their company exist in 25 years time? and anyhow this building should last hundreds of years,
i dont need the roofspace so i'm going to have a cold ventilated roof space, with insulation above the ceilings.
So I'll be able to walk around in the roof and look for leaks (but not sure about this - with no felt now i can see light through the gaps where slates have slipped, and wet patches underneath them, but with felt any dampness will flow down the felt and be invisible to me, hope this doesnt mean the timbers will rot... )
and im gonna have a few skylights put in, perhaps facing north so not too much burning sun, but how to insulate them i dont yet know...
[george]