I am aware that in large areas of the UK, no sarking or tarred felt is installed below the roof tiles. The tiles are just nailed straight onto the horizontal timber straps.
Thermal insulation in this situation is available from specialist firms who spray quick-drying plastic foam onto the inner faces of the roof-tiles.
My neighbour has recently had her 3-bed semi re-roofed with new Sterling-board sarking sheets, tarred-felt, new timber straps, and new cast concrete Marley-tiles, at a cost of £6500 inc Vat.
That will last another 50 years, even in our storm-lashed north end of Scotland.
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