what can I paint on my roof to go under the felt tiles

Hi I am building a shed and want to build it to a much higher standard than normal, but not quite up to the standard of a house, so it is quite big and much sturdier built than normal. On the roof I want to put those felt shingles that look like tiles, like these

http://www.roofshingles.co.uk/standard-products.html

but before I do I was wondering if there is any coating I can paint the plywood sheets with to further waterproof and protect the roof.

Is there maybe some flexible tar like stuff or what do you recommend?

It would be good if it were something that I could get from somewhere cheap like Wickes, thats if you thought it was good enough of course, I want something good not just something for the sake of it.

Any advice appreciated.

Use a layer of roofing underlay over the boarding, no need to paint anything on. should cost around £30-£40 for a 45 square metre roll.

Mind you, you need not do anything, the shingles will keep the roof watertight as they are

I think you'll find in the fixing instructions it does specify using an underlay first.. as just stated....

In all the felt shingle literature I have here (inc the Ruberoid Armourglass / Armourshield instructions) they all say an underlay is not always required (but should be considered especilly if the building is to be habitabl) , only really if you need to keep the boarding dry when stripping off old materials.