Run off rainwater from Shed roof

Creosote can still be bought in 50l drums - essentially for farmers. Creocote is not bad (smells like weaker creosote). But I use the Barretine on my oak porches. The neutral colour one absorbs into the wood invisibly, and then you can see raindrops just forming blobs on the surface and running away.
Cuprinol ducks back is supposed to last 5 years on fences. Rubbish. I have to do it annually.

 

Decking oil is an option .
A damp course membrane under the shed edges would help.