If the timber has been treated with a preservative then there is no reason the timber will rot.

Also if the wall is plastered with clay and lime, then these materials will help keep the timber dry….as long as you do not try and seal these materials ….they need to breathe…so you need a breathing paint.

If you use the lime plaster you then need to protect it with a breathable paint ….like “Breathecoat??? paint that acts like a membrane ….stopping rainwater entering but allowing moisture vapour out. Otherwise “the Natural Paint company??? supply a breathable clear coating…which also breathes and will stop water penetration.

I strongly suggest  staying away from cement plaster and rather using a Breathecoat paint coated lime plaster over a cob / or clay sand undercoat. ….or if you must use cement, then it is imperative you use a separating layer (e.g. plastic ticker tape) to keep the timber out of contact with the cement plaster at all times. Cement plaster is hardly waterproof anyway and will not stop water coming through….and thus is only as good as the paint applied to it to keep the weather out…..and with its higher moisture content is the same a putting the timber in contact with water all the time, whereas the lime plaster is hydroscopic and thus given a chance (with using breathable paints) will actually dry out and if it gets wet will dry out the timber.

If you use cement, the moisture content of cement is higher than timber (unlike lime or clay which has a lower ideal moisture content)….so it will rot the timber. This is the same reason why timber windows should be primed when they are buried in cement mortar and why a timber beam buried in cement is wrapped in plastic to separate the 2 materials.