Damp is something that strikes a chord with anyone who cares for a property and there is no doubt that the main enemy of historic buildings is water. In fact, it is almost the only enemy.
Historic buildings are usually as good as their roofs. A roof in a reasonable state of repair, combined with a working and well maintained water disposal system will greatly prolong the life of any building. In any programme of repairs, if the building can be kept dry, other repairs can follow.
Always use the best quality materials you can afford. Never use roofing felt in hidden valleys because it cannot be seen. If a problem occurs you will only know about it long after the damage has been done. Never use an external coating over existing roofs or gulleys as these invariably leak and it will be almost impossible to trace where this occurred.
If the building has a parapet or hidden inner roof slopes, it is more difficult to keep these under observation. Whenever an opportunity arises (perhaps when other building work is under way) it worth testing the effectiveness of the drainage system by pouring a bucket of water over the surfaces to see how well the water escapes.
Make use of existing roof dormer windows or skylights for inspections and if these do not exist consider the possibility of having a small unobtrusive hatch added to enable regular access to be gained to these inaccessible areas.
Rainwater disposal must not be neglected. A little money spent at an early stage will save very expensive repairs later, especially if dampness leads to an outbreak of dry-rot or death-watch beetle. Beetles are fussy feeders and they like nothing better than timber softened by persistent dampness.
Areas of hard paving often extend right up to the external walls. After wet weather it is worth checking that water is draining away from the building, not standing in pools, and it is preferable if the water flows easily to an existing functioning gulley.