The Building Research Establishment (BRE) recommends disused chimneys should be capped off at the top with a ventilated capping.

Where a disused chimney is left uncapped, it is open to the UK weather, which annually can be relied upon to deposit several feet depth of water down the average chimney. This can give rise to damp stains appearing internally and over time it contributes to steady internal deterioration of the fabric of the stack.

Blockage of the chimney by nesting rooks will not stop the rain getting in but it will tend to trap the moisture in the chimney by blocking ventilation and preventing the moisture from drying out.

Not to mention the dreadful unhygienic mess that nesting rooks can cause and the difficulty of removing the nests once established!