Mullions are the vertical stones between windows. They help support the lintels above. Like other stones, they should have been laid with the layers of stone running horizontally. Such long stones were hard to obtain, so the mullions were often laid with the stone layers running vertically. This means they often weather away (delaminate).

Problems can also occur at the base of the mullion, with additional wear from splashes from the cill.

Repair as for other stone defects. 

If the stone mullion is very badly worn, it should be replaced with a new stone mullion. Precast concrete mullions are currently not recommended and would not be permissible in listed buildings or conservation areas.

Professional help recommended?

Although the work may appear straightforward, ensure your builder or tradespeople have the skills for the job. If in any doubt, get professional help to specify and organise the repair.

Who pays?

These are part of the structure and normally a common repair.