Clip from Secrets of Althorp - The Spencers (2013) :
Narrator : What you see now is an illusion designed to look like grey brick. The exterior is made up of 106,000 thin clay tiles, called mathematical tiles. But centuries of wet English weather wreaked havoc with this outer shell"We must preserve history! But no, not that history." I suspect it is less about an interest in history and more about a dislike of change or a need for someone to boss people around.
Jan Bialek (Althorp Building Manager) : Originally, the mathematical tiles were fixed on a quite think lime mortar bed. And then iron nails would be fixed through the holes that are in there and the nails went through the lime mortar and into the red brick. The problem with the nails is that they are iron nails and they rust. And there's no secure fixing to the tiles. So the tiles are gradually coming loose and moving away
Narrator : The Earl embarked on a massive renovation project . A new lead roof and secure fixings for every tile on the walls.
Jan Bialek (Althorp Building Manager) : one or two people have asked "well, why didn't you restore it back to its original red brick?" Physically, it couldn't be done because the red brick was damaged behind.
Narrator : And there was another problem. Althorp's historical importance means that the house is protected by England's strict national heritage rules and regulations. The Earl would not have been allowed to strip the unique façade away from the building. …
(to whoever might be interested , the clip continues and explains how they fixed the mathematical tiles in place)