Twenty minutes to to kill in the National Theatre so a quick doodle. It’s always been my favourite building on the South Bank, but I was lucky enough to deal with the building in my previous architectural life at C20, and had the chance to explore behind the scenes.I love it all the more. Many people don’t realise that every set and prop used on the three stages here is cut, built, shaped and painted by a beehive of artists and technicians behind the fortress-like grey brick walls to the south. The exterior has a timeless sculptural quality, its concrete forms stain and weather just like the stone buildings all over London. I’ve just been reading that the architect, Sir Denys Lasdun, intended the building to age ‘so that in the end lichen grows on it and it becomes part of the riverscape’. This is the foyer, deep in the complex grid that moulds the building, the diagrid of the roof playing against the 90 angles° of the robust concrete walls. The rough-caste concrete is strokeabley beautiful, like an exotic lizard skin. I’ve always wanted to make a crayon rubbing of it. Random fact: each piece of wood was only used once on each side in the casting –so keen were the designers on achieving the perfect impression they wouldn’t re-use the moulds.