Most Celtic houses in Britain were simply constructed. They were generally round as in the remains of the house at Little Woodbury in Wiltshire, the wheelhouse at Jarlshof in Shetland, or the house on an unenclosed platform at Greenknowe in Berwickshire. … The roof of the Greenknowe house was conical. It had rafters fanning outwards from a high top, like the spokes of an umbrella. You can see why it is called a wheelhouse. The rafters were held at their lower ends on a horizontal ring of timber that rested on vertical posts with Y-shaped tops.