Common rafters without collar beams form most of this roof. There is not always a ridge board or beam where the rafter tops meet. Under the midsections of the rafters are purlins which support the common rafters and are supported by principal rafters. This roof ends in an octagonal hip.
English: Showing internal roof structure in gin gang at Beamish Museum, County Durham, England. Gin gang A gin gang is a structure built to enclose a horse mill, usually circular but sometimes square or octagonal, attached to a threshing barn. Most of the wood used here would have been seasoned oak originally, but much has been replaced during preservation, including apparently the main oak spindle-shaft of the mill, which appears to be unseasoned and cracked. Roof construction This renovated internal roof structure is based on a traditional space frame truss with its primary plane in line with the tie beam (or joist), and with members fixed between king post and rafters to support the semi-octagonal plan of the roof. There is one main transverse oak tie beam on which the king post of the main truss is based. The king post is in tension to prevent sagging of the horizontal tie beam, so neither the king post nor the tie beam are resting on the mill below. The roof construction is not structurally dependent on the horse mill, or connected with it. The top of the mill's main vertical spindle shaft and the end of the main power shaft are pivoted at the centre of their own separate tie beam which is below and parallel with the main roof tie beam, and set in the gin gang's side walls at either end. The mill's tie beam has to be stabilised with two massive oak beams which run, either side of the power shaft, from tie beam to barn wall. A large and basic engine like this can create great stresses from the torque engendered. Mill construction This is based on a central, vertical, heavy, 10ft high, oak spindle shaft, with its base pivoting on an iron framework set in the middle of the gin gang floor. Its top end pivots under the mill's own tie beam (if it didn't, it would fall over) but the tie beam cannot rest on the pivot, or the mill would not turn. The spindle shaft supports around its top end the main, 6ft-diameter, horizontal gear wheel, with the teeth on the top surface. The main power shaft, a massive oak pole, is girthed at one end by a secondary gear wheel which lies on the main gear wheel, thus turning the main power shaft on its long axis. The main power shaft runs from the main gear wheel through a hole in the barn wall and in the barn it powers the threshing machine and other machines via further cogwheels and belts. Such a machine would create great torque stresses, and if it were ever to work again it would require re-engineering to ensure smooth and safe running, and beams which are seasoned and not cracked. NB: This mill was rescued from Berwick Hills Low Farm, Northumberland, and has had some repairs since then. Horse power This is a powerful horse mill for four horses; probably English ponies, as modern heavy horses were not yet fully bred in the 1830s. Each of the four heavy oak beams connecting the central spindle shaft to the four horse shafts runs from beside the spindle shaft to the under-surface of the main gear wheel, and crosses the other three in a grid pattern between the spindle shaft and gear wheel, thus supporting the gear wheel and creating a rigid structure at the same time. They project for many feet beyond the main gear wheel, and require wooden stretchers to stabilise them. From the ends of these beams hang the horse shafts. The bottom ends of these shafts are quite low, which may indicate the use of ponies rather than early heavy horses.