The external and the internal covering of a building are very often not the same; the outer covering is then usually called a roof--the other, a vault or ceiling. In not a few Gothic buildings, however, they were the same; such buildings had what are known as open roofs--_i.e._ roofs in which the whole of the timber framing of which they are constructed is open to view from the interior right up to the tiles or lead. Very few open roofs of E. E. character are now remaining, but a good many parish churches retain roofs of the Dec., and more of the Perp. period. The roof of Westminster Hall (Perp., erected 1397) shows how fine an architectural object such a roof may become. The roof of the hall of Eltham Palace (Fig. 22) is another good example. Wooden ceilings, often very rich, are not uncommon, especially in the churches of Norfolk and Suffolk, but greater interest attaches to the stone vaults with which the majority of Gothic buildings were erected, than to any other description of covering to the interiors of buildings. The vault was a feature rarely absent from important churches, and the structural requirements of the Gothic vault were among the most influential of the elements which determined both the plan and the section of a mediæval church. There was a regular growth in Gothic vaults. Those of the thirteenth century are comparatively simple; those of the fourteenth are much richer and more elaborate, and often involve very great structural difficulties. Those of the fifteenth are more systematic, and consequently more simple in principle than the ones which preceded them, but are such marvels of workmanship, and so enriched by an infinity of parts, that they astonish the beholder, and it appears, till the secret is known, impossible to imagine how they can be made to stand. [Illustration: FIG. 22.--ROOF OF HALL AT ELTHAM PALACE. (15TH CENTURY.)] It has been held by some very good authorities that the pointed arch was first introduced into Gothic architecture to solve difficulties which presented themselves in the vaulting. In all probability the desire to give to everything, arches included, a more lofty appearance and more slender proportions may have had as much to do with the adoption of the pointed arch as any structural considerations, but there can be no doubt that it was used for structural arches from the very first, even when window heads and wall arcades were semicircular, and that the introduction of it cleared the way for the use of stone vaults of large span to a wonderful extent. It is not easy to explain this without being more technical than is perhaps desirable in the present volume, but the subject is one of too much importance for it to be possible to avoid making the attempt.