These project from the normal wall face more or less in accordance with the strain they have to resist. This strain may result either from the thrust due to the principals of a wooden roof, in which case they are placed with the same intervals of space as the principals, and so as to take up the strain; or they may serve to take up the thrusts imposed on the substructure due to the vaulting should this be employed.

They are economic, inasmuch as by their use the greatest strength is applied only where most needed, and the spaces between the buttresses may be filled by lighter walls, and also pierced by windows for light without danger to the stability of the general fabric. They also have a distinct value from a decorative point of view.

In proportioning a buttress, diagrams should be made illustrative of the direction of the thrust due to the weight of vaulting and roof, and a polygon of forces should be constructed giving the line of the resultant of the forces; and from this the proportion of the buttress and the positions of its set backs can be ascertained.

It is frequently the case that the resultant of forces is of such a nature that buttresses of great size would be required altogether disproportionate to the general features of the building. An avoidance of over heavy and unsightly masses of masonry can frequently be made by applying, as in mediaeval times, pinnacles over the head of the buttress, thus gaining weight without clumsiness. These additions may be made very important and valuable features, and add to the beauty of the general structure by careful treatment, combining both utility with ornament.

Fig. 150 illustrates the application of buttresses to the angles, and exhibits the set backs with their moulded, weatherings.

The apices are planned on a cruciform basis of major proportions, with a minor cross penetrating the major one and at right angles to it, the whole being surmounted by a pinnacle with carved crockets.

In church architecture cases arise wherein the buttress is employed to sustain the thrust of ribbed roofs in the aisles, but above the aisles rises the clerestory wall, carrying the nave roof, which is also vaulted.

To meet the thrust due to this vaulting, and to avoid Unsightly masses of masonry, flying buttresses are employed. These are half-arches springing from the buttresses of the outer aisle walls over the aisle roofs, and impinging on the clerestory walls at or near the springing of the ribs of the nave vaulting. They are employed to counteract the thrust caused by the main vault, or, more properly speaking, to transmit it to the outer buttresses. They lend themselves to considerable ornamentation.

Buttresses 198

Fig. 146.

The arches should be of flat sweep. The upper surface may be channelled, and utilised to lead the water from the clerestory guttering to the outer walls, or may be saddlebacked.

In certain situations and under certain conditions double flying buttresses are employed. These are more particularly applicable where thrusts are to be met at more than one point in the clerestory wall, generally where there is much difference in height between the aisles and the transepts. An illustration (Fig. 151) is given of a buttress of this kind.

Buttresses 199

Fig. 147.

Buttresses 200

Fig. 148.

It will be noticed that the buttress is carried well above the junction of the upper flier, neutralising by its added weight the outward thrust of the arch.