An elementary treatise on the construction of roofs

The present little work is intended to introduce the student of architecture to the first principles upon which the roofs of buildings are constructed and to enable him the more readily to understand the larger and more elaborate treatises upon this important subject. The work is divided into two parts, the first part treating upon the subject of wooden roofs, both ancient and modem, and also upon the method of ascertaining the strains which are exerted upon the several timbers of a trussed roof. The second part is devoted to the consideration of modem roofs constructed entirely of iron, a material now generally employed where wide openings have to be covered; and several examples of iron roofs of different design are described and illustrated, the principles of their construction being also explained.

1. The word Roof expresses the covering of a house or other building, by which its inhabitants or contents are protected from the injuries of the weather; it also helps to bind together and give firmness to the walls of the structure. A roof is not only an essential part of a house, but it is often made a characteristic feature of its design; as, for example, the roofs of palatial, public, and private buildings in France, which are denominated the “Mansard,” and more especially the picturesque roofs of the ancient chateaux of France, and the towers and castles to be found in various parts of Germany; in which much attention was often paid to their ornamentation. The timber roofs of many of the old buildings were made much heavier than was necessary for stability, and we consequently find that they have in some cases thrust the walls out of the perpendicular, where these have not been built of sufficient thickness, or strengthened by buttresses to resist their thrust.

In constructing a roof the object of the builder should be, so to arrange its several parts as to obtain a structure as light as possible combined with a maximum degree of strength.

2. In the present Treatise we shall limit the meaning of the word Roof to the framing of timber or iron by which the covering itself is supported, and propose to consider the different modes of construction that have been employed in roofs, according to the character and size of the buildings on which they are placed.

In examining the various examples of roofs which have been erected at different times, we cannot help remarking the great variety of Pitch, or angle of inclination of the rafters to the horizon, which occurs in them. Thus, in the ancient Greek and Roman architecture, as well as in the modem Italian buildings, we find that the pitch was always what is termed low, the angle of inclination being generally less than 30, or the height less than one-third of the span. This low pitch appears to have been maintained in most European buildings until about the twelfth century, when the taste for roofs of a higher pitch seems to have set in. In England the use of high-pitched roofs was chiefly confined to cathedrals, in which the timber framing served merely as a protection to the stone vaulting which formed the actual roof of the edifice; the pitch in these buildings was usually one of 60 degrees, the length of the rafter being equal to the span of the roof, and the height three-fourths of the span. In the ordinary parish churches of this country erected during the Gothic period, we seldom find roofs of so high a pitch as 60 degrees, the angle varying between 45 degrees and 55 degrees in the earliest roofs, and seldom exceeding 45 degrees in those of the fourteenth and fifteenth centuries, while many were of a much lower pitch even than this. The covering of these roofs being almost always of lead, a high pitch was quite unnecessary in order to keep out the weather, and was adopted out of purely aesthetic considerations.

In countries where the rains are very heavy, as in tropics, roofs of high pitch are not desirable, as the water pours off them too rapidly and the gutters are liable to be choked up, thereby causing the interior of the building to be flooded. They are also to be avoided in situations exposed to high winds or hurricanes, as the pressure of the wind on a high roof will often far exceed the normal load laid upon it, and will tend to strip off its covering; also, as the wind presses only on one side of the roof at a time, a racking strain is thereby thrown upon the framing which is extremely dangerous to the security of the structure. In climates where a roof is exposed to heavy falls of snow, a high pitch is generally preferable to a low one, as the snow is thereby prevented from accumulating upon it, but slides off at the eaves ; whereas in a roof of low pitch the weight of the snow may become greater than the framing is able to withstand.

The pitch of a roof should also be regulated by the nature of the material used for its covering, as in cases where tiles or slates are employed a very much higher pitch is necessary than where sheets of lead, zinc, or copper are used. The following Table is given by Tredgold to show the maximum pitch to be given to roofs according to the nature of the material of the covering, and also the pressure which such materials produce on every square foot of roofing:

The force of wind, which is here put down at 40 lbs. on the square foot of roofing, is in reality about the greatest pressure which in this country it ever exerts on a plain surface at right angles to the direction in which it is blowing; and as this direction is usually horizontal, or nearly so, it follows that the pressure which it exerts on a roof of high pitch must be much greater than on one of low pitch. The reader will find this matter discussed in “The Science of Building,” 2nd Edition,* page 188, where it is shown that the pressure on a roof of 60 degrees pitch is three-fourths of that on a vertical plane having the same area, when the wind is blowing horizontally; so that if we take 40 lbs. per foot as the pressure on a vertical surface, that upon a roof of 60 degrees is 30 lbs. per foot; upon one of 45 degrees it is 20 lbs.; while upon a roof of 30 degrees it is only 10 lbs. It will, therefore, be generally sufficient if we include in the pressure of 40 lbs. per foot for the “wind“ the effect of “other occasional forces,” as the load of snow, for example, which in roofs of low pitch may sometimes amount to a very considerable quantity.

3. We have now to consider in detail the various forms of framing which have been employed in the construction of timber roofs ; and these may be divided into two classes, namely, those which consist only of plain sloping beams, called rafters, placed across from wall to wall, on which the covering is laid; and those in which a system of framing called a truss is introduced, upon which the above-named rafters, which receive the covering, are made to rest.