Standard Mill Construction

The adaptability of the light forms of steel for framing trusses, especially when wide spans are needed, often compels their use; and in plants having safe occupancy, such as metal workers, they are not objectionable, provided adequate sprinkler protection with good water supply is available to prevent quick failure of the steel work due to heat from combustion of contents or roof. Similar protection is, of course, needed in shops with wooden trusses, if disastrous fires are to be prevented; but ex perience has shown that the steel-trussed roof will fail much quicker than would one of wood under similar conditions. Wooden posts are nearly always available, and should be given preference; but if light steel columns are neces sary, they should be well protected by insulating materials if in rooms containing combustibles, as the column is the vital part of the roof support.

The following suggestions show the best practice in saw-tooth roof construction to over come the difficulties and to make this type of roof a thoroughly satisfactory piece of work. What is good engineering from the view-point of the manufacturer can also be good fire pro tection engineering. Any design should be adapted to both if the best interests of the manu facturer are to be served.

It being desirable to avoid direct sunlight and at the same time obtain abundance of light perfectly diffused, the saw-teeth should face approximately north; and the glass should be inclined to the vertical, to take advantage of the brighter light in the upper sky, and to prevent cutting of the light by the saw-tooth imme diately in front. This also assures the diffusion of the light upon the floor rather than on the under side of the roof planking.

For the glass, an angle of 20 degrees to 25 degrees with the vertical, and an angle of ap proximately 90 degrees at the top of the saw tooth, will be about right, the variations to de pend on the amount of light required, and on the latitude. A sharper angle at the top is not needed, as it increases the cost, there being more roof to cover and larger spans. More glass is also required in proportion, and the light is not so good, more sky light being lost and too much thrown on under side of roof.

Double glazing, with space between, is pre ferred on account of its conducting qualities; but is not always necessary, except in the North.

Fig. 160. Saw-Tooth Roof Framing—Detail of Valleys.

The inside glazing should be factory ribbed glass, with ribs vertical and inside. Shadows cast by trusses are then almost unnoticeable.

Condensation gutters, as shown in the detail drawing, Fig. 160, are needed inside at the bot tom of the sash; and they should be drained through inside conductors, and not outside, under bottom of sash, as these latter admit cold air and are liable to freeze.

Valleys between the saw-teeth should be flat, 14 inches to 2 feet in width; and should pitch one-half inch per foot toward the conductors, which should be of ample size, and not much over 50 feet apart, preferably less. The neces sary pitch may be obtained by cross-pieces of varying heights on top of the trusses, thus avoid ing hollow spaces. Leaks, a common fault, may ordinarily be prevented by careful design of gutters, valleys, and sashes, and by insisting on good workmanship and materials. The roof

covering of asphalt or pitch should be contin uous through the valleys, and extend up to the glass.

Experience has demonstrated the advantage of a combination of direct radiation, with a fan sufficient only for ventilation and tempering the room. Heating pipes should usually be placed overhead, and directly under the front of the saw-teeth, and should run the entire length, and in this position assist in preventing condensation.

Where there is no moving shafting, some forced circulation is necessary. This is best ob tained by a fan, often driving air from a dry basement or outside as required, and discharg Mg it over heating coils to the floor above. In weave and similar rooms, is this especially necessary and advantageous in promoting health and comfort of employees, making greater efficiency possible.

Ventilation and cooling of these large areas with comparatively low stories must not be neglected. Ample vents are needed at top, in shape of large metal ventilators with double walls and tight dampers. They are recom mended instead of pivoted or swinging sash, which are apt to leak in driving storms, and which, when open, allow dirt to blow in off the roof. Good windows are advised in side walls, and experience has shown their value.

Framing of the saw-teeth may be in timber, steel, or reinforced concrete. The design should be such as to obstruct the light as little as pos sible; strong enough to hold wet snow without sagging; and stiff enough to carry shafting motors, etc., when they are to he overhead. When wood or steel is used, the roof planking should be 3 inches or over, spanning wide bays of 8 to 10 feet.

Hollow spaces in roofs should not be per mitted. They are very undesirable from a fire standpoint, and any condensation which may take place in them during cold weather soon rots both plank and sheathing.

Sheathing, even without spaces behind it, is more or less a bad feature, as it is readily com bustible; but, if used, should be applied directly to the under side of the roof plank, with only a layer of some insulating material between, so that there may be no concealed space. If 3-inch plank is sufficient for a flat roof, it should be for a saw-tooth; and, with good circulation of air, there should be no trouble, except in wet rooms, where condensation is bound to occur whether under a roof or the floor of the room above, un less large quantities of dry air are discharged into the room.

Saw-tooth roofs necessarily cost more, as there is practically the same amount of roofing as in flat roofs, and in addition there is the cost of windows, glazing, flashing, conductors, con densation gutters for the skylights, and a some what larger cost of heating. The addi tional cost of these items does not, however, fairly represent comparative cost, as there should be considered the total cost of the build ing compared with that of an ordinary one of sufficiently high stories and narrow enough to give the required light. When this is done, the slight additional cost is far outweighed by ad vantages of the saw-tooth type for work where good light is desirable.