In any large room which is to be used as a lecture hall the floor should not be perfectly level throughout, but should be so constructed as to be higher at the back end of the room than it is at the front. The fall of such a floor from back to front should be not more than 3/4 of an inch in 1 foot, and a fall of 1/2 an inch in 1 foot is much better. If the floor has a greater slope than this it becomes very noticeable when anyone attempts to walk over it.

Fig. 218. Building an Inclined Floor.

The simplest way to arrange for the slope is to construct what is known as an "inclined" floor, which rises steadily from front to back, so that a line drawn across it from side to side, parallel to the front or rear wall of the room, will be level from end to end. There are two methods of building an inclined floor, the difference between them being in the arrangement of the girders and floor joists. The two methods are shown in Figs. 218 and 219.

Fig. 218 shows the arrangement when it is necessary to have the girders run from the back to the front of the room, parallel to the slope of the floor. In this case the girders A are set up on an incline and the joists B resting on top of them are level from end to end. Each line of joists is at a different elevation from the lines of joists on each side of it. The floor laid on top of the joists will then have the required inclination. The slope of the girders must be the same as the slope required for the finished floor.

Fig. 219 shows the arrangement when it is desired that the girder shall run from side to side of the room, at right angles in the direction of the slope of the floor. The joists A will then be parallel to the direction of the slope, and are inclined to the horizontal, while the girders B are level from end to end. Each line of girders is at a different elevation from every other line of girders, and these elevations must be so adjusted that the joists resting on top of the girders will slope steadily from end to end.

Fig. 219. Building Inclined Floor When Girders Run at Right Angles of Slope.

When a simple inclined floor is employed, the seats must be arranged in straight rows, extending across the room from side to side, so that each line of seats may be level from end to end. This arrangement is not always desirable, however, and it is often much better to have the seats arranged in rings facing the speaker's platform. In this case a bowled floor must be built. A bowled floor is so constructed that an arc, drawn on the floor from a center in the front of the room, on or near the speaker's platform, will be perfectly level throughout its length. This means that the floor must pitch upward in all directions from the speaker's platform, or, in other words, it must be bowled. There are two methods of constructing a floor of this kind. The simplest way is to build first an ordinary inclined floor, which slopes from the front to the back of the room, and then to build up the bowled floor with furring pieces. This method should always be followed when it is necessary to keep the space beneath the lecture hall free from posts or columns. The second method is to arrange girders, as shown in the framing plan of a bowled floor in Fig. 220. These girders A are tangent to concentric circles which have their center at the speaker's platform, and each line of girders is at a different elevation. The elevations of the different lines of girders are so adjusted that the floor joists B which rest on them, will slope steadily upward as they recede from the platform. The girders may be supported on posts beneath the floor of the hall, and if the space under the floor is not to be used for another room, this is a very good method to employ.

Fig. 220. Framing Plan of a Bowled Floor Showing Arrangement of Girders.

Immediately around the platform there will be a space D, the floor of which will be level, and the slope will start several feet away from the platform.

If the floor is framed in this way it means that there will have to be a large number of posts in the space immediately beneath the floor, so many in fact as to make it practically impossible to make use of this space for another purpose. It would be necessary to put a post at each intersection of the girders which are arranged in concentric rings about the speaker's platform, so that the posts in the space below would also appear in rings parallel to each other and only a comparatively small distance apart. It is not possible to do away with absolutely all of these posts except as explained above, by building up on top of a plain inclined floor surface, but it is possible to do away with a large number of them if necessary, as will be explained. In Fig. 221 suppose that the space marked F is the flat space at the front of the room which we wish to floor with a bowled floor. We can place posts around this space under the floor as shown at the points marked A, and some more posts farther back from the front as shown at the points marked B. Between each set of points marked A and B we can run girders, resting at the front end on the post A, and at the other end on the post B. Other girders can be run from the posts A to the wall as, for example, the girder A C; and others again may be run from the points B to the walls as, for example, the girders B D. These girders can all be inclined so as to slope evenly toward the front from all directions, so that points on all the girders at a given distance from the center of the room at the front wall will be at the same level. The framing formed by the girders may now be filled in by joisting E, and the flooring laid on top of the joisting so as to form a solid floor surface on which the seats may be placed. The floor surface thus formed will slope towards a point in the center of the front wall and all the seats will face the platform in concentric rings, each ring being level from end to end. In the space beneath the floor there will be only a comparatively small number of posts, arranged in such a way that the space can be utilized for rooms if desired. All the posts marked B will be in a straight line and can be covered by a partition, so that only the posts marked A will be troublesome, and these are clustered together at the front where they can be easily concealed. The room shown in Fig. 221 has been purposely made somewhat different from the room shown in Fig. 220. In Fig. 221 the room shown is longer than it is wide while in Fig. 220 the room shown is wider than it is long. This gives rise to a slight difference in the appearance of the framing, but the principle is the same in both cases, and the two methods of procedure apply equally well to both rooms.

Fig. 221. Framing Plan for Bowled Floor of Longer Type than Fig. 220.