The lead is arranged as before, about 4 to 6 inches being turned up against the inside of the parapet wall and covered by an apron.
As the parapet is generally of considerable height, an overflow pipe should be inserted, as at OP, so that, in case of the gutter being flooded, the water may escape through the wall and not into the roof.
The end of the lead turned up the roof should be slightly higher than this overflow.
The necessary fall for a V gutter is obtained by lowering the bearer. It will be noticed that, as this brings it farther down into the angle between the slope of the gutter and the parapet wall, it has the effect of narrowing the gutter, which tapers in plan from the highest to the lowest level (see Fig. 456).
In arranging such a gutter, therefore, the points of exit for the water should be as frequent as possible, in order to avoid long lengths of gutter, for these spread out as they rise till they become very wide, and require a large quantity of lead. It will be noticed also, that as each drip raises the level suddenly, it has the effect of widening the gutter at the point where it occurs. An illustration of this is shown at d', Fig. 456.
Fig. 469 shows a gutter formed at the back of a chimney where it cuts through the roof.
Such a gutter must be made of a size proportionate to the area of roof that drains into it.
1 This name is sometimes applied to common arris gutters, formed with two boards fixed together at an angle.
If the chimney be against a wall, the water must, of course, be thrown off to one side only, and the gutter is broader at the end next the wall.
The apron at x in Fig. 469 is shown the full depth of the upturned flashing as an illustration of the remarks at p. 223.
Fig. 477 is a section of a V gutter formed between two roof slopes. The construction is similar to that just described, the fall being obtained by lowering the bearer. In the figure, the bearer is shown at the lowest point, and it will be seen that when it is raised to higher levels, in order to procure the necessary inclination, the gutter will be widened considerably.
It is evident that troughs have great advantages over V gutters, inasmuch as they remain throughout of a constant width, and do not require a large, unsightly, and expensive width of lead. Drips can be formed without widening the gutter, and down pipes are not required to be frequent.