Expansion joints are of no use when a gutter is built as in Figs. 53 and 54, where it is necessary to use a reglet to prevent leakage. Where this can be done away with, and the gutter-lining can be locked to the roof and cornice, an expansion joint can be used with some degree of success.

SCUPPERS

   One of the most important points of roof drainage design is often forgotten. This is proper provision for overflow by means of scuppers.

   A great many buildings have flat roofs enclosed by parapet-walls and inside drainage-systems. When the outlet becomes clogged water collects on the roof and not only causes an overload, but also works its way over the flashing and down into the building. The appearance of leaks of this character is often the first warning of the dangerous condition of the roof.

   On all roofs of this character — that is, where there is not ample provision for escape of the water when the leaders do not work, it is absolutely necessary to provide scuppers through the wall. These should be large enough to preclude any possibility of clogging (at least 4” x 12”), and should be unobstructed in any way by screens, etc. For a further description, see Figs. 36 and 58.

   When copper flashings are used the scuppers are completely lined with copper and this lining is soldered to the base flashing, the counter-flashing being worked around the hole, or omitted at this point.

   Scuppers must also be used to drain all balconies or similar small areas enclosed by a balustrade or wall.

EROSION

   In some localities it is general practice to omit gutters from small flat roof dormers, or similar construction, and allow the water from the dormer to fall to the roof below. When the roof surface receiving the falling water is flat, or nearly so, the force of the descending water is often great enough to drive it under the roof covering. A special flashing is sometimes used to overcome this difficulty. Such practice is not good as the wearing effect of the water erodes the metal. When this is galvanized iron or tin corrosion destroys the iron base as soon as the thin protective coat of zinc or tin is worn away. Sixteen-ounce copper subjected to this erosion has withstood it for sixteen years before failure. Gutters and leaders should be used whenever water from one roof discharges on another roof below.

COPPER AND OTHER METALS

   Dissimilar metals, when in contact in the presence of an electrolyte, set up galvanic action which results in the deterioration of the most electro-positive metal. Starting with the most electro-positive the commercial metals are listed as follows:

1. Aluminum (most Electro-positive)

2. Zinc

3. Steel

4. Iron

5. Nickel

6. Tin

7. Lead

8. Copper (most Electro-negative)

   This means that when iron (or steel) and copper are in contact with an electrolyte present (which may be water) the iron is corroded. When copper and lead or tin are in contact a tendency for similar action exists but it is very slight and produces practically no injurious results. This is because the difference in potential is much less and the corrosive action is negligible, especially where water is the electrolyte.

   Any possibility of galvanic action between copper and iron or steel should be carefully avoided by proper insulation. This insulation is effected in various ways, three of which are, (1) covering the steel member with asbestos, as is frequently done in skylight construction; (2) placing strips of sheet lead between the two metals, as when new copper gutters are placed in old iron hangers; and, (3) heavily tinning the iron, as is often done with iron or steel gutter and leader supports.

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