This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The destructive effect of water in causing rot of woodwork is well known, and precautions must be taken, in the construction of exposed surfaces, to lessen this result as much as possible. A few of these points are noted below.
The decay of veranda posts resting on the floor is due to capillary action; every time there is a shower or the floor is washed, some water finds its way under the post and is absorbed. This can be avoided by the interposition of an iron shoe, to keep the post from contact with the floor.
Water running down the faces of projections, as window sills, lintels, etc., at first drops off at the lower angle, but gradually forms a film across the projecting under surfaces; the drops are thus attracted into the stone or brick wall or the woodwork, as the case may be, causing disintegration of the mortar or decay of the wood. A simple preventive for this action is to groove or throat the under side, near the edge, causing the water to drip from the line of the groove. The necessity for this exists where there are horizontal projections exposing an under surface, such as water-tables, sill and lintel courses, copings, cap or drip members, and molded bands or cornices, whether of wood or stone.
The lower sash of windows, unless properly constructed and kept well painted, suffers through capillary action. If a film of water is allowed to form between the sills of the window frame and sash, it readily follows the vertical grain of the stiles; while the wind forces in rain until the inner sill is also wet. This may be prevented by forming grooves on the sill of the window frame and on the lower edge of the sash, as shown in Fig. 14.
Water absorbed between slates or shingles rusts the nails securing them, and also rots shingles. For this reason, slates too fine in texture are not as good as rougher ones, as the closer the contact of the surfaces, the better does capillarity act. Sawed shingles are not as good as split ones, the woolly surfaces retaining moisture, and being more favorable to its ascent. Shingled roofs should neither be close-boarded nor have the shingles underlaid with paper, in order that the air may circulate and dry them more readily, shingles should be well dipped in creosote before being laid, and afterwards thoroughly coated with it, to render them as impermeable as possible. Metal-roof flashings and valleys present surfaces between which water will creep up, and, unless they are made of sufficient width or height, the roof will not be watertight.