Some exterior doors and windows are arranged in such a way that the trim projects horizontally from the wall surface. In these cases, water tends to collect on the top of the wood trim, rotting the trim and leaking behind it.
Ideally, door and window flashing should be provided in these areas. The exposed edge of door and window flashing should be bent out to prevent water from dripping on the surfaces below. The flashing should tuck up under the siding or into a mortar joint in brick construction. (Most windows in masonry houses do not require flashing as the window frames are recessed.)
Door and window flashing is not required where the opening is protected by a roof overhang. As a general rule, if the distance from the window or door to the overhang is less than one-quarter of the overhang width, no flashing is needed.
Door and window sills should be sloped so that water drains away from, rather than toward the door or window. The sill should project far enough out so that water can drop off without wetting the area below. Good design incorporates a drip-stop (groove or projection on the underside of the sill) that prevents the water from being drawn back into the siding by capillary action.
Skylight Flashing
Skylights should be flashed in a similar method to chimneys. Unfortunately, many skylights are not installed at the same time as the roof and consequently, proper flashing details are omitted. Ideally, a skylight should sit on a curb or box which protrudes at least six inches above the roof surface (unless the skylight comes with a pre-manufactured flashing assembly). This allows for the installation of proper flashing and limits snow accumulation on the skylight.
Some skylights are simply a bubble which is intended to have no flashing. The bubble simply slides under the shingle material on the sides and uphill portion of the skylight. On the downhill side, the skylight overlaps the roofing material. The pitch of the roof, and the distance the skylight projects under the shingles, determines the effectiveness of this arrangement. Generally, this type of skylight is prone to leakage. Replacement with a better quality skylight and flashing system is the best solution.
It is not uncommon for condensation to form on the interior of skylights. This is often mistaken for flashing leakage problems.
Solarium Flashing
Leakage from solariums often occurs where the glass roof meets the conventional roof or original wall of the house. It is imperative that this area be properly flashed. Leakage also tends to occur where the glass roof meets the glass wall at the eaves of the solarium. Many solarium designs allow water to collect at a moulding or frame at the lower edge of the windows creating the roof. If these components are wood, they invariably rot.
Regardless of the material used, the puddle of water created in this location inevitably leaks through to the interior. Often, design modifications are required to correct a chronic problem. Caulking will work as a short term solution but rebuilding, in whole or in part, is often the only viable long term repair.
As with skylights, water stains from condensation may be difficult to differentiate from water penetration stains.
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