Causes of Leakage
Occidentally metal-wall flashing may warp and be drawn out of the groove or joint in the vertical surface or, when roll roofing is used, it may break at its junction with the vertical surface, allowing water to run down behind it.
The force of the wind may tear flashing loose from the face of the roofing allowing water to enter under it during heavy rains. Valley flashings, if too narrow, may allow water that is backing up to find its way under the roofing which can then leak and cause damage to carpeting, bathroom vanities, bathroom fixtures, hot tubs, shower doors, and flooring.
This seeping sometimes occurs when the valleys are dammed up with snow and ice. Flashing material in valleys may corrode or break, causing cracks or holes, through which water may enter and drip through the joint below.
How to Make Repairs
The approximate location of leaks in flashing may often be determined by wet spots on the walls or ceiling of the house. Carefully examine the flashing above and near such spots to ascertain the exact location and cause of leakage if possible.
If the leak is near a chimney or below the junction between the roof and a vertical wall or similar surface, see whether the flashing has become loose at any place. It may be that the mortar has fallen out and needs repointing after the flashing has been replaced. Elastic roofing cement and similar compounds are useful and effective in sealing cracks around flashing.
If the flashing that extends over the top of roofing, as at the base of a vertical surface, becomes loosened, it should be nailed down after the underside of the lap has been well coated with elastic cement. The elastic cement serves to seal the spaces around nails and the cracks along the edges, which otherwise might allow leakage. It is also best to use short nails for this purpose in order to avoid penetrating through the roof boards.
When exposed metal flashing signs signs of rusting, it should be painted with a good metal paint to preserve it from further corrosion. This paint coat should be examined at regular intervals and renewed when it shows signs of wear. Often indoor bathroom features can cause excess steam and moisture to affect flashing, so in areas where you have a hot tub, steam shower, or sauna be especially wary.
To make valley flashing watertight, it is advisable to cover with elastic cement that portion which is to be overlapped, immediately before applying the roof covering. This should seal the space between the two and prevent water from backing up over the edge of the flashing. If the flashing in a valley is too narrow, or if it is corroded or broken, it will probably be necessary to replace it with new pieces of metal. This is not difficult in an open valley, but is rather troublesome in a closed one.
In a closed valley covered with shingles, it is quite difficult to repair leaks in the flashing by pushing pieces of metal up under the shingles to cover the leaky spots, unless the metal is folded into a wedge-shaped point. Folding makes the task of pushing the sheets past such obstacles as nails easier.
If nails interfere too much, they can be cut off under the shingle with a plumber's chisel or flat, sharp cold chisel and later replaced with new nails. The size of the sheets to be used for flashing depends on the pitch of the roof and the exposure of shingles to the weather. Insert a piece of flashing under top layer of first course of shingles at the eaves and over the top of the old flashing and slide it up until the upper point of the sheet is at least 2 inches above the butts of the second course of shingles.
Then insert another sheet under the second course of shingles pushing it up on top of the old flashing. The lower point of this piece will show below the butts of the second course of shingles. Continue this process until you reach the top of the valley or until you are satisfied that all broken flashings have been covered. This includes flashings that may have been affected by steam or moisture from a swim spa, hot tub , or bathtub.
On a well-nailed wooden-shingle roof, the sheets of flashing may not require nailing, as the pressure of the roofing material should hold them in place, but if the shingleles are loose, or if the roofing is of flexible shingles, it will probably be necessary to nail each sheet at a point which will be covered by the sheet above it.
If a larger sheet is used more of the flashing will be exposed at the base of the shingle courses and the nails in the lower flashing will be completely covered. If a good material is used the method just described makes a permanent repair and covers the cracks or holes rusted out in the angle of the original flashing.