Moisture behind sidingfinding the water before it finds the frame

Water gets behind siding mostly at openings and intersections: window heads and sills, roof-to-wall joints, porch ledgers and the bottom of the wall. If the barrier and flashing behind are missing or damaged, it soaks the sheathing. Fixing it means finding and flashing the entry point, then repairing what got wet.

Many Boston houses have replacement windows installed without new head flashing or sill pans. Water runs down the face of the siding, gets behind the trim and soaks the wall below. The fix is to open the trim, add the missing flashing and repair the damaged wood.

Porch roofs on triple-deckers send water down walls unless kick-out flashing diverts it into the gutter. Porch ledgers attached without flashing trap water against the house. Both cause long stripes of rot that are often hidden behind siding.

In Boston's long heating season, warm moist air from kitchens, baths and laundry can travel into wall cavities and condense on cold sheathing. Good ventilation, air sealing and, when re-siding, exterior insulation reduce that risk.

Moisture damage in one location is repaired once the source is fixed. Moisture in many locations suggests the barrier is failing across the house, which often makes re-siding the better route.

Triple-deckers have stacked porches front and back, and every porch floor and roof meets the wall at a joint that can leak. Those joints are among the most common moisture sources in the city. Bay windows add many corners and small roofs, each with its own flashing. Flat roofs with parapets on some three-families send water down walls at scuppers and downspouts.

Older single-family houses have their own hot spots: the bottom of walls where soil has built up over the years, window sills where paint has failed, and dormers where roof and wall meet. Rowhouses in Charlestown and South Boston often have narrow back walls with little sun, where moisture lingers.

Original clapboard over board sheathing could dry outward because nothing sealed it in. When asbestos shingles, aluminum or vinyl went over it, sometimes with felt or foil in between, the wall lost some of that ability. Moisture from small leaks or from inside the house now has fewer ways out. When a contractor removes those layers, they often find damp, stained boards in spots that never showed a problem from the outside.

Damp building materials can grow mold within a day or two if conditions are right, and mold inside walls can affect indoor air. The fix is always to stop the water first, then remove or clean affected materials and dry the wall. If you see mold on interior walls, mention it on your call so the contractor checks both sides.

Window replacement without siding removal is common in Boston, and it is a frequent source of hidden leaks. Insert windows sit inside the old frame and rely on the old flashing, if any exists. Full-frame replacements remove the old frame and need new flashing integrated with the barrier, which is hard to do without removing some siding. Ask how the window installer handles flashing, or coordinate window and siding work together.