Why is there water behind my siding?

Water behind siding is a sign the layer behind it is failing

Siding is the outer skin, not the waterproofing. Kaycan's installation guide says: "Vinyl siding should be installed over a continuous water resistant barrier to stop the intrusion of incidental water."^[1] That barrier, with the flashing that ties into it, does the real work. So when you find water behind siding, the question is which part of that system has been bypassed.

The usual suspects, roughly in order of how often they cause it:

• Flashing at openings. The guide adds: "Code-compliant flashing should be integrated with the water Resistant barrier and applied around windows, doors, and other openings."^[1] Missing, cracked or badly lapped flashing around a window is the classic leak.
• Corners and roof meetings. It also says: "Flashing should also be applied to inside and outside corners, and the intersection of walls and roofing to prevent water infiltration."^[1] Where a roof lands on a wall, water runs down and in.
• Gutters and drainage. EPA's moisture guidance says: "Moisture controls include site drainage, gutter systems, above- and below-grade drainage planes, effective flashing, condensate drainage and humidity controls."^[2] A clogged gutter that overflows down the wall, or a downspout that empties against it, soaks one spot again and again.
• Things attached to the wall. Hose bibs, dryer and bath vents, light fixtures, and sprinkler heads aimed at the wall all pierce or wet the cladding.

Rain is the main driver. EPA notes that "in most climates, rain is the largest source of water in buildings,"^[2] and a leak that only appears in storms almost always traces to the exterior.

Work out the pattern first

Where and when the water appears tells you most of what you need.

Only in heavy or wind-driven rain points to flashing, a roof meeting or an overflowing gutter. Water after the sprinklers or a shower points to a head spraying the wall or a vent leaking. Water that never stops, or that shows at the bottom of a wall, can be a pipe, a hose bib or a condensation problem inside the wall. A stain directly below a window or a roof edge usually has its cause directly above.

Check from the ground. A good look at gutters during rain, plus a slow walk around every window, door and corner, finds a surprising share of causes. Stay off ladders and roofs; a second-storey wall or a roof meeting is a job for someone with the right equipment.

Dry it and fix the easy causes

Speed matters. EPA says: "It is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth."^[3] Inside, run fans and a dehumidifier and open the area to air. Outside, you are limited to fixing the source, because the wall cavity cannot dry while it keeps getting wet.

You can sensibly handle the simple causes yourself: clean clogged gutters, point a downspout extension away from the wall, redirect a sprinkler head, and replace a failing hose bib washer. Do not seal the siding seams to keep water out. Kaycan's guide says not to caulk overlapping joints between panels or the trims around them.^[1]

Dry does not mean fixed. EPA warns: "If you clean up the mold, but don't fix the water problem, then, most likely, the mold problem will come back."^[3] On size, it says: "If the moldy area is less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself".^[3] Anything bigger, or any mold inside an opened wall, is not a weekend job.

Stop and call a pro if

• The water is coming through at a window, door, corner or roof meeting. That is a flashing repair, which means removing siding and often trim. A siding contractor or carpenter should do it.
• The sheathing or framing behind the siding is soft, dark or crumbling. That is rot, and it needs cutting out and replacing before any siding goes back. How to patch a hole in vinyl siding covers the same rule.
• You can see stains, bubbling paint or damp on the inside wall. The water has already passed the barrier.
• Mold covers more than about 10 square feet or sits inside the wall.
• A pipe is the suspect. A leak from a supply line or drain in the wall is a plumber's job.
• The cause is up high. Anything that needs a ladder past a step, or any work on the roof edge, belongs with a pro.

Getting help in Virginia

Start with a siding contractor for flashing and siding work, and a plumber if a pipe is involved. Who repairs siding, a siding contractor or a handyman explains which to call, and how much siding repair costs in Virginia covers what to expect on a quote. Ask for the cause in writing, the repair method, and whether wet sheathing will be replaced.

Loclis lists licensed Virginia businesses by trade. Browse siding, see more home repair answers, or, when you are ready, describe the leak once and local pros quote it.