Water Damage Behind Siding in Portland, Oregon
Water behind siding does its damage out of sight. The stain you can see is usually below and to the side of where the water is getting in.
Water damage behind siding is one of the most expensive siding problems because it stays hidden. Siding sheds most of the rain; the house wrap and flashing behind it deal with the rest. When one of those layers is missing or installed backwards, water that gets behind the siding soaks into the sheathing and framing, and in Portland, where November through January each average about five inches of rain or more, it can stay wet for months before anyone sees a stain.
Portland Siding Pros passes these requests to independent local siding contractors, who find the source and quote the repair.
Five ways water gets in
- Roof-to-wall corners without a kickout flashing. The most common single cause. Look below any spot where a lower roof edge ends against a wall.
- Windows and doors. Missing sill pan, side or head flashing, or house wrap tucked behind the flashing instead of over it.
- Torn or missing house wrap. Old building paper deteriorates; wrap gets torn during other work.
- Penetrations. Light fixtures, dryer vents, hose bibs and deck ledgers that were caulked instead of flashed.
- No way out. Without a drainage gap, water that gets behind the siding has nowhere to go but into the wall.
How contractors find the source
Because water runs down and along the sheathing, contractors trace upward from the damage. They look at the roof edge, window head and anything else above the stain, check with a moisture meter, and open the siding at the worst spot. A hose test, wetting one detail at a time while someone watches inside, can confirm a suspected leak.
What the repair involves
Repair means opening the wall past the damaged area, replacing wet or rotted sheathing, rebuilding the house wrap shingle-fashion, installing the missing flashing (kickout, sill pan, head flashing) and reinstalling or replacing the siding. A siding repair covers one location; if the same missing details are on every wall, siding replacement fixes them all at once and adds a drainage gap.
What Oregon’s code expects
Oregon’s residential code requires a water-resistive barrier behind siding, flashing at windows, doors and roof-to-wall intersections, and a kickout flashing where a roof edge meets a wall. By default it also requires a drainage space of at least 1/8 inch behind the siding, with listed exceptions. Many older houses were built before those details were standard, which is why water damage shows up on houses of every age.
Related problems
Water behind siding is what turns into dry rot in sheathing and trim, and it is why T1-11 and hardboard siding fail from their edges. On older houses with original wood, see historic wood siding repair.
What does it cost to fix?
It depends on how much sheathing has to be replaced, which nobody knows until the wall is open. Portland repair figures and material costs are on the siding cost page with their sources.
What you may notice
Streaks, stains or bubbling paint on siding below the corner where a lower roof meets a wall.
Soft or swollen siding and trim under window corners.
Stains, peeling paint or mold on the inside face of an exterior wall, or damp drywall at a window.
A musty smell in a room with no visible leak.
Siding that bulges or feels soft when pressed, with no damage visible on its face.
What causes it
Siding sheds most rain, and the layers behind it deal with the rest. Water gets past them where a roof edge dies into a wall without a kickout flashing, where house wrap is torn or lapped the wrong way, and where windows were set without head or sill flashing.
No kickout flashing where a roof edge ends against a wall, so roof runoff pours down behind the siding.
Windows and doors installed without sill, jamb and head flashing, or with the flashing lapped the wrong way.
Torn, missing or reverse-lapped house wrap or building paper behind the siding.
No drainage gap, so water that does get behind the siding cannot drain out.
Penetrations such as light fixtures, hose bibs, vents and deck ledgers that were never flashed.
How soon it needs attention
Water behind siding wets the sheathing and framing, which do not dry quickly in Portland's wet months. The longer it runs, the more of the wall needs rebuilding, and the more likely mold grows inside the wall. Have the source found before another rainy season if you can.
Services that fix it
Siding Repair
Targeted work on part of a wall: rotted boards and trim cut out, torn or missing flashing fixed, storm and ice damage patched, and matching boards woven in.
Siding Replacement
Tear-off to the sheathing and a complete new wall skin: rot repair, house wrap, flashed windows, a drainage gap where it is required, new trim and new siding.
Historic Wood Siding Repair
Repairing original lap and shingle siding on older Portland-area houses: consolidating and patching sound boards, milling matches for missing ones, and working lead-safe.
Common questions
What is a kickout flashing?
A kickout (or diverter) flashing is a small angled piece of metal at the bottom of a roof-to-wall step flashing, where the roof edge ends against a wall. It throws roof runoff away from the wall and into the gutter. Without it, water runs down behind the siding at that corner. Oregon's residential code requires one at those intersections.
Can water behind siding be fixed without replacing all of it?
Usually. The contractor opens the wall around the leak, repairs damaged sheathing, rebuilds the house wrap and flashing, and puts the siding back or replaces those boards. If the same missing details are found on every wall, replacing the siding lets all of them be corrected at once.
Will house wrap stop all water behind siding?
House wrap is the backup layer; it only works if it is continuous, lapped so water runs out over each lower layer, and tied into flashing at every window, door and roof edge. A drainage gap between the wrap and siding lets water that gets behind the siding drain and the wall dry.
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