Rotten siding at the roofline: it's usually a roof problem

The joint that fails first

Every place a roof runs into a wall is a channel. Water coming down the roof hits the wall and has to go somewhere. On a house built right, a piece of metal called step flashing sits under each course of shingles and up behind the siding, so the water runs down the metal and out onto the roof. At the bottom of the wall a small piece called a kickout diverts it into the gutter instead of behind the siding.

When the flashing is missing, rusted through, bent, or buried behind a repair somebody did without pulling the siding, the water goes behind the wall instead. The siding looks fine for a while. Then the paint lifts, the bottom boards go soft, and by the time it shows, the sheathing behind it is often wet too.

How to tell it's roof water

• Where the damage is. Peeling, staining or soft boards concentrated near the roofline, or in a line running down from where a roof meets a wall, point to the roof. Weathering from rain and sun is spread across the whole wall.
• A stain inside upstairs. A water stain on an upstairs ceiling or wall near an outside corner often lines up with a bad wall-to-roof joint outside.
• Black streaks below the joint. Water finding its way out at the bottom of the wall leaves a trail.
• No kickout. Look where the roof edge meets the wall above the gutter. If the water has nowhere to go but down the wall, that's the problem.

Chimney chases: the same problem, four sides

A chimney chase is a wood box wrapped in siding, and it meets the roof on every side. Its flashing has to be right all the way around, and the siding on it takes weather from every direction. It's one of the first places we see rot. The job below is one of ours: the chase was stripped back to the framing, wrapped, re-flashed to the roof, and re-sided.

The before and after photos are a Silver Ridge job.

Fixing it right

Pull the damaged siding. Check the sheathing behind it, and replace what's soft. Install new step flashing under the shingles and up the wall, with a kickout at the bottom. Then the house wrap, then the new siding, cut to leave a gap above the shingles so water and debris don't sit against the bottom edge. Paint or caulk on top of a bad joint doesn't fix it, and new siding nailed over old flashing rots on the same schedule the old siding did.

This is why the same crew should do the roof side and the wall side. If the roofer says the flashing is fine and the siding contractor says the wall is fine, the water keeps going where it's been going.

Small now, bigger later

A failed joint that's caught early is a few feet of flashing and a few boards. Left through two or three winters, it's sheathing, sometimes framing, sometimes the drywall and insulation inside. If you see the signs above, the useful next step is to have someone look at the flashing from the roof, not just at the siding from the ground. Here's how we handle siding, and what a roof repair involves.