T1-11 nearly always fails at its bottom edge and horizontal joints, where rain soaks into raw plywood. Rot in the lowest few inches can be cut off, Z-flashed and replaced with a strip; higher rot needs a new sheet. Expect roughly $900-$6,500 including materials (Portland-area market data, November 2026) . Soft studs need siding and rot repair from a pro.
I'm Slava, with 7 years spent working on houses with my own hands. What follows is how to read T1-11 damage before anyone picks up a saw.
Why does T1-11 siding rot at the bottom first?
T1-11 is panel siding sold in big sheets, usually 4 by 8 feet, with vertical grooves milled into the face. Most is plywood; some panels sold under the name are OSB or other engineered wood, which swells faster when wet. The grooves are the thin spots in every sheet.
The bottom edge is end grain: the exposed layers of the plywood. It wicks water like a paper towel dipped in a puddle, and an edge left unpainted when the sheet was hung has nothing to slow that.
What keeps it wet is near the ground: soil, bark mulch or a patio against the siding, rain bouncing off concrete, a downspout emptying beside the wall, a sprinkler hitting the house. On west-side lots, walls sitting under tall Douglas firs get little sun from fall into spring and stay damp for days after each rain. When water works between the plies, the face veneer peels off in layers. That's delamination: the damage is inside the panel, not just in the paint.
How can you tell whether T1-11 can be repaired or needs a new sheet?
Work up from the ground, pressing an awl into the panel every couple of inches until it stops sinking in, and pencil a mark there. Repeat on both sides of each vertical seam, because water runs down seams and they often go soft higher than the open face. Also look for lifting veneer, a bulge where stacked sheets meet, rust streaks under nail heads and soft spots below window corners.
Over a garage or crawl space, check from inside too: dark stains or punky wood on the studs or sill plate change the job.
| What you find | Usual fix |
|---|---|
| Soft wood only in the bottom 2-6 inches | Cut off a strip, add Z-flashing, fit a new piece or a band board |
| Soft wood running up a seam or under a window | Replace the full sheet |
| Face veneer peeling, core still hard | Scrape, prime and paint; a new sheet if it's peeling across the panel |
| Studs, sheathing or sill plate soft behind the panel | Framing repair first, then the siding |
Those heights are rough guides, not hard limits .
Can you cut off just the rotted bottom of T1-11?
You can, provided the rot stays low. Done right, this standard patch also removes the cause, because the new bottom edge sits higher and drier. In order:
- Snap a level line a little above the highest soft spot.
- Set the saw blade to the thickness of the siding, so it cuts the panel and leaves the building paper and studs behind it alone.
- Pull off the cut strip and the old nails.
- Look at the paper or house wrap. Patch any tears, lapping the patch so water runs outward.
- Prime the fresh cut on the upper panel. It's raw end grain now, the same weak spot as before.
- Slide the Z-flashing up behind the upper panel, so its lower leg hangs over the face of the new piece.
- Prime the new piece on both faces and every edge, then fasten it into the studs.
- Leave a small gap at the flashing so water can drain, then paint.
The new piece can be a strip of matching panel or a flat trim board, often called a water table or belly band. A strip leaves a seam across the wall. A band board carried around the whole house makes that line look planned .
Fasteners should be made for outdoor use, hot-dipped galvanized or stainless, so they don't rust and streak the paint . Keep the new bottom edge clear of soil, mulch and patio; that gap is the point.
When does a whole T1-11 sheet need replacing, and is T1-11 structural?
A strip won't do when rot climbs a vertical seam, sits at the joint where one sheet stacks on another, wraps around a window or door, or when the face has peeled across most of the panel. Then the whole sheet comes off, and the new one has to match the old thickness and groove spacing, gets primer on both faces and all four edges before it goes up, and any horizontal joint gets Z-flashing .
On many houses T1-11 is nailed straight to the studs with only paper behind it. There the panel is more than a skin: it's also what keeps the wall from racking sideways. So a careful repair removes one sheet at a time and nails the new one back the same way.
Soft studs, sheathing or a soft sill plate turn it into framing work. The state Building Codes Division generally exempts siding repair and repairs with matching materials from permits, but not cutting or removing a load-bearing support, so once rot reaches the frame the city or county may want one. You'll hear that from us before the wall is opened further .
Should you caulk T1-11 seams and the bottom edge?
Not where water has to get out. The joint where the upper panel sits over the Z-flashing should stay open, and the bottom edge shouldn't be sealed down to a slab or to trim below it. Water behind the siding drains through those gaps; caulk there traps it against the wood. Paint those edges instead.
The vertical seams between sheets are lapped, a shiplap joint, and normally aren't caulked either. Joints where panels butt into window and door trim usually are . Caulk is upkeep; it won't rescue a panel that's already soft.
Is T1-11 worth repairing, or should you switch to another siding?
Fixing it is sensible when the damage sits in one or two spots and the rest of the wall is sound. Switching gets worth a look once several walls are peeling apart, or OSB-type panels have swelled at every edge. The usual alternatives are vertical fiber-cement panels grooved to copy the T1-11 look, engineered-wood panels, or lap siding over new sheathing.
Covering old T1-11 with new siding is a full re-side decision, with flashing to work out at every window, door and corner. How long T1-11 lasts depends more on paint, clearance and gutters than on the panel . We're set up for repairs: a strip, a few sheets, a corner, one section of wall .
What does T1-11 siding repair cost around Portland?
The figures below are market ranges for the Portland area, gathered in November 2026; Slava hasn't confirmed them as our prices.
| Work (price row) | Basis | Low end | High end | Typical job | |
|---|---|---|---|---|---|
| T1-11 strip or panel repair (row: siding repair) | with materials | $900 | $6,500 | $1,800 | |
| Rotted trim or band board (row: wood rot repair, trim) | with materials | $500 | $4,000 | $1,500 | |
| Sealing trim joints (row: exterior caulking and sealing) | labour only | $300 | $1,500 | $700 | |
| Painting the repair (row: exterior trim and fascia painting) | labour only | $400 | $2,000 | $900 |
Where a job lands depends on feet of wall, strip versus full sheets, flashing and trim, damage behind the panel, and second-storey height. A fresh coat on a patch rarely matches sun-faded siding, so painting often runs to the nearest corner; a full repaint is a separate job under exterior painting. For the breakdown across all kinds of siding, see what siding repair costs.
Do lead paint rules apply to T1-11 on an older house?
They can. T1-11 went up on houses before 1978 as well as after, and Oregon guidance presumes any Oregon home built before that year carries lead paint until testing proves it doesn't. A one-foot strip off a 30-foot wall is already 30 square feet.
We don't disturb lead paint, so on older houses we test first or pass the job on . Plenty of Cedar Mill houses and Beaverton's older streets predate that cutoff; most of Bethany came after it. The details are in Oregon's lead paint rules.
How do you keep repaired T1-11 from rotting again?
Most of it is about water near the ground:
- Keep soil, bark mulch and patio surfaces well below the bottom edge. Siding trades often suggest 6 to 8 inches; that's common practice, not a code number.
- Aim sprinklers away from the wall.
- Make sure downspouts carry water out into the yard rather than dropping it beside the siding.
- Keep gutters clear so they don't spill down the wall. Under Cedar Mill's firs, needles fill them fast, and gutter cleaning is a much smaller job than a new sheet.
- Repaint before the bottom edge goes bare.
Spring, when winter's damage shows, is a good time to walk the house with an awl .