T1-11 Siding Replacement in Portland: Panels & Sheathing
Grooved plywood siding fails from its edges, the bottom edge first. Whether you can swap a few panels or need new siding depends on how far it has gone and what is behind it.
T1-11 siding replacement in Portland, Oregon usually starts with the bottom edge. T1-11 is plywood with vertical grooves cut into the face, sold in 4-by-8 sheets, and it takes on water where its edges are exposed: the cut bottom edge near the ground, the horizontal joint where one panel sits on another, and the corners of openings. Once water gets into the plywood layers, the outer veneer peels away and the panel softens from the edge inward.
Portland Siding Pros passes T1-11 repair and replacement requests to independent local siding contractors, who inspect and quote directly.
Why does T1-11 fail from the bottom?
The face of a T1-11 panel is protected by paint or stain; its edges are raw plywood end grain. At the bottom of a wall, that edge is the first thing splash and standing water reach. Where two panels meet horizontally, water runs down the face and behind the upper panel unless a Z-flashing sends it out. Around windows, water collects at the corners. Keep those three places in mind when you look at your own wall.
Is your T1-11 structural?
This is the first question for any T1-11 job. On many houses, garages and additions, T1-11 was fastened directly to the studs and acts as both the siding and the wall’s structural sheathing. That changes the repair:
- If it is structural, replacing it with lap siding means new sheathing goes on first, which adds cost and may bring in a building permit. Ask the contractor how they will handle it and whether a permit is needed.
- If there is sheathing behind it, the old panels can come off and any siding can go on over new house wrap.
Repair a few panels, or re-side?
Damage confined to the bottom row or a couple of seams can be repaired: cut out the bad sections, add Z-flashing, install new panels with sealed edges and clearance above grade, and paint. When panels are peeling across several walls, or the soft edges have spread upward, siding replacement usually costs less over time than repeated patching.
Options once the T1-11 comes off
- New T1-11 or engineered wood panel keeps the same look.
- Fiber cement panel with battens gives a board-and-batten look that will not rot.
- Lap siding in fiber cement, vinyl or engineered wood changes the look of the house entirely.
Whatever goes on, the wall should get continuous house wrap, flashed openings and a drainage gap where Oregon’s code requires one.
Is it T1-11, or something else?
Swollen lap boards with mushroomed lower edges are more likely hardboard siding; soft solid-wood boards and trim point to dry rot. Grooved vertical panels that peel in layers are T1-11.
Pricing a T1-11 job
Angi’s T1-11 guide puts installed T1-11 panel siding at a few dollars per square foot; replacement with fiber cement or lap siding costs more. The siding cost page has every figure with its source, and the quote should break out any sheathing added where the old panels were structural.
What you may notice
Outer plywood layers peeling or splitting away along the bottom edge of the panels.
Swollen, soft or crumbling panel bottoms near the ground, a deck or a roof line.
Dark staining or rot along horizontal seams where one panel sits above another.
Soft spots at the corners of windows and doors.
Paint or stain that will not stay on the lower part of the wall.
What causes it
T1-11 is plywood with vertical grooves, and it lets water in through its edges: the bottom cut edge near grade, horizontal joints that lack Z-flashing, and panel corners at windows. The veneers then separate and the panel softens from the edge inward.
Unsealed bottom edges close to soil, patios or roofing, which wick water into the plywood layers.
Horizontal joints with no Z-flashing, letting water run behind the upper panel.
Window and door openings flashed poorly or not at all.
A finish left to wear through, so the face veneer takes on water.
How soon it needs attention
T1-11 softens from the edges inward, so damage is usually worse than it looks from the face. Where the panels also serve as the wall's sheathing, softening at the bottom weakens more than the siding. Get it assessed before the damaged edges spread along the wall.
Services that fix it
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.
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.
Fiber Cement (Hardie) Siding
James Hardie and other fiber cement lap, panel and shingle siding, primed or factory-finished, installed to the maker's clearance and fastening rules.
Common questions
How can I tell if T1-11 is holding up the wall?
On many houses and additions, T1-11 was nailed straight to the studs and serves as both siding and structural sheathing. From outside, look at a panel edge or an opening; from inside, an unfinished garage or attic wall may show the back of the panels on the studs with no separate sheathing. A contractor can confirm by removing a trim piece.
Is it possible to swap only the damaged panels?
Yes, if the damage is limited to a few panels, usually the bottom row. New panels should get Z-flashing at horizontal joints, sealed edges and proper clearance above grade. The patched area will be visible until painted, and T1-11 patterns vary, so matching the groove spacing matters.
Which sidings are used in place of T1-11?
New T1-11 or engineered wood panel for a matching look, fiber cement panel with or without battens, or lap siding in fiber cement, vinyl or engineered wood. If the T1-11 was structural, sheathing is added before a non-structural siding goes on.
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