Wall Sheathing Rot and Moisture Damage Repair

Sheathing rot repair for Calgary homes. When old siding, stucco or capping comes off over soft OSB or plywood, we cut back to sound wood, replace sheathing, damaged studs and wet insulation, and fix the flashing or clearance fault that let the water in.

(587) 324-8484 How the job runs

An exterior wall with siding removed, showing dark, water-stained and crumbling oriented strand board beneath a window.

The Problem It Solves

Sheathing rot is usually found, not reported. The siding looks tired but whole, then a section comes off during a repair or re-side and the OSB behind it breaks apart in the hand, dark and soft, with nails pulling straight out. Siding hides it well: vinyl looks dry on its face, aluminum capping hides the wood it covers, and stucco can hold water against the wall while its surface looks sound. “Dry rot” is the everyday name, but the wood rotted because it stayed wet.

The water almost always comes from a detail rather than from rain landing on the open wall. A roof that meets a wall without a kickout flashing sends water behind the siding at the bottom of the roof line. A window with no head flashing, or failed sealant at the sill, feeds the wall below it. Stone veneer buried in landscaping, or siding run down to a deck or grade, wicks water into the sheathing. Each source leaves its own pattern: a triangle under a roof junction, a stripe below a window, a band along the base of the wall.

Left alone, rot moves past the sheathing. Studs soften at the bottom plate and under windows, insulation gets wet and stops insulating, and mould grows in the cavity. Sheathing also stiffens the wall, and new siding can’t be fastened properly to board that won’t hold a nail. LP’s installation instructions say plainly not to apply siding over rain-soaked or buckled sheathing, and the principle holds for every product: new cladding over a rotten wall buys a few years of looks and hides the problem again.

How It Works

The repair is a removal and a rebuild. We open the wall wider than the visible damage, probe sheathing and framing until we reach sound, dry wood, and cut back to framing lines so new panels land on solid support. Damaged studs are reinforced or replaced, wet insulation comes out, and cavities dry before they are closed. New sheathing goes on with gaps between panels; the City of Calgary’s climate-ready guidance says sheathing shouldn’t be tightly sealed, because those gaps let trapped moisture dry out. The same guidance says plywood has better water resistance and drying ability than OSB, which makes it worth considering on a wall that has already rotted once.

Then the water source gets fixed, because that is the real repair. Kickout flashings go in where roofs meet walls. Window heads get flashing that runs up behind the membrane and slopes out with a drip. Sheathing membrane is replaced and lapped so upper sheets shed over lower ones, and sealant goes back between siding and frames. The CCMC evaluation for James Hardie cladding puts moisture risk in the wall largely down to workmanship on the membrane, flashing, caulking and attachment, and calls a high level of quality control at every stage imperative.

How the Job Runs, Step by Step

Open and map the damage

We remove siding beyond the visible damage in every direction and mark the extent of soft sheathing and framing on the wall. It is worth asking for photos at each stage, so you can see what was found before anything is covered again.

Trace the water path

Rot follows water, so we follow the staining upward and outward to its source: a roof-wall junction, a window head or sill, a penetration, a buried base. If the source isn’t obvious, we investigate further rather than guess and close the wall.

Cut out and dry

Rotted sheathing is cut back to framing lines, wet insulation is removed and studs are assessed. Where framing damage goes beyond a local repair, we stop, explain what we found, and confirm the approach with the municipality before continuing.

Rebuild the substrate

Studs are repaired or replaced, cavities are re-insulated once dry, and new sheathing is fastened with gaps between panels. Where the interior vapour barrier is exposed, it is kept continuous on the warm side of the wall.

Membrane, flashing, cladding

Sheathing membrane is lapped at least 100 mm with upper sheets over lower, flashing is installed at the leak source, and cladding is reinstalled or replaced to the manufacturer’s instructions with proper clearance to grade and roof.

What Decides the Specification

How much comes out is decided by the wood, not by the estimate. Sheathing that is stained but firm and dry can stay; sheathing that is soft, delaminated or won’t hold a fastener comes out. Older homes often have board sheathing rather than panels, which changes how a patch ties into what remains. Thickness follows Article 9.23.17.2. of the NBC(AE) and has to suit the cladding going back on: James Hardie’s Canadian instructions, for example, allow HardiePlank to be fastened directly to OSB of at least 11.1 mm (7/16 in). The City of Calgary expects replaced components to perform as well as, or better than, what they replace.

The second driver is what goes over the new sheathing. Rigid foam isn’t a substitute; the City’s guidance says rigid insulation shouldn’t be used alone as wall sheathing because it doesn’t protect against high winds. Whether to add a rain screen is decided here too. The code doesn’t require one on a Calgary house, but on a wall that has already rotted, a drained gap behind the cladding is often worth it.

Alberta Code Notes

Alberta’s Permit Regulation exempts re-siding with a similar type of siding only where there is no structural change and health or safety isn’t at risk. Replacing sheathing and studs can count as a structural change, so a rot repair can take a job outside that exemption. The authority having jurisdiction decides, whether that is the City of Calgary, Airdrie, Okotoks, Cochrane or Chestermere, and we confirm before the wall is closed. The rebuilt wall still needs two planes of protection under the NBC(AE): the cladding, and a second plane that intercepts water getting past it and dissipates it to the exterior, with flashing at horizontal junctions and substrate changes.

Two code clearances bear directly on rot. Cladding adversely affected by moisture, such as untreated wood, plywood or OSB, needs 200 mm between it and finished ground, and 50 mm above a roof surface. On houses whose building application was submitted on or after November 1, 2016, the City of Calgary requires renovations to comply with the energy-efficiency requirements of Section 9.36, which shapes the insulation that goes back into opened cavities.

General guidance only. The authority having jurisdiction and the product’s own evaluation report decide what applies to your job.

Around Calgary

Rot turns up in every quadrant, but the causes differ by era. Post-war homes in communities such as Killarney/Glengarry often carry original sheathing behind stucco or later over-cladding, and every layer added is another place to trap water. 1960s and 1970s homes in areas like Lake Bonavista may have aluminum capping over wood fascia and window frames, which can hide rot until the wood is gone. Newer 2000s homes in Evergreen and Bridlewood aren’t exempt: missing kickout flashings and stone veneer bases set too close to landscaping are common sources of wall rot on houses of that age.

Common Mistakes to Avoid

New siding over soft sheathing: Cladding can’t hold on board that won’t grip a fastener, and the wet wood keeps decaying behind it. LP’s instructions rule out siding over rain-soaked or buckled sheathing for good reason.
Fixing the board, not the leak: New sheathing under the same missing kickout flashing will rot on the same schedule. The water path is traced and closed before the wall is rebuilt.
Sealing sheathing joints tight: Closing every gap between sheathing panels removes the space that lets trapped moisture dry out, which the City of Calgary’s guidance specifically warns against.
Capping over the damage: Wrapping rotted fascia or window frames in aluminum hides the rot and keeps it wet. Capping is a cover, not a flashing, so the wood is repaired first.

When to Choose Something Else

If the stain is on interior drywall and the exterior sheathing proves sound, the water may be coming from plumbing, the roof or condensation inside the house, and a plumber, roofer or building envelope consultant is the better first call than a siding crew. If rot runs through studs, plates and floor framing, or the wall has moved out of plumb, get a structural assessment from a qualified professional before anyone rebuilds it. And if a moisture reading shows damp sheathing with no decay, fixing the leak and letting the wall dry may be all it needs.

Often Done Alongside

Weather-Resistive Barrier (WRB) & Housewrap

Tyvek, building paper or self-adhered membrane, lapped, taped and flashed.

Rain Screen & Cavity Ventilation Systems

Furring or drainage mat leaving a drained, vented gap behind the cladding.

Hardie Board & Panel Sectional Repair

Cracked or failed Hardie boards, panels and trim replaced, and the cause fixed.

Where We Do This Work

Sheathing & Rot: Questions We Get

How can I tell if sheathing is rotten without removing siding?

Often you can’t be sure until a section comes off. Warning signs include siding that bows or feels soft when pressed, fasteners backing out, staining or peeling paint below windows and roof junctions, a musty smell or damp drywall on an exterior wall, and swollen trim at the base of the wall. We remove a small test section at the most likely spot, which tells you far more than looking from the lawn.

Does sheathing rot repair need a permit?

It can. Alberta’s Permit Regulation exempts re-siding with similar siding only where there is no structural change, and replacing sheathing or studs may count as structural work. The City of Calgary, Airdrie, Okotoks, Cochrane and Chestermere each decide within their own boundaries. We check with the municipality once the extent of the damage is known, and before the wall is closed, so any inspection happens while the repair is still visible.

Is plywood better than OSB for the replacement?

The City of Calgary’s climate-ready guidance says plywood has improved water resistance and drying ability compared with OSB, so on a wall that has already rotted it is worth considering. Both are used as wall sheathing, and either will rot again if the leak isn’t fixed. We match the thickness to the existing wall and to what the new cladding needs, and leave gaps between panels so the wall can dry.

What happens if you find mould in the wall?

Mould on framing and sheathing is a sign the wall has been wet for some time. Mouldy insulation and badly affected sheathing come out along with the rot, and the cavity is left to dry before it is closed. If growth is extensive or has reached the interior of the house, we recommend a mould remediation specialist before the wall is rebuilt, because that is a separate trade with its own procedures.

Can siding be installed in a Calgary winter?

Often, with limits. Vinyl gets stiffer and less forgiving in the cold, and the industry installation manual calls for a larger gap at trim and openings below about 4 °C so panels have room to expand in summer. Primed fibre cement has to be painted within James Hardie’s time limit, and paints and sealants may have application temperatures of their own, so finishing can wait for milder days. Snow, ice and wet sheathing are dealt with before membrane or siding goes on, and we will tell you plainly if a job is better started in spring.

Considering Sheathing & Rot?

Call Calgary Siding Solutions or send the details. We will look at the job, tell you whether sheathing & rot is the right fix, and price it in writing.

(587) 324-8484 Send the Details