The 10 mm Gap That Decides Whether Your Siding Lasts

Where the requirement came from

British Columbia has the most demanding residential wall requirements in Canada for a specific and expensive reason. Through the 1980s and 1990s, thousands of buildings in the Lower Mainland were built with cladding fastened directly to the sheathing over building paper. Water that got past the cladding, and water always gets past the cladding, had nowhere to go and nothing to dry it. The result was widespread structural rot and repair bills that reshaped the industry.

The response was a Code change. Building permits issued after December 15, 2006 by most British Columbia municipalities west of Hope and on Vancouver Island require the installation of a capillary break between the cladding and the building paper or wrap. In British Columbia, the requirement is for a gap of 10 mm, which is 3/8 of an inch on a tape measure.

The North Shore sits squarely inside that geography. It is also among the wettest parts of it, sitting where Pacific weather meets the mountains, which is why local building departments take the detail seriously and why a re-siding permit here carries expectations that a comparable job in the interior would not.

The practical consequence for a homeowner is simple to state and easy to miss. If your house was clad before 2006, the wall behind your siding is almost certainly not built the way a new wall has to be built, and replacing the siding means building a different assembly rather than reinstalling the old one.

What the cavity actually does

The gap is doing three jobs at once, and understanding them explains why the details matter more than the material.

First, drainage. Water that gets behind the cladding hits the drainage plane and runs down the cavity to an opening at the bottom of the wall, where it leaves the building. Without a cavity, that water sits in contact with the sheathing.

Second, drying. Air moves through the cavity, which lets the back face of the cladding and the face of the water-resistive barrier dry after they get wet. This is why a vented cavity outperforms a merely drained one, and why the openings at top and bottom matter.

Third, the capillary break itself. Two surfaces in tight contact pull water between them by capillary action, the same effect that draws water up a paper towel. A 10 mm separation is wide enough that capillary action cannot bridge it. This is the reason the gap has a minimum dimension rather than just being a recommendation to leave some space.

Pressure moderation is a fourth benefit on exposed walls. A cavity vented to the outside equalises air pressure across the cladding, which reduces the force pushing water inward through any gap in the siding.

How the assembly is built, layer by layer

Working from the sheathing outward, a compliant wall has an order that does not change even though the products do.

The water-resistive barrier goes on the sheathing first, lapped like shingles so water running down the face always crosses a joint on the outside. This layer is the drainage plane, and it is the actual waterproofing of the house. The siding is a rain shield, and the barrier behind it is what keeps the building dry.

Then the strapping, most commonly vertical wood furring, which creates the cavity and gives the cladding something to fasten to. Vertical strapping creates continuous drainage paths. Where horizontal strapping is needed for a particular cladding, it has to be notched, sloped or cross-strapped so water is not dammed at every row.

Insect screen and a vent at the base of the cavity, so the cavity can drain and breathe without becoming a home for wasps or rodents. This is a cheap detail and it is left out constantly.

Then the cladding, fastened through the strapping into the framing, with fasteners long enough to account for the cavity depth. This is where a re-siding job quietly gains cost: every fastener is longer, every window and door needs new extension jambs, and every trim detail moves outward by the depth of the cavity.

• Water-resistive barrier lapped shingle-fashion over the sheathing.
• Strapping creating a continuous drained and vented cavity, generally vertical.
• Screened openings at the base of the cavity, and venting at the top.
• Head flashing above every window and door, with end dams.
• Sill pan and back dam at every window opening.
• Cladding fastened through the strapping into structure with fasteners sized for the added depth.

The details that decide the outcome

A rainscreen wall does not fail in the middle of a wall. It fails at the holes, and all of the holes are put there deliberately.

Windows are the first one. A window in a cavity wall sits proud of the sheathing, which means the opening needs a sill pan that drains outward onto the drainage plane, a back dam so water cannot run inward, and head flashing with end dams so water arriving at the top of the window is thrown clear rather than routed into the wall at the corners. A window installed without a sill pan will eventually deliver water directly into the rough opening no matter how good the cladding is.

Penetrations are the second. Hose bibs, dryer vents, electrical, gas lines, light fixtures, deck ledgers. Every one crosses the drainage plane and every one needs to be flashed or sealed so that water running down the plane goes around it rather than through it.

Deck ledgers deserve their own sentence, because on the North Shore they are a leading cause of hidden rot. A ledger bolted through the cladding into the rim joist creates a horizontal line of fasteners on the wettest part of a wall. The correct detail is a flashed ledger or, better, a deck structure that stands free of the house.

Transitions are the third. Where the wall meets the roof, where it meets a deck, where cladding material changes partway up an elevation, and at inside and outside corners. Each of these is a junction between two systems, and junctions are where installers make judgement calls that were never drawn.

What you find when the old siding comes off

This is the part of the job that cannot be priced accurately in advance, and any quote that pretends otherwise is guessing. Until the cladding is off, nobody knows the condition of the sheathing behind it.

On North Shore houses of a certain age the pattern is consistent. The field of the wall is usually fine. The damage clusters below windows, at the corners, behind deck ledgers, under leaking gutters and at the base of the wall where splashback and grade contact have been at work for decades.

The honest way to handle this is an allowance in the contract with a stated unit rate for sheathing replacement and framing repair, so that when the wall is open the extra work is priced at a rate you already agreed to rather than negotiated on the spot. A fixed price with no allowance is either padded to cover a worst case you may not have, or it is going to become a change order.

Removing siding is also the point where an older house reveals its other secrets, and one of them is regulated. Building materials from before the 1990s can contain asbestos, and on the exterior that includes some stucco, some siding products and some sealants. The survey belongs before demolition rather than after.

The moment to add insulation

Here is the argument for doing more while the wall is open, and it is the strongest one in renovation work.

Stripping cladding exposes the outside face of the sheathing, which is the only time in a building's life when exterior insulation can be added without gutting the interior. Exterior insulation sits outside the framing, so it covers the studs rather than stopping at them, which addresses the thermal bridging that limits what cavity insulation alone can do.

It also warms the sheathing, which reduces the risk of condensation inside the wall. The physics is favourable and the marginal cost is far lower than the same work done as a standalone project, because the cladding removal, the strapping and the trim adjustments are already in the scope.

The constraint is dimensional. Adding insulation outboard of the sheathing moves the cladding further out, which means deeper window and door extensions, revised roof overhang relationships at the eaves, and attention to whether the fastening through the assembly still reaches structure properly. These are solvable, and they are much easier to solve on a drawing than on a ladder.

If a Step Code target, a heat pump, or an energy rebate is anywhere in your plans for the next few years, the re-siding job is the moment those plans get cheap. Doing it later means paying to remove cladding you just paid to install.

How to read a re-siding quote

Two quotes for the same house can differ enormously and both be honest, because they are describing different scopes. These are the lines that tell you which one you are reading.

Does the quote name the water-resistive barrier and the strapping as separate items, or does it say siding and a square footage rate? A quote that does not mention the cavity is probably not pricing one.

Does it include window flashing, sill pans and head flashing as line items? On a re-siding job, existing windows either get re-flashed properly or they do not, and the difference is invisible the day the job ends and decisive ten years later.

Is there a stated allowance and unit rate for sheathing and framing repair? This is the mark of a contractor who has done this work before and is telling you the truth about what is unknowable in advance.

Does it address the venting at the base and top of the cavity, and insect screening? These are small lines that indicate whether the estimator understands the assembly or is copying a template.

On our own projects the envelope scope is written this way deliberately, because the parts of this job that determine whether it lasts are the parts a homeowner will never see once the siding is on.