New siding in Oregon goes on over three layers the homeowner never sees: a water-resistive barrier such as house wrap over every wall, flashing at every window, door, trim board and roof-to-wall corner, and, by default, a drainage space of at least 1/8 inch between that barrier and the siding. Together they let rain that gets behind the siding drain back out before it reaches the sheathing.
All three come from the Oregon Residential Specialty Code, and the siding makers add their own requirements on top. This guide walks through each layer as the 2023 code writes it, what James Hardie, LP and the vinyl industry require, why it matters in a Willamette Valley winter, and what a siding bid should say about it.
Which code applies. Section numbers here are from the 2023 Oregon Residential Specialty Code. The Building Codes Division made the 2026 edition effective on October 1, 2026, but outside its administrative chapter either edition may be used until April 1, 2027, and the 2026 codebook was not yet published when this was written. Ask which edition your job follows.
What goes behind new siding in Oregon?
From the framing out, a re-sided wall is built in this order:
| Layer | What it does | Rule |
|---|---|---|
| Sheathing | The structural skin the rest is fastened to; rotted sections are replaced before anything goes over them | Replacing sheathing needs a permit in Salem |
| Water-resistive barrier (house wrap or felt) | Stops water that gets past the siding from reaching the sheathing | R703.2: at least one layer over every exterior wall |
| Flashing | Turns water out at windows, doors, trim, deck ledgers and roof-to-wall corners | R703.4 |
| Drainage space | A gap of at least 1/8 inch so water behind the siding can run down and out | R703.1.1, unless an exception applies |
| Siding and trim | The visible cladding, installed to the maker’s instructions | R703.3 and the section for the material |
Oregon writes the envelope rule as a system. Section R703.1.1, an Oregon amendment that replaces the model code’s own section, says the wall must be built so that water entering it can drain to the outside, and that the barrier, the drainage space, the flashing and the siding must be integrated so that together they do that. A good wrap with careless flashing, or a gap with no way out at the bottom, does not meet the intent.
Do you need house wrap under siding?
Yes, under every kind of siding. Section R703.2 calls for at least one layer of water-resistive barrier over the studs or sheathing of all exterior walls, continuous up to the top of the walls, and finished at windows, doors and other penetrations so it ties into the flashing. No siding is exempt from it on the grounds that it sheds water well.
The siding makers agree. The Polymeric Exterior Products Association’s 2022 vinyl siding installation manual, the reference behind the Vinyl Siding Institute’s program, says vinyl is an exterior cladding, not a water-resistive barrier, and must go over one. It says vinyl is designed to let the wall behind it breathe, which it calls a supplemental rainscreen that cuts the water reaching the barrier, not a substitute for one. James Hardie and LP require a barrier too, as the maker table below sets out.
So when a bid says nothing about the barrier, that is a gap in the bid, not a sign the wall does not need one. On a tear-off, ask whether the quote includes a new barrier over the whole wall or plans to keep the old one, and why.
House wrap, Tyvek or building felt: what the code accepts
Oregon’s code lets the barrier be one of several materials. Section R703.2 lists No. 15 asphalt felt meeting ASTM D226 Type 1, sheet barriers meeting ASTM E2556, and other approved materials installed to their maker’s instructions, alongside barriers tested under two further standards.
How it goes on matters as much as which product it is. Felt and E2556 barriers are applied horizontally, each upper layer lapped over the one below by at least 2 inches, with at least 6 inches of lap where ends meet. Lapping the other way, upper course tucked under the lower, sends water behind the wrap rather than over it.
Tyvek and other brand-name wraps. DuPont’s Tyvek HomeWrap is a common choice and meets ASTM E2556 Type II. DuPont describes it as vapor permeable, so moisture in the wall can dry outward, and says it withstands up to 120 days of sunlight before it should be covered; past that, it suggests adding a second layer or replacing it. James Hardie makes its own wrap, HardieWrap. What to ask about is the product’s own installation instructions and exposure limit, because a wrap left uncovered through a delayed summer job can be past its limit before the siding goes on.
Oregon’s rainscreen rule: the 1/8-inch drainage space
Oregon does not adopt the model International Residential Code’s water-resistance section; its own section R703.1.1 requires a space of at least 1/8 inch between the water-resistive barrier and the siding, formed with any noncorrosive furring strip, drainage mat or drainage board.
That space is what people mean by a rainscreen. Water that gets past the siding has room to run down the face of the barrier and out at the bottom of the wall instead of being held against it, and air in the gap helps the back of the siding dry. The Forest Products Laboratory’s Wood Handbook makes the same point for wood siding, as covered on the cedar siding page.
The three ways to make the gap:
- Furring strips. Thin vertical strips, often wood, fastened over the barrier at the studs, with the siding nailed to them. The siding makers set the rules for this. LP’s instructions, for its 38 Series lap siding over foam sheathing, call for Southern Pine furring at least 3/4 inch by 3 1/2 inches, no more than 16 inches apart. James Hardie’s Technical Bulletin 19 (February 2025), on furring over foam and other layers that cannot hold a nail, ties furring thickness to the fastener and recommends nominally 4-inch-wide wood furring.
- Drainage mat. A mat installed over the barrier that holds the siding off it, with the siding fastened through it.
- Drainage board. A board product with the drainage space built into it.
A furred wall sits farther out than the old one, so window and door trim, sills and corner boards need detailing for the extra depth. That is simplest to plan during a full tear-off.
When the drainage gap is not required
Section R703.1.1 lists six exceptions:
| Exception | What it covers |
|---|---|
| 1. Drainage-enhanced barrier | A barrier meeting R703.2 that is manufactured to enhance drainage and meets the 75 percent drainage-efficiency requirement of ASTM E2273 or another recognized national standard |
| 2. Pan-flashed windowsills | Sills with pan flashing that drains through the wall to the face of the siding, detailed in the construction documents |
| 3. Detached accessory structures | Sheds, detached garages and similar buildings |
| 4. Matching repairs and additions | New siding that matches the existing, in the same plane with no change of direction or control joint, over an R703.2 barrier |
| 5. Concrete or masonry walls | Designed under chapter 6 and flashed under R703.4 or R703.8 |
| 6. Tested assemblies | A wall assembly shown to resist wind-driven rain in an ASTM E331 test with set sample size, pressure and duration |
Exception 1 is the one that turns on which house wrap goes on. Some wraps are sold with a grooved or textured face for drainage: DuPont, for example, describes the vertical grooves on Tyvek DrainWrap as giving enhanced drainage behind claddings such as fiber cement and primed wood, with a drainage efficiency above 98 percent in the ASTM E2273 test. DuPont also lists plain HomeWrap above 90 percent in the same test, but the exception asks for a barrier manufactured to enhance drainage as well as the test result. Whether a given wrap qualifies is the building official’s call; on a job with no permit, ask the contractor to show you the product literature it relies on.
Exception 4 is why a small siding repair does not normally need the wall furred out: patching matching boards into a wall in the same plane, over a sound barrier, is exempt. It does not stretch to a full re-side in a new material.
Stucco has its own, stricter rule. Under section R703.7.3.2, stucco over wood-based sheathing in Marion, Polk and the other western Oregon counties the code lists needs a space or drainage material at least 3/16 inch deep on the outside of the barrier, or drainage of at least 90 percent efficiency. That matters only if part of the house is stucco.
Flashing: windows, kick-outs and Z-flashing
The barrier only works if water that runs down it is turned out at every interruption. Section R703.4 requires approved corrosion-resistant flashing, applied shingle-fashion and extending to the face of the siding, at these places:
- Window and door openings, to the window maker’s flashing instructions or another method the code allows; where the maker gives none, pan flashing at the sill, sloped or sealed to drain outward, with protection at the head and sides (R703.4.1).
- Continuously above all projecting wood trim.
- Where porches, decks or stairs attach to a wood-framed wall or floor.
- Wall and roof intersections, with a kick-out flashing at every one, such as where a lower roof ends against a wall.
- Elsewhere: where chimneys or masonry meet a framed wall, under and at the ends of copings and sills, and at built-in gutters.
Z-flashing is the shape the siding makers draw above horizontal trim such as window and door heads and band boards, and it is also used where a deck ledger meets the wall. Section R703.5.1 allows it at horizontal joints in panel siding such as T1-11, over solid blocking or sheathing. Missing Z-flashing at those joints is one of the ways T1-11 siding fails from the edges in. James Hardie’s HZ10 instructions leave at least 1/4 inch between the Z-flashing and the siding above it and say not to caulk that gap, so water can get out.
Kick-out flashing sits at the foot of a roof-to-wall joint and turns roof runoff into the gutter instead of down the wall. James Hardie draws it at least 4 by 4 inches, and LP tells installers not to run siding or trim inside it. Where one is missing, the wall below that corner is the first place to look for rot.
What James Hardie, LP and the vinyl industry add
The code is the floor. Each maker’s instructions add rules of their own, and the warranty depends on them. The table draws on James Hardie’s HardiePlank HZ10 instructions and HZ10 trim instructions, LP’s SmartSide lap siding instructions (12/08/2023, as posted by a dealer) and the vinyl manual:
| James Hardie (fiber cement) | LP SmartSide (engineered wood) | Vinyl (industry manual) | |
|---|---|---|---|
| Barrier | Required to local code, with penetration and junction flashing; Hardie takes no responsibility for water infiltration | Required unless code exempts it; LP disclaims liability for barrier problems | Required; vinyl is not a barrier |
| Flashing | Over horizontal trim in every installation method; kick-out at least 4 by 4 inches | Metal or other 50-year material; 4-inch upper leg; integrated with the barrier | Code-compliant flashing tied into the barrier at openings, corners and roof lines |
| Furring | Thickness set by the fastener; nominal 4-inch wood furring as best practice (Technical Bulletin 19) | 3/4 by 3 1/2 inch Southern Pine over foam sheathing, spacing by series | Furring used to even out a wall or on non-nailable surfaces |
| Warranty link | Instructions warn that not following them may affect warranty coverage | Warranty remedies not available if the instructions are not followed | Follow the siding maker’s instructions for the product |
Two practical points follow. A fiber cement job in Salem should be built to James Hardie’s HZ10 documents, since Salem-area ZIP codes are in that zone and its trim instructions warn they are not valid in parts of Oregon. And LP says that where its instructions, the code and the designer disagree, the most restrictive requirement applies, a sensible rule to hold any bid to. The siding pages go further into each material: fiber cement siding, vinyl siding, and a side-by-side in the best siding for Salem’s climate.
Why it matters in the Willamette Valley’s wet season
Salem’s rain comes in one long season. NOAA’s 1991-2020 normals for Salem’s airport put about 40 inches in an average year, and about 31 of those inches fall from October through March. The same normals count about 91 days a year with a tenth of an inch of rain or more, and about 66 of them come in those six months. June through September brings about 3.35 inches combined.
What that means for a wall: some water gets behind any siding, and in a Salem winter the wet days come close together, with little drying weather between them. The USDA Forest Products Laboratory’s Wood Handbook says continuous protection of walls from rainwater will prevent decay from developing. A barrier lapped the right way, flashing that turns water out at each interruption, and a gap that lets the wall drain and dry are how a re-sided wall gets that protection. What causes dry rot explains what happens to sheathing when it does not.
A tear-off leaves sheathing and wrap open to the weather, so weigh the best time to install siding in Salem alongside the wrap’s own exposure limit.
What to look for in a siding bid
A bid that only names the siding leaves out the parts that decide how long the wall lasts. Ask for these in writing:
- The barrier. Product name, whether it covers every wall, and how it is lapped and taped.
- The drainage method. Furring, drainage mat or drainage board, and its thickness, or which R703.1.1 exception the contractor is relying on and how it is met.
- Flashing, by location. Window and door heads and sills, every kick-out (count the roof-to-wall corners yourself), Z-flashing over trim and at deck ledgers, and what the flashing is made of.
- Sheathing. How rotted sheathing will be priced if it turns up, and that replacing it in Salem needs a permit; Salem’s siding permit rules cover when.
- Trim, soffits and fascia. Whether the eaves are in the job; soffit and fascia repair in Salem covers what to ask about them.
- Which maker’s instructions the installation follows, for fiber cement the HZ10 documents, and which code edition.
- The contract. Oregon’s ORS 701.305 requires a written contract showing the contractor’s CCB number for residential work over 2,000 dollars, and the license can be checked on the CCB license search.
These layers are part of what a re-side costs, so a bid that leaves them out is not comparable with one that includes them. The figures this site can source are on the siding cost page, and the full job order is laid out step by step under siding replacement.
Describe your project on the request page, with the year the house was built and the siding on it now. Hold any bid you get to the list above, and check the contractor’s license on the CCB license search before you sign.
This page is general information, not professional advice about your specific situation. Have it assessed by a qualified professional before making decisions.