A rainscreen is a small gap behind your siding that lets water drain and the wall dry out, and house wrap is the water-resistive barrier (WRB) on the sheathing underneath. Oregon’s residential code requires exterior walls to be built so water that gets in can drain back out, which in practice means a barrier plus a drainage space of at least 1/8 inch, or a barrier made to drain on its own.
The simple version
Siding sheds most rain, but not all. Wind pushes water behind it, through joints, around windows and under trim. The wall has to handle what gets past the siding. Think of three layers:
- Siding shields the wall from most water.
- Water-resistive barrier (house wrap, felt or similar) stops the rest from reaching the sheathing.
- Drainage gap gives water a path to run down and out, and lets air in to dry the back of the siding.
Flashing ties them together at windows, doors, rooflines and the bottom of the wall.
What Oregon code says
Section R703.1.1 of the Oregon Residential Specialty Code requires exterior walls of regulated residential buildings to be constructed so any water entering the wall assembly from the exterior can drain out. As quoted in local forms from Eugene and Woodburn, the envelope consists of:
- an exterior veneer (the siding),
- a water-resistive barrier,
- a space of not less than 1/8 inch between the barrier and the veneer, which can be formed with furring strips, drainage mat or drainage board, and
- integrated flashings.
There are exceptions. A separate space is not required if the water-resistive barrier is manufactured to enhance drainage and meets the 75 percent drainage efficiency requirement of ASTM E2273. Some barriers and sheathing products are built this way. The rule reportedly traces back to moisture failures in Oregon homes in the late 1990s and early 2000s, and the state’s rainscreen requirement has been in place for years, since the 2014 code according to one manufacturer news release.
Oregon’s current residential code is the 2023 ORSC, in effect for new permits since April 1, 2024. I could not read the 2023 text of R703.1.1 directly, and the quoted versions are from the 2021 edition. Edition numbers and section references differ between sources, so confirm the current wording with the Building Codes Division or your local building department.
Code applies whether or not a permit is needed. See our permits guide.
Rainscreen vs. house wrap: not the same thing
- House wrap / WRB: a sheet or coating on the sheathing. It resists liquid water and, depending on the product, allows vapor to pass. James Hardie sells its own weather barrier products; there are many other brands. Seams and penetrations must be taped or flashed and lapped like shingles, top over bottom.
- Rainscreen: the gap, made with furring strips, a drainage mat or a drainable wrap. Behind a fiber cement or cedar wall, it helps water run out and the back of the siding dry.
You can have a good wrap and no gap, which may not meet the code. Or a gap with sloppy flashing, which still leaks. It has to work as a system.
Why this matters in Oregon
Rain here is long and slow. Walls take on water for months, and the moisture sits in shaded or north-facing areas. Without a drainage path, water sits against sheathing and framing, and that is where rot, mold and failed paint begin. Gorge wind-driven rain makes it worse in East County. If you have ever pulled off old siding and found black, damp sheathing behind a bottom course, you have seen what no gap looks like. Our signs of dry rot guide lists what to watch for.
What to ask a siding contractor
- What water-resistive barrier do you use, and how do you flash windows and doors?
- How do you create the drainage space, or is the barrier a drainable one?
- Is there kick-out flashing at the ends of roof edges?
- How do you handle the bottom of the wall: a starter strip, a bug screen or vent so air and water can move?
- What clearance do you leave at roofs, decks and the ground? Hardie’s instructions call for clearances. Check the current Hardie installation guidance.
- Is the cost of this in my estimate? (See the cost guide.)
If a bid skips these answers, it may be skipping the work. A cheaper price for no wall system is not a bargain; it is a deferred repair.
What a rainscreen does and does not do
It does:
- give water an exit,
- let the back of cedar or fiber cement dry,
- reduce the chance of trapped moisture.
It does not:
- replace flashing,
- fix a leaking window,
- make siding waterproof,
- remove the need for maintenance.
Quick answers
- Does my old house have a rainscreen? Most 1950s to 1990s siding does not. Older walls were nailed over felt, if anything.
- Can I add one when I re-side? Yes. That is the best time.
- Is house wrap alone enough? Not under the code, unless the barrier is made to drain and meets the standard.
- Does fiber cement need a gap? The code applies to the veneer, whatever the material.
- Does cedar need one? Yes, even more so, since cedar benefits from drying out. See Hardie vs. cedar and the best siding for Oregon rain.
Get it done right
Dave Shaffer is a second-generation sider with about 40 years of hands-on experience, and he is on every Six-Point job. Ask him how he builds the wall behind your siding. We serve Gresham, Corbett and the rest of East County and the Portland metro. CCB #153635, licensed, bonded and insured.
Call or text Dave: 503-260-7647. Free estimates.