Hardwood Siding and Rainscreen Specification Guide
A successful wood facade is a coordinated wall assembly, not a species attached to sheathing. This guide converts profile, moisture, drainage, ventilation, attachment, finish and procurement decisions into a specification suppliers and installers can price consistently.
Key Takeaways
- The cladding sheds rain; the water-resistive barrier and flashings manage water that passes it
- Specify finished dimensions, grade, moisture, lengths and appearance—not species alone
- Coordinate profile, cavity, furring and fasteners as one tested wall assembly
- Approve a full-size mockup and a written maintenance intent before ordering
Profile decision matrix
Profile names do not standardize dimensions—require a mill drawing and approved sample.
| Profile family | Visual / joint strategy | Critical coordination |
|---|---|---|
| Open-joint rainscreen | Expressed gaps; WRB is visually exposed | Requires UV-rated inner layers and tested attachment |
| Shiplap / channel | Overlapping joint with defined reveal | Allow movement; do not turn the lap into the drainage plane |
| Tongue and groove | Tight linear appearance | Exterior use needs drainage behind and movement at boundaries |
| Bevel / lap | Traditional overlapping courses | Course exposure, nailing and flashing control performance |
| Board and batten | Vertical boards with covered joints | Preserve drainage behind horizontal attachment members |
Start with the wall, not the species
Wood cladding is the outer screen of a layered wall, not the water-resistive barrier. A durable specification identifies the cladding, attachment system, drainage and ventilation cavity, water-resistive barrier, flashing, insect screens, structure and interior control layers as one coordinated assembly.
Exposure, building height, wind pressure, wildfire requirements, climate, overhangs and wall orientation change what the assembly must do. Confirm current code, tested assemblies and the design professional’s details before selecting a profile.
- Define cladding as a screen, not the drainage plane
- Preserve continuity at windows, doors, decks, roofs and penetrations
- Provide a clear route for bulk water to drain out
- Coordinate cavity fire blocking and insect screening with code
Write a measurable lumber specification
Species name alone does not control the delivered product. State botanical identity where trade names are ambiguous, grade and grading rule, heartwood or sapwood limits, exposed faces, grain orientation, moisture condition, finished dimensions, tolerances, lengths and permitted defects.
Require a physical mockup that shows board range, profile, joint, corner, fastener, flashing and finish. Approve the range rather than one ideal sample, because natural color and grain variation are part of solid wood.
- Botanical and common name
- Written grade rules and exposed faces
- Finished—not nominal—dimensions and tolerances
- Target moisture range at delivery and installation
- Length tally, waste factor and permitted substitutions
Design drainage, ventilation and drying
A drained wall accepts that some rain passes the cladding and gives it a path back outside. Building Science Corporation notes that the cavity acts as a capillary break and drainage path; ventilation can also help remove retained moisture. Cavity depth and vent area must follow the selected system, code and project exposure.
Keep the cavity continuous at field areas while detailing safe terminations. Do not block drainage with horizontal furring, sealant or insulation. Cross-furring or a tested drainage matrix may be needed when vertical siding would otherwise require horizontal attachment.
- Flash and drain every interruption
- Screen openings without choking airflow
- Slope horizontal projections away from the wall
- Keep end grain out of standing water
- Maintain required clearance from grade, paving and roofs
Engineer attachment for load and movement
Fastener schedules depend on cladding weight, board width, profile, substrate, furring, wind pressure and edge distances. Dense hardwoods commonly require predrilling and corrosion-resistant fasteners; coastal and chemically aggressive exposures can justify Type 316 stainless.
Open-joint clips and proprietary systems must be installed as tested. For face-fastened boards, locate fasteners so boards can move without splitting, and never use a generic spacing copied from another species or profile. The engineer or system manufacturer should establish attachment capacity and spacing.
Treat finish as a maintenance system
Sunlight and wetting change exterior wood. A penetrating finish can slow moisture exchange and preserve color, but no clear exterior coating makes maintenance disappear. Decide whether the design intent is maintained color or natural weathering, then document prep, all-face or field application, cut-end treatment, recoat trigger and access.
The USDA Forest Products Laboratory emphasizes water, sunlight and weathering as primary exterior finish concerns. Use a finish proven compatible with the species and mockup. Keep drainage joints open—do not caulk intentional rainscreen gaps.
Issue a quote-ready package
Comparable bids require drawings and a material schedule. Ask suppliers to identify every assumption and separate lumber, milling, finishing, accessories, packaging and freight.
- Wall area by orientation and elevation
- Profile drawing with finished dimensions and reveal
- Species, grade, moisture, lengths and appearance rules
- Corner, base, top, opening and penetration details
- Furring, clips and fastener metallurgy
- Finish system and factory-versus-field scope
- Certification documents, lead time, freight and claim process
Frequently Asked Questions
Technical sources and scope
Reviewed September 2026. This guide supports specification planning; current code, project drawings, tested assemblies and manufacturer instructions control.
- USDA Forest Products Laboratory—Wood Handbook
Moisture, dimensional movement, fastening, durability and exterior finish principles.
- USDA FPL—Moisture Relations
Wood-water relationships and dimensional change.
- USDA FPL—Finishing Wood
Exterior exposure, sunlight, water, weathering and finish performance.
- Building Science Corporation—Rain Control Theory
Independent building-science basis for screened, drained and ventilated wall assemblies.
- American Wood Council
Current wood construction standards and code resources; project code and engineering remain controlling.