5/4x4 Cumaru Rainscreen Siding

A 5/4x4 Cumaru board weighs approximately 1.70 lbs per linear foot. Cumaru has a density of about 68 lbs/ft³, making it one of the densest commercially available hardwoods. Always factor weight into your structural and shipping calculations.

Yes — Cumaru is rated Durability Class 1 (Very Durable) and is naturally resistant to rot, decay, insects, and fungal attack without chemical treatment. It is Class B fire-rated and performs exceptionally well in full exterior exposure including siding applications. While ground contact is possible, we recommend elevating Cumaru boards at least 6" above grade for maximum lifespan.

Cumaru outperforms composite in hardness (3,540 vs ~200 Janka), lifespan (up to 50+ years vs 25-30 years), and heat resistance. Unlike composites, Cumaru won't melt, warp in extreme heat, or fade significantly. While initial cost is similar ($8-15/LF installed), Cumaru costs less over its much longer lifetime. Composites do offer lower upfront maintenance.

Cumaru requires minimal maintenance. Without treatment, it naturally weathers to an elegant silver-gray patina over 6-12 months. To maintain the original warm brown color, apply a UV-inhibiting penetrating oil (like Ipe Oil or Penofin) once or twice annually. Cleaning involves periodic washing with a mild soap solution. No sanding, staining, or sealing is required for structural durability.

Cumaru is DIY-friendly with proper preparation. Key requirements: (1) Always pre-drill all screw holes — Cumaru is too hard for direct screwing. (2) Use only 305 or 316 stainless steel fasteners — standard hardware will corrode and stain. (3) Face-screw with stainless steel trim-head screws. (4) Allow boards to acclimate on-site for 5-7 days before installation. A compound miter saw with a carbide-tipped blade is recommended.

A nominal 5/4x4 Cumaru board has actual finished dimensions of 1" × 3.5" (25.4 x 88.9 mm). The "5/4x4" is the nominal (rough-cut) size — after kiln drying and precision surfacing, all hardwood lumber finishes slightly smaller. This is standard across the lumber industry, similar to how a nominal 2×4 softwood measures 1.5" × 3.5". Always use the actual dimensions (1" × 3.5") when calculating material quantities and coverage for your project.

We responsibly, legally, and ethically source all our Cumaru. We work only with suppliers who follow responsible forestry practices with full chain-of-custody transparency. Note: We are not FSC certified. Cumaru trees grow naturally in diverse tropical forests and regenerate well under managed harvest. The extremely long lifespan (up to 50+ years) means fewer replacements compared to softwood or composite alternatives, reducing long-term environmental impact.