Quick answer: For most outdoor projects, ACX plywood gives you the best balance of looks and durability. Use CDX when appearance doesn’t matter (like shed framing), and save marine-grade for boats or premium builds. Your choice comes down to three factors: budget, whether people will see the finished surface, and how much weather exposure it’ll face.
What Makes Plywood “Exterior Grade”?
Exterior plywood uses waterproof glue between its layers. That’s the key difference from interior plywood, which will literally fall apart when wet because the layers separate.
You’ll see an “X” on exterior plywood (like CDX or ACX)—that X means waterproof adhesive. But here’s the thing: while the glue is waterproof, the wood itself isn’t. The wood still absorbs moisture, which is why proper sealing and finishing are critical for outdoor projects.
Common applications include decks, sheds, outdoor furniture, roof decking, and garden structures. For indoor projects, Baltic Birch plywood offers superior finishing characteristics, but it’s not suitable outdoors.
Types of Exterior Plywood
CDX Plywood – The Budget Option
CDX is what you’ll see on most construction sites. It’s not pretty—expect visible knots, rough patches, and some splits—but that’s fine when building something nobody will see.
The letters indicate quality: C-grade face, D-grade back, and X for exterior waterproof glue. D-grade is the lowest you’ll find, with knotholes up to 2.5 inches and plenty of imperfections.
Best uses:
- Roof decking under shingles
- Shed walls and framing
- Wall sheathing
- Subflooring in covered structures
- Any structural application you’ll cover with siding
When properly protected, CDX plywood can last 20+ years in covered applications. Just don’t use it for furniture—the rough surface won’t finish nicely, no matter how much sanding you do.
ACX Plywood – The Sweet Spot
This is where most DIYers land for outdoor furniture and visible projects. ACX has an A-grade front face (smooth and nearly knot-free) with a C-grade back.
That smooth front face makes all the difference. You can stain it, paint it, or apply clear finish, and it’ll look professional. The back side has some imperfections, but if only one side will be visible, why pay for two perfect faces?
Best uses:
- Outdoor furniture (tables, benches, planters)
- Visible deck surfaces
- Pergolas and gazebos
- Garden boxes
- Any project where appearance matters
ACX plywood provides excellent weather resistance and is the most popular choice for outdoor DIY projects.
Marine Plywood – Premium Quality
Marine plywood is the top tier. It’s completely knot-free with no voids between layers. Both faces are smooth and attractive (typically AA, AB, or BB grade).
The “no voids” part is crucial for boats—any gap between layers can trap water and lead to rot. It’s also why marine plywood bends smoothly for curved applications without cracking.
Best uses:
- Boat building
- Curved outdoor structures
- High-end furniture where both sides are visible
- Projects requiring maximum longevity
Marine plywood is the premium option. For most outdoor furniture and structures, ACX provides excellent value. Learn more about the different grades and types of marine plywood available.
Pressure-Treated Plywood
Standard plywood gets treated with chemical preservatives (usually copper-based) to resist rot, mold, and insects. This is the only option for ground-contact applications.
Best uses:
- Any wood touching soil
- Foundation walls
- Sill plates
- High-moisture environments
- Areas with termite concerns
The chemical treatment makes it unsuitable for food-contact surfaces like cutting boards or counters. Always wear protective equipment when cutting treated lumber.
How to Choose the Right Plywood
Step 1: Consider Your Budget
CDX is your most affordable option and works great for structural applications. ACX gives you that smooth, finished look for a moderate investment. Marine plywood is the premium choice—go this route only for boats or when appearance and maximum durability are essential.
Step 2: Will People See It?
If you’re covering the plywood with siding, shingles, or other materials, save money with CDX. If one side will be visible (like outdoor furniture), go with ACX. If both sides need to look perfect, marine plywood is your answer.
Not familiar with plywood grading? Our guide to understanding different grades of plywood breaks down the grading system.
Step 3: What’s the Weather Exposure?
Full exposure to elements requires ACX or marine with proper sealing. Partially protected areas under eaves can use CDX or ACX. Ground contact or extreme moisture demands pressure-treated plywood—nothing else will hold up.
Step 4: Match Thickness to Your Project
For outdoor furniture and cabinet sides, 1/2″ (12mm) works well. Roof decking and lighter subflooring need 5/8″ (15mm). Heavy-duty furniture, deck surfaces, and structural flooring require 3/4″ (19mm). Check out our plywood thickness guide for specific span requirements.
Critical maintenance tip: Regardless of which exterior plywood you choose, proper sealing makes or breaks longevity. Apply exterior-grade primer and paint, or use exterior stain and sealant on all surfaces and edges. Seal cut edges immediately—exposed end grain absorbs water like a sponge. Even affordable CDX can last decades with proper finishing.
For project-specific guidance, check out our guide on choosing the best wood for outdoor furniture. You’ll also want to check out our wood finishing and sealing products to protect your investment.
Frequently Asked Questions
What’s the difference between exterior and marine plywood?
All marine plywood is exterior plywood, but not all exterior plywood is marine grade. Marine plywood features completely knot-free construction with no voids between layers—critical for boats where water intrusion can cause serious problems. Standard exterior plywood like CDX or ACX can have knots and voids while still using waterproof glue. Marine plywood is significantly more expensive than ACX, so save it for marine applications or premium projects where both appearance and structural integrity are paramount.
Can you use interior plywood outdoors?
Absolutely not. Interior plywood uses non-waterproof glue that delaminates when exposed to moisture. Even painted or sealed, interior plywood will fail outdoors because water penetrates through cut edges and fastener holes, causing layers to separate. Always use exterior-grade (X-marked) plywood for outdoor applications. Using interior plywood outdoors guarantees premature failure—often within a year or two.
How long does exterior plywood last outside?
With proper installation and maintenance, CDX lasts 15-25 years in covered applications (5-10 years fully exposed), ACX lasts 20-30 years with proper finishing, and marine plywood can go 30-50+ years. Pressure-treated plywood lasts 20-40 years even touching the ground. Lifespan drops dramatically without proper sealing. Standing water, trapped moisture, and unsealed edges cause most premature failures. The quality of your finish and maintenance matters more than the plywood grade itself. Learn how to maintain and extend the life of marine plywood with proper care.
Is CDX plywood waterproof?
CDX is water-resistant, not waterproof. It uses waterproof glue, so layers won’t separate when wet, but the wood itself absorbs moisture. CDX needs protection through proper installation (good drainage and ventilation) plus finishing with paint, stain, or sealant. Best used in covered applications or as structural elements you’ll protect with siding. For fully exposed surfaces, ACX or marine plywood with proper sealing performs better.
What plywood should I use for outdoor furniture?
ACX plywood (3/4″ thickness) is ideal for most outdoor furniture. It provides a smooth A-grade face that takes stain or paint beautifully and offers excellent weather resistance. Marine plywood makes sense if both sides will be visible or you need curved components that won’t crack when bent. Skip CDX for furniture—its rough, blemished surface doesn’t finish well no matter how much you sand. Always seal all surfaces and edges with exterior-grade finish, and reapply yearly for maximum longevity.
Ready to Start Your Project?
For most outdoor projects, ACX plywood hits the sweet spot between appearance and value. Use CDX to save money on hidden structural work, and upgrade to marine plywood only for boats or premium builds.
Here’s the real secret: proper sealing and maintenance matter more than plywood grade. I’ve seen CDX structures last 25+ years with good finishing, and marine plywood fail in under 10 years when left unsealed. Always seal edges before installation, apply appropriate exterior finish, and maintain that protective coating.
Need help selecting the right plywood? Our team can assess your project and recommend the optimal type. Contact us for guidance on choosing the right exterior plywood and protective finishes for your outdoor project.
