Compost Bin Materials: What to Build With, What to Skip, and the Size That Heats Up

Most guides to compost bin materials give you a shopping list and stop there. That is backwards. The material decides how long your bin lasts and how much your back hates you. It has almost nothing to do with the compost that comes out the other end.

It is late September. Leaves are starting to drop, which means a lot of us are about to have more carbon in the yard than we know what to do with. This is bin building season, and it is a good time to get the decision right once instead of rebuilding in the spring.

So here is the plain version. What to build with, what to skip, and the one dimension that matters more than every material choice put together.

What are the best compost bin materials?

The best compost bin materials are whatever you can get cheaply that will hold a cubic yard and last a few winters in your climate. For most people that means untreated wood pallets, wire mesh, or concrete block.

Colorado State University Extension puts real numbers on the common builds:

• Pallet bin: $5 to $25. Fasten the sides with wire and deck screws, and hinge the front panel so you can get in.

• Three chambered wood and wire system: $20 to $80.

• Commercial plastic bin or tumbler: $80 to over $100.

Extension also notes a trade off worth knowing before you pick: wire bins dry out faster and usually need a plastic cover or tarp, while wood bins hold moisture better but cost more. That single sentence should steer your choice more than any product review. If you live somewhere dry and windy, wire alone will fight you. If you live somewhere wet, wire is a gift.

Are wood pallets safe compost bin materials?

Yes, as long as the pallet is stamped HT or carries no international stamp at all. Do not build with a pallet stamped MB.

Pallets that cross borders have to be treated against pests under an international standard called ISPM 15, and they get marked. According to USDA APHIS, that mark has five parts: the accredited agency's symbol, the facility number, the treatment code, the two letter country code, and the IPPC symbol (the little wheat stalk). The treatment code is the part you care about.

• HT means heat treatment. APHIS specifies a minimum wood core temperature of 56 degrees Celsius (about 133 F) for at least 30 minutes. Nothing is added to the wood. Heat, then done.

• MB means methyl bromide fumigation. That is a pesticide. It is rare on North American pallets now, but MB stamped pallets still arrive on imported freight.

A pallet with no international stamp was almost certainly built for domestic use and never treated at all, which is fine. Look at the wood itself while you are checking: skip anything with chemical staining, spilled oil, or food residue you can still smell. A pallet that hauled something unpleasant is not bin material.

Pallets also solve a problem you would otherwise have to design around. They come with air gaps already built in. Stand four of them on edge, wire the corners, and you have a roughly 40 inch cube with ventilation on all four sides.

Which wood lasts longest in a compost bin?

Heartwood from naturally decay resistant species lasts longest. Western red cedar, redwood, white oak and black locust are the usual picks.

A USDA Forest Service above ground durability study in Wisconsin put western red cedar heartwood, redwood heartwood and sapwood, and white oak and red oak heartwood all in the top class, lasting more than 20 years in that test. That is a specific test in a specific climate, not a warranty, but the ranking holds up.

Pine is the other honest answer. It is cheap, it is everywhere, and a pine board sitting against wet compost is food. Expect a few seasons, not a decade. Clemson Cooperative Extension notes that redwood, cypress, cedar and oak are naturally decay resistant but expensive, which is the whole trade in one line.

There is a middle path that costs nothing. The University of Arkansas Extension wood and wire three bin plan uses cedar, or pine coated with latex paint, and drops the front slats into 1 inch vertical slots so they lift out. Build it that way and the boards that rot are the ones you can pull and replace in ten minutes. You replace a board, not a bin.

Is pressure treated lumber safe for a compost bin?

The chemistry people worry about is not the chemistry that is on the shelf anymore. Residential pressure treated lumber has not been CCA for about 20 years.

Clemson lays out the timeline: in 2002 the wood preserving industry voluntarily phased out arsenic and chromium containing preservatives, and by 2004 the EPA restricted CCA (chromated copper arsenate) for residential use. What replaced it is micronized copper azole (CA) or alkaline copper quaternary (ACQ). Copper, no arsenic, no chromium.

University of Maine Cooperative Extension addressed this exact question for compost bins and raised beds. Their answer: with CCA, the copper and chromium were low risk and arsenic was the problem, mostly through soil dust and soil particles rather than uptake by plants. With ACQ, the products "are not known to have toxicity issues or to leach into soils." They also make the point I would make, that it has always been a personal choice, and that hemlock or cedar are fine alternatives.

If you want a belt and suspenders version, Clemson suggests a plastic liner inside raised beds or keeping vegetables 12 inches away from the wood edge. For a compost bin, where nothing is rooted in the soil against the boards, that concern gets smaller still.

My own approach is boring. I build with untreated wood because it is cheaper and I never have to think about it. But if you have treated scrap in the shed, current chemistry is not the thing I would lose sleep over. Old gray decking from before 2004 is a different story. Leave that out.

One hard line, though. The EPA's home composting guidance lists treated wood among the things you should not compost. Framing a pile with treated lumber is a different question from feeding the pile treated sawdust. Do not toss the offcuts in.

What about wire mesh, concrete block, plastic tumblers and straw bales?

All four work. They just trade different things away.

• Wire mesh and hardware cloth. Cheapest and fastest to build, best airflow of anything on this list, and it takes about 20 minutes with a roll of mesh and three stakes. The catch is drying. Colorado State recommends a plastic cover or tarp on wire bins for exactly this reason.

• Concrete block. Essentially permanent, and the mass helps hold temperature through a cold snap. Heavy to move, so site it once. Lay the lower courses with gaps every other block, or set blocks core side out, so air can still reach the bottom of the pile.

• Plastic tumblers. Easy to turn, tidy, and good at keeping animals out. The honest limitation is volume. Most consumer tumblers hold well under a cubic yard, which means they rarely get hot. Treat a tumbler as a convenient cold composter and you will not be disappointed.

• Straw bales. The cheapest insulation you can buy, which makes them genuinely useful for composting through a cold winter. Stack them into a three sided box. They will break down and join the pile within a season or two, so plan on it as a one year bin.

How big does a compost bin have to be to actually heat up?

About 3 feet on every side. Colorado State University Extension states it plainly: "Three feet by three feet by three feet is considered the minimum size to allow for heating." That is 27 cubic feet, or 1 cubic yard. University of New Hampshire Extension gives the same figure.

Here is why that number is not arbitrary. Microbes generate heat throughout the volume of the pile. The pile loses heat through its surface. Cornell Waste Management Institute describes the balance directly: pile size has to weigh heat generation, which depends on volume, against heat loss through surface exposure. Too small and it cools faster than it can warm itself. Too large and the middle overheats or goes anaerobic.

The arithmetic is worth doing once:

• A 2 foot cube holds 8 cubic feet and has 24 square feet of surface. That is 3 square feet of heat loss for every cubic foot of heat production.

• A 3 foot cube holds 27 cubic feet and has 54 square feet of surface. That drops to 2 square feet per cubic foot.

• A 4 foot cube holds 64 cubic feet and has 96 square feet of surface. Now you are at 1.5.

Going from a 2 foot bin to a 3 foot bin cuts your heat loss per unit of heat production by a third. That is the whole story of why small bins stay cold.

There is an upper end too. Cornell notes that for windrow shaped piles the ideal height usually falls between 1 and 3 meters (roughly 3 to 10 feet), with dense fast rotting material like grass and food scraps at the low end and porous slow material like leaves at the high end. For piles relying on ambient air rather than forced aeration, the maximum airflow path is about 2 to 3 meters. In a backyard, anything much over 5 feet deep starts starving in the middle.

Colorado State adds a useful seasonal note: with brown heavy material, a larger bin measuring 5 feet by 5 feet may keep things running through the winter months. If you are building this month to handle leaf drop, that is worth knowing before you cut lumber. If you are still deciding whether you want a hot pile at all, our guide to picking the right home composting method covers the choice between hot, cold, tumbler and worm systems.

For reference on what "hot" means: EPA says a well maintained backyard pile can reach 130 to 160 F. Colorado State puts microbial activity between 70 and 140 F, best breakdown at 120 to 130 F, and warns that above 160 F you start killing the organisms doing the work for you.

Do you need a lid, a solid base, or air gaps?

Air gaps yes, bare soil yes, lid depends on your rainfall.

Air gaps. One inch between horizontal boards is plenty. The Arkansas Extension plan builds exactly this way, with slats dropped into 1 inch vertical slots. You want air moving into the sides of the pile, not a sealed wooden box.

Base. Put the bin on bare soil whenever you can. It lets excess water drain and lets soil organisms move up into the pile. If you are stuck on concrete or decking, EPA recommends starting with a 4 to 6 inch layer of bulky browns at the bottom, which gives you drainage and an air channel under the pile.

Lid. In a wet climate, a lid or a tarp keeps the pile from drowning. Cornell puts optimum moisture at 50 to 60 percent, notes that below 30 percent bacterial activity is inhibited, and that above 65 percent decomposition slows and odors start. EPA's field test is simpler: the pile should feel like a wrung out sponge. In a dry climate, the lid is holding moisture in rather than keeping rain out, and a wire bin absolutely needs one. If your pile has already gone sour, our walkthrough of what to do about a smelly compost pile covers the usual causes.

What mesh size keeps rodents out of a compost bin?

Quarter inch hardware cloth. The National Park Service rodent exclusion manual specifies 1/4 inch, 16 to 19 gauge to exclude both mice and rats. Half inch mesh will stop rats but not mice.

The numbers behind that: a mouse gets through an opening about 1/4 inch across, roughly the width of a pencil. A rat needs about 1/2 inch. If the animal can fit its head through, the body follows.

Two practical notes. First, line the floor as well as the walls, or at least the bottom 18 inches, because rodents dig in from underneath. Second, and more important, do not build the attraction in the first place. EPA's do not compost list includes meat, fish, bones, dairy, fats, oils and grease, cooked food beyond small amounts, and pet waste. Keep those out and the mesh becomes a backup rather than your only defense. If animal bedding is part of your stream, read our guide to composting manure safely before you add it.

What most guides skip about compost bin materials

The bin material barely affects the compost. A cedar bin and a chicken wire hoop holding the same cubic yard of the same material at the same moisture will produce roughly the same compost on roughly the same schedule.

What actually moves the needle is short and boring:

1. Volume. One cubic yard minimum, or the pile cannot hold its own heat.

2. Airflow. Cornell notes oxygen in the pile needs to stay above roughly 5 percent to keep things aerobic. That comes from particle mix, turning, and gaps in the walls.

3. Moisture. 50 to 60 percent, per Cornell. Wrung out sponge, per EPA.

4. Carbon to nitrogen ratio. Cornell puts the target at about 30 to 1 by weight. EPA's kitchen version is simpler: add at least 2 to 3 times the volume of browns to the volume of greens.

So choose your material on a completely different axis. Not "which one makes better compost," because none of them do. Choose for how long it will stand up to your weather and how easy it is to load and empty. A bin you can open from the front and fork out without climbing is worth more than any species of wood.

For what it is worth, I run two pallet bins. One of them falls apart every three or four years. Replacing it costs an afternoon and a handful of screws, and the compost has never noticed. You don't have to be extreme to be ready.

Frequently asked questions

What is the best material for a compost bin?

Untreated wood pallets are the best value for most people, at roughly $5 to $25 per Colorado State University Extension. They come with built in air gaps, they are easy to source, and they hold more than a cubic yard. Wire mesh is cheaper and faster but dries out and needs a cover. Concrete block lasts longest. All of them make the same compost.

Are all wood pallets safe to use for a compost bin?

No. Check the stamp. Under the international ISPM 15 marking that USDA APHIS describes, HT means the pallet was heat treated (wood core held at 56 degrees Celsius for at least 30 minutes, nothing added), while MB means it was fumigated with methyl bromide. Use HT pallets or unstamped domestic pallets. Skip MB, and skip any pallet with chemical staining or spilled oil.

Is pressure treated wood safe for a compost bin?

Modern pressure treated lumber is not the same product people remember. Clemson Cooperative Extension notes the industry voluntarily phased out arsenic and chromium preservatives in 2002, and EPA restricted CCA for residential use by 2004. Today's lumber uses micronized copper azole or ACQ, and University of Maine Cooperative Extension says ACQ products are not known to have toxicity issues or to leach into soils. Avoid pre 2004 CCA lumber, and never put treated wood scraps into the pile itself.

How big does a compost bin need to be to get hot?

At least 3 feet wide by 3 feet deep by 3 feet tall, which is 1 cubic yard. Colorado State University Extension calls that the minimum size for heating, and University of New Hampshire Extension agrees. Below that, the pile loses heat through its surface faster than the microbes inside can produce it. A well managed pile at that size can reach 130 to 160 F according to EPA.

What size mesh keeps rats and mice out of a compost bin?

Quarter inch hardware cloth in 16 to 19 gauge, which is what the National Park Service rodent exclusion manual specifies for excluding both mice and rats. Half inch mesh stops rats only, because a mouse can pass through a 1/4 inch opening. Line the base as well as the walls, and keep meat, dairy, fats and pet waste out of the pile so you are not advertising.

If the leaves are already coming down where you are, you have everything you need to start. Four pallets and some wire, or a roll of mesh and three stakes. Make it 3 feet on a side, put it on dirt, leave gaps in the walls, and let the leaves pile up. You can build the handsome cedar version next year, once you know where you actually want the bin to live.

Start with what you have. Build from there.

You never know, but you can always be ready.

Sources: Colorado State University Extension, Making Compost, University of New Hampshire Extension, Composting for the Home Gardener, Cornell Waste Management Institute, Excessive Pile Size, Cornell Composting, The Physics of Composting, Cornell Composting, Compost Chemistry, US EPA, Composting At Home, USDA APHIS, Export ISPM 15 Compliant Wood Packaging Material, Clemson Cooperative Extension, Treated Wood in the Landscape, University of Maine Cooperative Extension, Is Pressure Treated Wood Safe, University of Arkansas Extension, Wood and Wire Three Bin Turning Compost Bin, USDA Forest Service, Comparative Durability of Untreated Wood in Use Above Ground, National Park Service, Rodent Exclusion Manual.