Coroplast is corrugated plastic with hollow flutes running through it. It is waterproof and belongs outdoors. Foam board is a foam core between two paper facing sheets. It looks better indoors and delaminates permanently once it gets properly wet. They are not interchangeable, and the difference is the facing material, not the thickness.
These two get confused constantly, including by people selling them. Both come in 4mm. Both print full colour. Both look flat and rigid in a product photo. And one of them turns into a warped ruin after a single afternoon of Florida rain.
The short answer
| Coroplast | Foam board | |
|---|---|---|
| What it is | Corrugated polypropylene, hollow vertical flutes | Foam core, paper or plastic facing both sides |
| Waterproof | Yes, completely. Absorbs nothing | No. Facing absorbs, core delaminates |
| Outdoors | Months to a season | Days, and only if dry |
| Stakes into ground | Yes, legs slide inside the flutes | No. Nothing to slide into |
| Surface | Slight ribbing visible in raking light | Perfectly flat, better for photography |
| Weight | Heavier | Lighter |
| Typical use | Yard signs, directionals, real estate, campaigns | Easels, trade show boards, presentations, retail |
If the sign is going in the ground or outside, the answer is coroplast and there is nothing more to decide. Everything below is for the cases where it is genuinely a choice.
Real thickness and real prices
Both materials are sold in millimetres. Here is what each one actually measures, and what each costs at three sizes for a single sign printed one side.
| Material | Thickness | 6 x 24 in | 12 x 18 in | 18 x 24 in |
|---|---|---|---|---|
| 4mm coroplast | 4mm / 0.157" | $16.48 | $11.80 | $17.10 |
| 4mm foam board | 4mm / 0.157" | $18.58 | $20.88 | $25.70 |
| 6mm coroplast | 6mm / 0.236" | $20.92 | $24.95 | $31.58 |
| 8mm coroplast | 8mm / 0.315" | $22.15 | $27.40 | $35.25 |
| 10mm coroplast | 10mm / 0.394" | $24.80 | $27.98 | $37.00 |
Prices as of September 2026, single sign, printed one side, standard turnaround. Per-sign cost drops substantially at higher quantities. Check the product page for current pricing.
Two things in that table are worth pausing on.
Foam board is the more expensive of the two 4mm options at every single size. At 18 x 24 it is 50 percent more than coroplast. You are paying for a flatter, better looking surface, not for durability. Anyone who tells you foam board is the budget option has it backwards.
12 x 18 in 4mm coroplast at $11.80 is the cheapest sign in the range, cheaper even than the smaller 6 x 24 rider size. That is not a mistake. 12 x 18 is the volume size the industry gangs and runs constantly, so it carries the lowest setup share. If your message fits in 12 x 18, that is the size to order.
Why foam board fails outside, specifically
It is worth understanding the mechanism, because it explains why there is no partial solution.
Foam board is a sandwich. The middle is extruded foam, which is closed cell and does not much care about water. The outside faces are usually paper, bonded to the foam with adhesive. That paper is what your artwork prints on.
When the facing gets wet it swells, and the adhesive bond between paper and foam releases. The two materials then expand and contract at different rates as they dry, which is what produces the characteristic curl and bubbling. It is permanent. The board does not flatten out again once it dries, because the bond that was holding it flat is gone.
This is why you cannot rescue an outdoor foam board sign by laminating it or by spraying it. Water gets in at the cut edge, which is exposed foam and exposed paper on every single sign, and works inward from there.
Coroplast has no equivalent failure mode. It is a single extruded polypropylene material with air in the middle. Soak it, freeze it, leave it in a sprinkler, and it is unchanged. What eventually limits a coroplast sign is the ink fading under UV, not the substrate failing.
Flutes, and why only coroplast takes a stake
The flutes are the hollow channels running through coroplast, like miniature corrugated cardboard. They are the reason a coroplast sign needs no drilled holes to mount in the ground: the two vertical legs of an H-stake slide up inside the flutes and grip by friction.
This only works if the flutes run vertically relative to how the sign will stand. On a portrait sign they usually already do. On a landscape sign, the sheet may need to be cut with the flutes running the short dimension instead, which is a different production orientation. That is what an H-Stand Compatible option controls when you see one on an order form.
Get it wrong and the stake legs meet a solid wall of plastic and will not go in at all. People then drill the sign, which weakens it exactly where the load is.
Foam board has no flutes. There is nothing for a stake to slide into, and pushing a stake into the foam core just crushes it. Foam board signs go on easels, in frames, taped to walls, or on a stand. Not in the ground.
How long each lasts, honestly
Coroplast outdoors: the plastic itself lasts years. Ink is the limit. In full Florida sun expect good colour through a season and visible fading beyond that. In shade, considerably longer. Campaign and open house signs are almost always retired before the material is.
Foam board outdoors: one dry week is realistic. One rainstorm can be enough. Under cover, on a porch or in a window, much longer, because the enemy is liquid water rather than humidity.
Foam board indoors: indefinitely. It is a genuinely good indoor material and this is where it earns its price premium. A trade show board or a reception sign in foam board looks better than the same artwork on coroplast, because there is no flute ribbing catching the light.
Where foam board is the right answer
It is easy to read the above as foam board being the worse material. It is not. It is the better material in its own place.
- Trade show and event boards. Lighter to carry, flatter to look at, no ribbing under booth lighting.
- Anything photographed. Coroplast flutes show as faint vertical lines in raking light. On a board that appears in photos, that is a real drawback.
- Easel and A-frame inserts. Light enough not to tip a lightweight easel.
- Presentation and pitch boards. The flat surface reads as more finished and more premium in a room.
- Retail and point of sale. Indoors, dry, and appearance matters more than durability.
- Window displays. Behind glass, so weather is not a factor.
Where coroplast is the only answer
- Anything staked into ground
- Anything outdoors for more than a couple of dry days
- Real estate and open house signage
- Political and campaign signs
- Event directionals and parking signs
- Construction and job site signs
- Anything zip-tied to a fence
What about thicker coroplast?
Thickness buys stiffness, not waterproofing. All corrugated plastic is equally waterproof regardless of grade.
- 4mm is the standard and correct choice for almost everything up to 18 x 24 that comes down within a few months.
- 6mm and 8mm hold their plane through a full season and across wider panels that would otherwise start to bow.
- 10mm is for large exposed panels only. Note from the table that it roughly doubles the 4mm cost, so it is a real decision rather than a small upgrade.
Common questions
Is coroplast or foam board better?
Neither in the abstract. Coroplast for anything outdoors or staked in ground, because it is genuinely waterproof and takes an H-stake. Foam board for anything indoors where appearance matters, because it is flatter and has no flute ribbing.
Can foam board be used outside?
Not for long. The paper facing absorbs water and delaminates from the foam core, and the warping that follows is permanent. A few dry days is realistic. One rainstorm can end it.
Is foam board waterproof?
No. The foam core resists water but the facing sheets usually do not, and water enters at every cut edge. Coroplast is the waterproof option.
Which is cheaper, coroplast or foam board?
Coroplast, at every size we print. At 18 x 24 foam board is about 50 percent more. Foam board costs more for a better looking surface, not for better durability.
Can you put a stake in foam board?
No. There are no flutes for the stake legs to slide into, and driving a stake into the foam core crushes it. Foam board needs an easel, a frame or a stand.
What does 4mm coroplast mean?
The sheet is 4 millimetres thick, about 0.157 inches. It is the industry standard weight for yard signs and the most widely stocked grade.
Does coroplast show lines in the print?
Slightly. The flutes can be seen as faint vertical ribbing in raking light, particularly on large flat areas of solid colour. From ten feet away it is invisible. In a photograph taken at an angle it is not.
How long does a coroplast sign last outdoors?
The material lasts years. The ink fades first. Expect good colour for a season of direct sun, longer in shade.
Is foam board the same as foam core?
In practice yes, the terms are used interchangeably for the same foam-and-facing sandwich. Branded variants exist with tougher plastic facings instead of paper, which perform better but are a different product and price.
Ordering either one
For coroplast, decide your mounting before you decide your grommets. On an H-stake you want no grommets at all, because the legs go inside the flutes. Zip-tying to a fence means corner grommets. Large exposed panels want grommets every two to three feet so wind load spreads across many fixings.
For foam board, decide how it is being displayed, since nothing about the board itself will hold it up.
Either way, artwork gets a free hand check before production. On signs the most common thing we catch is type that will be unreadable at the distance the sign is actually viewed from.
More detail on thickness, grommet patterns, stake counts and sizing for reading distance is in the complete yard sign buying guide.