Hay Tarp Buying Guide: Size, Material & Anchoring for Bale Stacks
Updated: May 27, 2026
Why Hay Tarps Matter: Dry Matter Loss Data
Uncovered hay loses 10–35% dry matter over a single storage season from surface spoilage, weathering, and leaching. Properly tarped hay loses 1–5% dry matter. The economics are significant:
| Storage Method | Dry Matter Loss | Notes |
|---|---|---|
| Uncovered outside | 25–35% | Highest loss — surface spoilage, runoff |
| Tarped outside (round bales) | 5–15% | Depends on tarp quality and anchoring |
| Tarped outside (square bales, good tarp) | 3–8% | Best outdoor option |
| Inside barn (ventilated) | 2–5% | Standard indoor storage |
| Inside barn (sealed) | 1–3% | Best quality preservation |
A 5% dry matter loss on 100 tons of hay = 5 tons of lost feed value. At $200/ton, that’s $1,000 saved by proper tarping.
What Size Tarp for Hay?
Use the formula:
Tarp length = stack length + (height × 2) + (tuck-under × 2)
Tarp width = stack width + (height × 2) + (tuck-under × 2)Standard tuck-under: 2 ft per side (minimum). Use 3 ft for windy locations. For the full step-by-step formula and stack shape variations, see How to Calculate Hay Tarp Size.
Common square bale stack sizes:
| Bales Stacked | Stack Footprint | Stack Height | Tarp Needed |
|---|---|---|---|
| 4 rows, 20 bales | 20 × 8 ft | 8 ft | 32 × 28 ft → buy 30×40 |
| 4 rows, 40 bales | 40 × 8 ft | 8 ft | 48 × 28 ft → two 25×30 |
| 6 rows, 60 bales | 40 × 15 ft | 10 ft | 54 × 39 → two 25×40 |
| 8 rows, 100 bales | 50 × 20 ft | 12 ft | 68 × 48 → custom |
What to Look for in a Hay Tarp
Thickness (Mil Rating)
- 5 mil: Minimum for hay storage. Light wind and UV exposure.
- 6–8 mil: Recommended for most farms. Handles moderate wind and 2–4 seasons of UV exposure.
- 10–12 mil: Heavy duty. Windy areas, multiple seasons of use.
Avoid tarps below 5 mil for hay — they tear at grommet points under wind stress. For a full material comparison (PE vs canvas vs vinyl vs silage film), see the Tarp Types Guide.
UV Stabilization
UV stabilization (UV inhibitor added to the PE coating) is essential for outdoor hay storage. Unstabilized PE tarps degrade in 1 season in direct sunlight — they become brittle and develop cracks that allow water in.
Look for: “UV stabilized,” “UV treated,” or “UV protected” on the label. Not the same as “UV resistant” (a weaker standard).
Grommet Spacing and Size
- Standard grommet spacing: 18–24 inches on all edges
- Grommet size: 3/8” or larger for hay tarp anchoring
- Reinforced corners: Double-layered PE at each corner grommet
Grommets are the first failure point on hay tarps — the tarp tears at the grommet before the main body fails. Look for aluminum (not steel — steel rusts and transfers rust stains to hay).
Color
- White or silver: Reflects heat — keeps hay cooler under the tarp. Reduces UV degradation from inside the tarp.
- Black: Absorbs heat — not recommended for hay. Can overheat surface bales and cause quality loss.
- Blue / green: General purpose. Moderate heat absorption. Fine for hay.
Anchoring Methods
Tuck-under (standard): Tuck 2–3 ft of tarp under the bottom bale row on all sides. The weight of the bales is the primary anchor. This is the most secure method for square bale stacks.
Bungee net: A net of bungee cords draped over the top of the tarp and attached to stakes or the bottom bale row. Good supplement to tuck-under in windy areas.
Tire weighting (round bales): Place used tires on top of the tarp at regular intervals. More practical for loose-stacked round bales where tuck-under isn’t possible. For silage tarps, continuous tire weighting across the entire surface is mandatory — see the Silage Tarp Guide for details.
Stakes and rope: Pass rope through grommets and stake into the ground. Use ratchet straps for high-wind areas. Not sufficient alone — always combine with tuck-under or tire weighting.
Tarp Maintenance
Before storage season:
- Inspect all grommets — replace tarp if more than 2 grommets have pulled through
- Patch any tears with PE tarp repair tape before laying the tarp on hay
After storage season:
- Remove and fold clean
- Dry completely before storage (moisture + heat = mold inside folded tarp)
- Store away from UV exposure (in a barn or shed — not outdoors on a fence post)
A properly maintained 8 mil UV-stabilized PE hay tarp lasts 4–6 seasons.
Use the Tarp Size Calculator — select Hay / Silage Stack for instant tarp size calculation.
See also: How to Calculate Hay Tarp Size and Tarp Types Guide.