How many bows the tunnel needs
Bows sit at a fixed spacing along the length, plus one extra to close each end. That "+1" is the piece people forget when they order pipe.
- Standard spacing. 4 ft on center suits most home tunnels. A 30 ft tunnel at 4 ft is 30 ÷ 4 = 7.5, rounded up and plus one, so 9 bows.
- Snow and wind. Tighten to 2–3 ft where snow load matters; each closer bow adds strength.
- Pipe per bow. One bow's arc length is R × θ, where the radius comes from the width and peak height. Widen the tunnel or raise the peak and every bow gets longer.
Sizing the poly film
Cover is the curved roof plus the two end walls, then a margin for the overlap that wraps each end and buries into the base. Roof area for a semicircular tunnel is about π × width × length ÷ 2.
| Tunnel (W × L) | Ground area | Approx. film needed |
|---|---|---|
| 12 × 24 ft | 288 ft² | ~950 ft² |
| 12 × 30 ft | 360 ft² | ~1,150 ft² |
| 14 × 48 ft | 672 ft² | ~2,050 ft² |
| 20 × 96 ft | 1,920 ft² | ~5,800 ft² |
Ground area is just width × length — it sets your growing space. Film always runs well above it because the roof is curved and the ends need covering too.
Where beginners get it wrong
- Under-buying pipe. Bows are only part of it — you also need a ridge purlin and two base rails running the full length.
- Skipping the anchor. Without wiggle-wire and buried base rails the film flogs loose and tears in the first real wind.
- No venting plan. A closed tunnel can hit 130°F on a sunny day. Roll-up sides or open ends are not optional in summer.
- Orientation. Run the ridge north–south so light falls evenly across the beds through the day.
Common questions
How far apart should hoop house bows be?
Four feet on center is the usual spacing for PVC or EMT bows. Drop to 2 to 3 feet in areas with real snow or wind load; going out to 5 or 6 feet saves pipe but risks collapse under a heavy snow.
How much poly film do I need for a hoop house?
Add the curved roof area to both end walls, then add about 10 percent for overlap where the film wraps the end bows and attaches to the base. A 12 by 30 foot tunnel needs roughly 1,000 to 1,200 square feet of 6 mil film.
How long does hoop house plastic last?
4 mil film lasts about 2 to 3 years, 6 mil UV-treated film 3 to 4 years, and 8 mil premium film 4 to 5 years. UV exposure, not tearing, is what usually ends its life, so buy film rated for greenhouse use.
