Greenhouses & polytunnels

Greenhouse or Polytunnel: How to Choose the Frame and Covering and Order Well

Updated 21 October 2026

Greenhouse, polytunnel and roof shape

A polytunnel (or cold frame) is a low structure for early seedlings, opened from the top. A greenhouse is tall, and you can stand and work inside. Among greenhouses, the arched type is the most common: simple, cheap and good at coping with weather. A “house” type with a pitched roof gives more headroom, and a lean-to stands against a wall and saves space.

Choose the size to suit your plot and your crops: tomatoes and cucumbers need a height of 2 m or more, while for seedlings a low polytunnel is enough. The length is usually a multiple of the spacing between arches, so 3×4, 3×6 or 3×8 m are typical sizes.

The frame and covering decide everything

• Frame: galvanised tube, for example 20×20 or 20×40 mm; the wall thickness and the spacing of the arches matter (the closer the arches, the stronger the structure).
• Polycarbonate: 4 mm multiwall suits summer use, 6–8 mm suits a year-round greenhouse; look for UV protection.
• Plastic film and shade net: cheaper, but need replacing every year.
• Foundation: a timber beam or a metal base frame, otherwise the wind can lift the greenhouse.

What to put in your request

• type — greenhouse or polytunnel, arched, house type, lean-to;
• size — length, width, height in metres;
• covering — polycarbonate (thickness), film, glass;
• frame — tube (size), aluminium, wood; spacing of arches;
• fittings — doors, vents, automatic vent openers, shelves, heating;
• installation — with foundation, assembly only or delivery only;
• city and budget.

Example: “I need an arched 3×6 m greenhouse, 2 m high, 6 mm polycarbonate, 20×20 galvanised tube, two vents with automatic openers. Installation on a timber base, Toronto. Budget up to CA$840.”

How to compare offers

The same “3×6” doesn't mean the same product. Check the tube diameter and thickness, the arch spacing, the density of the polycarbonate, UV protection and the fixings. Ask whether the price includes doors, vents, thermal washers and end panels, and whether the covering has a warranty. Count delivery and installation separately, because transport for long parts can cost a good deal.

On BlaBlaPrice you don't drive around warehouses: one request, and makers and sellers reply in a chat with the specification, price and terms. They can see where their offer ranks and can improve their terms to win. The system shows the best-value offer first, with seller reviews alongside. Your contacts stay hidden. Learn more on the “How it works” page.

Common mistakes

People choose a thin frame, and then the greenhouse bends under wind and rain. They don't plan ventilation, and plants overheat in summer. They put the greenhouse in a shaded spot. They forget about watering: take a look at irrigation systems and hoses. To fill it, you will need soil and young plants.