How to assemble a greenhouse kit

Inspired by a helpful YouTube guide. We based this walk-through on Vitavia Greenhouses' "How to Assemble a Greenhouse Kit". Vitavia are a well-established UK greenhouse manufacturer and the video gives a thorough step-by-step of their aluminium frame system, which is broadly representative of most kit greenhouses sold here.

1. Choose the right spot

A greenhouse needs as much light as possible — south-facing or south-west-facing is ideal. Avoid overhanging trees, which drip on the glass and drop leaves into the gutters. Leave enough clearance on at least one side to open the door fully and move in and out with a wheelbarrow.

Mind you, a sheltered spot matters too. A position exposed to the prevailing wind puts the structure under constant stress and makes ventilation harder to manage in summer.

2. Prepare a level base

A greenhouse must sit on a level base or the frame will rack and the glazing will not fit. You have several options: a concrete slab, paving slabs on a compacted hardcore bed, or a timber frame on ground anchors. Whichever you choose, check for level in both directions before you start building.

An out-of-level base by even a centimetre or two will cause panels to bind, doors to stick, and glazing bars to sit at odd angles. Worth spending time on this part before anything else.

3. Unpack and sort all the parts

Lay out all the extrusions, glazing bars, fixings and panels before you start. Match each component against the parts list in the instructions. Aluminium extrusions can look almost identical in a pile — check the lengths carefully.

Keep bolts, washers and glazing clips separate in small tubs. Using the wrong bolt length in the wrong place either strips the thread or pokes through the other side of the extrusion.

4. Build the base frame first

Assemble the base frame on the prepared surface and check it for square by measuring the diagonals. Equal diagonals means a square frame. If one diagonal is longer than the other, push the longer corners gently towards each other until they match, then fix the frame down.

Bolt it to the base using the anchor bolts supplied or rawlbolts into a concrete slab. A greenhouse that is not fixed down is a serious hazard in a gust of wind.

5. Erect the frame panels

Most kit greenhouses build up in panels — gable ends first, then the side walls, then the roof sections. Keep all bolts finger-tight until the entire frame is standing. Tightening as you go makes it difficult to correct alignment later.

Once all panels are in place and the frame is square, go round and tighten everything up properly. Work systematically from the base upwards.

6. Fit the glazing

Glass or polycarbonate sheets slide into the glazing bars from the top downwards in most kits. Glass first, then secure with the W-wire or glazing clips supplied. Do not force a panel — if it will not slide in, the bar above is probably not aligned correctly.

Wear gloves when handling glass. Cut edges are sharp. Polycarbonate panels are lighter and more forgiving, but protect the open ends of the flutes with aluminium tape to stop insects nesting inside them.

7. Fit the door, vents and gutters

The door hangs on a track and usually clips on at the end. Adjust the top roller and the bottom guide until the door slides smoothly without wobbling. Roof vents hinge at the top and are held open by a notched vent arm — check the hinge screws are tight before leaving them.

Clip the gutters into the eaves brackets and fit the downpipes. Gutters are sometimes an afterthought, but they really do extend the life of the base by directing rainwater away from the foundation.

8. Anchor and seal the structure

If the manufacturer supplies ground anchors, use them. Drive them into the ground at each corner and fix the base frame to them. In exposed gardens, additional anchors mid-span on each side are worthwhile.

Run a bead of greenhouse sealant along any joint between the base and the frame where rain could get in and pool. A wet interior floor in winter promotes disease and makes the greenhouse miserable to work in.

When to call a handyman

Call Richard if the base needs building from scratch, the kit is heavy and you need an extra pair of hands for the frame, or the glazing is proving stubborn. A greenhouse built properly in a day beats one half-built over three weekends.