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Whether you’re using an existing foundation or building your own from concrete, brick or paving slabs, it needs to be level. Check it’s flat with a spirit level, and make sure the diagonals are the same length (if it’s not in line, the glass panes won’t fit). When building a base, check the exact size of the frame before starting: although often described as 8x10ft or 6x8ft, these dimensions are approximate and frames are usually bigger.
It’s a good idea to put all the clips and bolts into separate pots before you start, so you don’t lose them. Lay out the pieces of frame, assemble each section and attach together. Then carry the sections to your base and screw them in place. Fasten roof sections, roof bars and corner strengthening bars, but don’t over-tighten. At at this stage, you’re just holding things together until you add the glass, at which point further adjustments may be needed.
Having assembled the door, you can fit the door runners when the rest of the frame is complete. Then slide the door onto the upper, then lower, tracks. Adjust the height of the top track until the door is running smoothly. If you have them, this is a good time to install automatic window openers and louvre vents. These help to keep an even temperature inside the greenhouse, avoiding spikes of heat in summer, which could scorch tender plants.
Buying a greenhouse from a supplier, then assembling it yourself can save you hundreds of pounds on construction costs - and the bigger the structure, the more you'll save.
Line the glazing bars with foam glazing tape or rubber strips. These will form a seal between the greenhouse frame and glass and help cushion the panes. You can use a pane to check your corners are square, and a spirit level to make sure everything is level. This will ensure the glass is securely and tightly supported. Glaze the roof first, while you can still move around freely inside and outside the structure. Once the roof is on, fix the wall panes.
Tighten it up as you’re building - you need to keeping checking the bars with a spirit level. It is vital to keep the uprights and horizontals square or the glass won’t fit snugly. You may need to make minor adjustments as you go. Once you’ve fitted all the glazing, you can go round the again and tighten all the bolts. Take care not to over-tighten, as bolts can snap and you may even crack the glass. Once everything above ground is secure, you can finally tighten the base fittings.