Technical Installation · 7 min read
Greenhouse Installation Done Right: Siting, Base and Glazing
Glass forgives nothing. How to install any garden greenhouse properly — where it should stand, why the base must be dead level and square, and the assembly rules that keep every pane whole.
Level, Square, Then Glass
Installing a Greenhouse, Done Right First Time
Glass shows up every shortcut in siting, base and frame. Here's how we'd put one up — where it should stand, why the base must be perfect, and the assembly rules that keep every pane whole.
We've been hand-building garden buildings in our West Midlands workshop since 1995, and three decades have taught us that buildings forgive in proportion to their materials. Timber flexes and absorbs small errors. Glass forgives nothing. Put up true, square and unhurried, a greenhouse serves for decades; rushed onto a lumpy base, it binds at every vent and cracks panes for sport.
Everything below applies to any garden greenhouse, ours or another maker's. We've deliberately left out glazing systems, part counts and model-specific steps: the manual for your specific building is the authority — where it disagrees with any general guide, including this one, the manual wins. What manuals rarely cover is everything around the build: siting, base, weather and the first season — that's this article.
Siting: Decide This Before Anything Arrives
Light through the whole day
Watch the spot across a full day before you commit. A corner that's sunny at nine and shaded from noon is a half-day greenhouse — open sky from morning to evening beats the convenient corner.
Ridge running east to west
In winter the sun sits low, so run the ridge east–west with the long side facing south to catch that low winter light — June has light to spare, February doesn't.
Shelter from the prevailing wind
Wind steals heat and tests glass. A hedge or fence a little distance upwind takes the edge off the gusts without stealing the light — shelter, never shadow.
Well clear of overhanging trees
Branches drip long after rain, drop leaves and sap onto the glass, cast moving shade, and occasionally drop themselves. Give every tree a wide berth.
The base: stricter than any shed
For a greenhouse this isn't advice, it's physics: the base must be dead level and dead square — non-negotiable. A timber shed has forgiveness built in; a glazed frame has none. A low corner racks the frame, a racked frame turns rectangular openings into parallelograms, and glass — which does not bend — either refuses to go in or goes in under stress and cracks on the first cold night. Doors bind, vents jam, and draughts appear where panes no longer meet their seals.
We won't re-teach base building here — we've already done it properly: our base preparation guide covers slabs and concrete, the foundation options guide compares every route honestly, and if you're going the timber-frame way — tanalised (pressure-treated) timber, carrying the same 20-year anti-rot guarantee as our buildings — the timber base installation guide walks it end to end. Whichever you build: check level in several directions, check square by the diagonals, and don't start assembly until both pass. Still working out where the building can physically go? Start with measuring up your garden.
Assembly: The Rules That Keep Glass Whole
Your manual owns the bolt-by-bolt order; these principles hold for every glazed building we've seen go up well.
- 1
Wait for a calm, dry day
Glass in a gust is a sail with edges, and wet glass is slippery glass. If the forecast is doubtful, split the job: frame today, glazing when the wind drops.
Careful: Never glaze in wind. Not "take care in wind" — don't glaze in wind.
- 2
Two people, start to finish
One holds while one fixes; one steadies the pane while one seats it. Frames rack when worked single-handed, and glass gets set down "just for a second" in exactly the wrong place.
- 3
Lay out and read before you build
Unpack, check the parts against the manual's list, and read the whole sequence once with a cup of tea. A glazed building is far harder to un-build than a shed.
Tip: Store the panes flat on soft ground well away from the work — never leaned against a fence or the half-built frame.
- 4
Build the complete frame first — no glass yet
Assemble the full frame with every fixing snug rather than tight, so the sections can still shift a few millimetres. The frame gets finished and proven before a single pane comes near it.
- 5
Square it, anchor it, then tighten
Measure both diagonals corner to corner; when the figures match, the frame is square. Nudge until they do, fix the frame down to the base, and only now tighten everything. The frame must be square BEFORE the glass goes in — glass will not pull a crooked frame straight, it will simply refuse or break.
Tip: If the diagonals won't come right, re-check the base before blaming the frame — nine times out of ten the problem started underneath.
- 6
Glaze without hurry
Work pane by pane in the order your manual gives, gloves on, no reaching over unfixed glass. A pane that won't seat is information — the opening isn't true — so stop and find out why. Never force glass, and never rush the last panes because rain is coming — rain is a reason to stop, not hurry.
Careful: Forcing a pane that doesn't want to fit is how greenhouses draw blood. If it resists, the frame is telling you something.
- 7
Doors and vents last
Adjust doors and roof vents once the glazing is in and the frame has taken its final shape. Everything should move at a fingertip's push — a door or vent that needs a shove is out of true. Find that now, not on the first hot day of July.
Ventilation and Watering: Plan the Daily Routine In
Air, from day one
- Roof vents aren't optional extras — even a bright spring morning can overheat a shut greenhouse before lunch.
- Automatic vent openers earn their keep every day you're out by mid-morning.
- Aim for airflow low and high — an opening low down, vents at the ridge — so air moves through, not just in.
- In summer the door is a ventilator too: open early, close late.
Water, within reach
- Keep a firm, clear path to the door — wide enough for a full watering can in each hand.
- Site within easy reach of a tap or hose, or stand a water butt beside the house; rainwater is free and plants prefer it.
- Leave room to turn round inside — summer watering is a daily job.
- Think about where spilt water goes — a solid path beats bare earth turning to mud by June.
First-Season Setup
The building's up, the glass is whole — these jobs make the first year sing.
- Fit staging (benching) along one side — plants at working height, and room below for pots, compost and the can.
- Sort shading before high summer: external shade netting or a wash-on shading paint stops June and July scorching seedlings. Off again in early autumn — the low-light months need every ray.
- Hang a max–min thermometer and learn the house's moods before trusting anything tender to it.
- Nip round the fixings after the first few weeks of sun and wind, and re-seat any pane or clip that has crept.
- Wash the glass before the first winter — clean glass is free light, and winter is when you'll want it.
Greenhouse Questions, Answered Straight
Is a greenhouse base really stricter than a shed base?
Yes — it's the honest headline of this guide. A shed's timber frame absorbs small errors; glass doesn't bend, so level and square are pass or fail. If the base fails the spirit level or the diagonals, fix it before assembly — an afternoon's work now, or a binding door and cracked panes every winter after.
Can one person put up a greenhouse?
The frame, possibly, slowly. The glazing, no — one person steadies every pane while the other seats it, and a frame worked single-handed tends to rack. Plan for two throughout, on a calm, dry day.
What do I do if a pane won't go in?
Stop. A pane that won't seat means the opening isn't true — usually a frame tightened before it was squared, or a base problem beneath. Loosen off, re-square by the diagonals and try again. Forcing glass never ends with the glass winning.
What if I want growing space with more shelter and storage?
That's a potting shed's job — glazing for the growing side, solid tanalised timber for the rest, with the 15-year anti-rot guarantee our pressure-treated buildings carry. Have a look at our <a href="/shop/type/Potting%20Sheds">potting sheds</a>, or <a href="/contact">talk to us</a> about which suits your garden.
Glass, done the careful way
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