Greenhouse
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Most assembly problems begin before a part is unpacked, because the base sets the geometry for everything that follows. Mark out the footprint, check the diagonals to confirm it is square, and remove turf, stones and roots. A frame built on soft ground will settle and twist within a season, and that twist eventually cracks panes. Take the ground firm and level before any base material goes down.
For a hobby structure on ordinary garden soil, a timber or aluminium rail base on paving slabs or compacted hardcore is enough. Set the rails on a bed of sand for drainage, level them on both axes, and fix each rail with anchor pegs. For a long tunnel on an exposed plot, dig strip footings under the wall lines and set the base on piers every two metres. Write down the finished base dimensions before assembly, because the door and panels are built to those numbers.
| Base type | Where it works | Time |
|---|---|---|
| Timber rail on slabs | Level domestic gardens | Half a day |
| Compacted hardcore | Tunnels and simple kits | One day |
| Concrete footings | Large or exposed sites | Several days |
| Soil screw anchors | Obstructed ground | Half a day |
Open the boxes and check the contents against the parts list before starting, because a missing glazing bar discovered on day two stops the build. Lay components out in the order they will be used and follow the supplied instructions. The usual order is base assembly, corner blocks, eave rails and side frames, the ridge, the roof, then the glazing, the door and the vent covers. Building the frame square is the most important step, so check the diagonals twice.
Glass panels should sit on a continuous supporting bar along their bottom edge rather than floating between bars, and each pane needs a little free space at the edges so the frame can move without stressing the glass. Cut new panes slightly smaller than the opening, work from the bottom row upwards, and bed each pane on putty or glazing tape rather than silicone alone. Putty holds the pane and can be repointed years later, whereas silicone alone lets the glass creep and rattle. Work the tacks in from the corners outward.
Polycarbonate is handled differently. Cut with a fine saw and a clean blade so the flutes are not crushed, and drill holes slightly larger than the fixings to allow for the expansion between summer and winter. Fit the UV-stable face outward, since the fluted side of most multiwall sheets is not UV-stabilised. Place fixings on the flat ribs rather than across the hollow section, and use an aluminium capping strip or a seated washer to stop wind and water driving through the hole.
Staging turns a garden greenhouse from a shed into a growing space, and its height decides how much work your back will do. A working height of 75 to 80 cm is comfortable for most people standing, and it puts the crop closer to the light and away from cold air on the floor. Staging along both side walls with a narrow path in the middle gives more usable area than a single central bench, provided the structure is at least 180 cm wide.
Use slatted or mesh tops so air can move under the trays, and treat timber with a horticultural-safe preservative. Galvanised or powder-coated steel staging lasts longer and will not rot at the edges where it is repeatedly splashed. Plan power and water at bench level rather than floor level, because that is where you will plug in a propagator or run an irrigation line.
| Staging material | Durability | Best use |
|---|---|---|
| Galvanised steel | Very long lasting | Heavy pots and benches |
| Treated timber | Medium, needs upkeep | Potting benches and shelves |
| Mesh or slatted top | Long lasting | Seed trays and cuttings |
| Solid top | Long lasting | General crops |
Ventilation is the most important operating control in a small greenhouse, and most crop failures trace back to a structure that was too warm and too wet for too long. Roof vents remove hot air most effectively because warm air rises, so a ridge vent does more than a side vent of the same size. The rule of thumb is roughly one square metre of opening per ten square metres of floor, and vents only work if air can also enter through a cracked door. Automatic vent openers driven by a sensor remove the need to remember.
Shading lowers leaf temperature and water loss on hot days. Internal blinds mounted below the glazing are more effective than external ones, because they intercept sunlight before the glass converts it to heat. Shade during the brightest hours and ventilate at the same time. Humidity is the other half of the problem: condensation on the glass in the morning is normal, but persistent damp air encourages grey mould and botrytis. Remove dead leaves, space plants so air moves between them, and water in the morning.
| Control | Setting | Prevents |
|---|---|---|
| Ridge vent | Opens at 18 to 22 C | Heat build-up and scorch |
| Door and side vent | Cracked when possible | Stagnant humid air |
| Internal shade blind | 50 to 70 percent shade | Leaf scorch and wilting |
| Watering time | Early morning | Wet foliage overnight |
Hand watering with a can fitted with a fine rose is the most common method in a small structure and it has real advantages, because you see the compost and the plant every time. The drawback is that it is easy to water unevenly, especially in grow bags or large pots, and it takes real time with a full bench. A hose with a quality lance lets you move quickly between door and water butt, but many growers overwater because the flow is hard to judge.
Drip irrigation is the best compromise for a domestic greenhouse. A main pipe along the ridge or eaves feeds short drip lines to each pot, grow bag or bench, and because the water arrives slowly at the compost surface little is lost to evaporation or runoff. Capillary matting wicks water from a lower reservoir into the base of pots, but it needs cleaning to avoid carrying root disease between crops. Add a timer at the main supply and run the system manually before leaving it to a schedule.
| Watering method | Water use | Maintenance |
|---|---|---|
| Can and rose | Low if careful | Time only |
| Hose and lance | Medium | Minimal |
| Drip line with timer | Low | Flush lines regularly |
| Capillary matting | Low | Replace mat to avoid disease |
Most heated garden greenhouses use a small fan heater rated at roughly 1 to 3 kW, enough to hold a modest temperature above freezing on a cold night. Heaters with thermostats let you set a target instead of switching on and off, and models that distribute air along the length of the structure avoid a hot spot near the glass. Position the unit near one end at about bench height, on a stable surface with clear space around it.
Safety matters more than the heat output. Every heater needs an independent cut-out that switches it off if it overheats or tips, and a greenhouse full of dry plant material and plastic glazing is no place for a heater without one. Run the supply in outdoor-rated cable with weatherproof connectors, protected by a residual current device, and keep the plug outside where it stays dry. Electric heaters draw considerable current, so check that the circuit can carry the load. A thermometer at plant height tells you more than the thermostat setting alone.
The most effective pest control measure in a garden greenhouse is a physical barrier. Greenfly, whitefly, thrips and vine weevil all enter through open vents and gaps, and a greenhouse with insect screens over the vent openings removes most of that pressure. Screens are mesh stretched in a frame over the vent, and they are worth making yourself, since the mesh must overlap the vent edge to stop insects working around it. Check the screen every spring, because a torn corner undoes the system.
Doors matter more than most growers realise. A sliding door on a rail gives full width access without a door swinging into the staging, which matters in a narrow structure. Make sure it closes completely, that the threshold is not a trip hazard, and that the track is kept clear of grit, because a jammed door lets in pests and cold air. Birds are a common problem, so check the ridge and eaves for gaps where they can nest. Slug barriers around the base and a rodent-proof base will each prevent a recurring problem.
Cleaning the covering is not cosmetic. Dust on the outside reduces the light reaching the crop, and algae and moss on the north side hold moisture against the frame and block gutters, a common cause of a greenhouse that floods after heavy rain. Wash the covering in spring and autumn with a mild detergent and a soft cloth, and use a non-abrasive pad on polycarbonate, since scouring leaves a scratched surface that traps dirt. Keep the gutters clear of leaves.
Deal with repairs as they appear rather than saving them for winter. A cracked glazing bar, a slipped panel or a loose ridge clip lets water into the frame and rots the timber beneath. Replace broken panes with the correct thickness and re-bed them in putty, and tighten loose glazing bar fixings before they distort the panel. Check the frame for corrosion at the base and confirm the anchors are firm in the ground.
| Task | Frequency | Reason it matters |
|---|---|---|
| Wash glass and covering | Spring and autumn | Maximum light, clean panels |
| Clear gutters and ridge | Monthly in leaf fall | Prevents overflow and rot |
| Inspect frame and anchors | Twice a year | Catches rust and movement early |
| Test vents and door | Before winter | Keeps heat in and pests out |
| Service heater and wiring | Before cold season | Prevents failure during frost |
Glass and polycarbonate transmit sunlight, and the covering then traps the re-radiated heat. A closed greenhouse can rise 20 C above outdoor temperature on a still, sunny day, which is why ventilation is not optional. Roof vents and shading help.
Low humidity around the soil surface, combined with air that never moves, allows botrytis and grey mould to establish. Water in the morning, remove fallen leaves, space plants so air passes between them, and open the vents whenever possible. Lifting pots off a wet floor also helps.
Most breaks come from a frame that has twisted, so keep the base level, the anchors tight and the diagonals square. Avoid leaning tools against the glazing bars, and remove any panel with a star crack before it spreads. Bracing long sides also helps on exposed plots.
If the frame is sound, replacing the covering is often more economical than replacing the whole structure, and polycarbonate cuts heat loss considerably while making the panels lighter. Fit an aluminium capping strip over the fixings, allow for expansion, and check the frame will take the new sheet thickness.
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