7 Step Greenhouse Shading Setup with Bayliss Autovent on a 8-Metre Run

Step 1: Measure the Ridge Vent Throw First

Start with the roof lights, before ordering shade cloth. A Bayliss MK7 Autovent rated to lift 7kg will open a standard 600mm x 600mm hinged roof light to about 380mm of throw at 25C ambient. On an 8-metre run with three or four ridge vents, that open area decides how hard the shade cloth has to work.

Measure each vent aperture and the distance from the wax cylinder bracket to the vent arm pivot. A ridge vent at full throw removes more heat than one stuck halfway, so the amount of fabric needed over the glass changes with the actual travel of the vent, not just the length of the greenhouse.

The MK7 piston starts moving at about 17C and reaches full extension near 25C, with peak lift complete before 30C. If the shade is so dense that the interior stays below 22C on a clear day, the Autovent may never open fully. The result is stale, humid air over the crop even though the glass feels protected from direct sun.

Orientation matters as well. A north-south ridge takes afternoon load differently from an east-west ridge, and the door end of a long run often behaves differently from the far gable.

Step 2: Match Shade Density to the Crop

Green woven shade netting sold for amateur greenhouses commonly appears in 30, 40, and 50 percent densities. Tomatoes, peppers, and aubergines usually cope with 30 to 40 percent shade over an 8-metre run without losing fruit set. Ferns and propagation benches under glass are better suited to the 50 percent grade.

A single roll over the whole structure is a common error on a long house. The seedling bench and the fruiting crop do not ask for the same light. For a mixed planting, split the cloth: 30 percent over the tomato section near the door, 50 percent over the seedling bench at the far gable.

Rionet and Coolaroo both sell knitted shade cloth in two-metre widths, which fits an 8-metre run neatly as four panels with a short overlap at each seam. Knitted cloth also resists fraying after cutting. Cheaper extruded netting often unravels at the edge within one season.

Beth Chatto’s drought-planting principle from The Dry Garden belongs in the decision if a border runs cool and dry. Drought-adapted ornamentals such as Stipa or Verbascum want more light than salad crops, so they should not sit under the same dense fabric used for propagation or leafy growth.

Step 3: Clip the Cloth to the Glazing Bars

Clips gripping the aluminium glazing bar carry shade cloth without stressing the panes. Use bar clips at 400mm centres along an 8-metre run, roughly twenty clips per side for external shading. Internal shading on a wire run needs fewer fixings, but it leaves more heat trapped against the glass, so external cloth is the stronger choice for a structure that overheats.

Run a 3mm galvanised wire along the ridge and a second wire along the eaves. Tension both with a turnbuckle, then hang the cloth from the wires with plastic hooks. The cloth can then slide back on bright, cool spring mornings and be drawn across by late May.

A staple-gunned panel removes that flexibility. It also tends to bag in the middle of an 8-metre span where no intermediate support exists.

Step 4: Set the MK7 Spring for the Wind It Actually Sees

The MK7 ships with an adjustable tension spring to hold the vent closed against wind lift. On an exposed 8-metre run, ridge vents can chatter and slam in a gusting easterly, and that movement fatigues the wax cylinder mounting over a season.

Wind the spring tension up by two or three turns on the most exposed gable vent. Leave sheltered vents looser, so each one opens at roughly the same interior temperature despite different external pressure on the roof light.

Check the piston rod for grit twice a season. A fouled rod can stick at half-throw. From the path, the vent may look open, but the wax has not completed its stroke and the interior temperature can climb beyond the point the shade cloth can control.

Wipe the rod with a dry cloth and leave oil off it. Lubricant collects dust and makes sticking more likely. The Gardman and Exhart copies of this mechanism use a different piston compound that drifts in calibration faster, while a genuine Bayliss cylinder holds its temperature band more reliably across three or four years.

Step 5: Change the Watering Once the Shade Goes Up

Shade cloth reduces transpiration, so the watering schedule needs to move with it. A Hozelock automatic watering kit running from a 2-litre-per-hour dripper line can overwater a 30-percent-shaded tomato bed if the controller stays on its pre-shade timing.

After the shade goes up in late May, drop the controller from two cycles a day to one. The surface of the compost gives a better reading than the timer once the glass is covered and the root zone cools.

Lay the drip irrigation before the shading. Threading dripper line under a tensioned shade panel on an 8-metre run is awkward, and the work often dislodges clips that were already sitting correctly.

Run the main line along the back wall. Take 4mm spur lines to each grow bag or border station. Fit a pressure-reducing valve at the tap so the Hozelock end-line drippers deliver more evenly across the full length.

Without that valve, the drippers nearest the tap discharge faster than those at the far gable. The difference is easy to miss until the near bags are wet and the end plants are still asking for water.

A 50mm layer of mulch over border soil makes the reduced watering last longer. It also keeps the root zone cooler than bare compost under the same shade.

Step 6: Put the Jobs in Seasonal Order

Mulch goes down first, in April, while the soil is still holding winter moisture. The drip line follows, then the shade cloth in mid to late May as the sun angle steepens, with the Autovent already installed and calibrated from the previous autumn.

That order keeps the June work out of the hottest part of the job and avoids crawling over established dripper line while fixing shade fabric above it.

Step 7: Let the Crop Show Whether the Cloth Is Right

Pale, hard, slow-growing tomato leaves under heavy shade point to too little light. Remove a panel or swap a 50-percent section for 30. Bleached or curled leaf margins on a bright day point the other way: the shade went up too late, or the density is too low for that orientation.

Sweet pea Spencer cultivars grown for cut flowers in a cooler lean-to section show heat damage quickly. Spencer types bleach their bloom colour under direct glass heat above 25C and hold petal substance better under 40-percent cloth.

Dense shade can leave the Autovent only half-open, and that is where botrytis becomes a risk on the same Spencer blooms that were shaded to protect their colour. The cloth has done its job on direct sun, while the roof light has not moved enough air through the house.

Pest pressure shifts under shade as well. Cooler, moister border soil favours slug activity. A Nemaslug nematode application watered into the shaded bed in spring suppresses the population before the canopy closes over and makes hand-picking impractical.

On a long run, separate panels make adjustment easier than one continuous sheet. A 30-percent cloth can remain over the hot tomato end while a 50-percent section stays over propagation, and the Autovent spring can be set differently on exposed and sheltered vents.

Even with those adjustments, the door end can still read hot while the gable end stays damp, even after both halves have been shaded.