Kit review - Glasshouse structures

Replacing lettuce grower PR & AJ Mercer's glasshouse at Moss Farm Nurseries has boosted crop management but there were challenges on the way, Sally Drury reports.

PR & AJ Mercer: glasshouses are packed with germinating and young lettuce plants - image: HW
PR & AJ Mercer: glasshouses are packed with germinating and young lettuce plants - image: HW

You might think that replacing one glasshouse with another would be a relatively easy job. Take one down and construct the other in its place. Sounds simple, but what if the replacement glass is a used block sourced from the Netherlands? At PR & AJ Mercer - lettuce producer at Moss Farm Nurseries - constructing two acres of glass to tie in neatly with the existing facility was something of a challenge.

Located on the rich fertile soils of the important growing area around Banks, north of Southport, in Lancashire, Paul and Amanda Mercer have been building their business since they moved to the site in 2002. Today they receive ready-seeded blocks from Sunnyside and grow the lettuces on until ready for transplanting. From Christmas until September, the glasshouses are packed with germinating and young lettuce plants, mostly Iceberg and Little Gem but with some Cos too.

The main objectives of the houses are to germinate and bring on the seedlings. Spared from the variability of the weather, the lettuces have to be kept frost-free in the colder months and cool in the hotter months. Irrigation also needs to be controllable so glasshouses are essential for the main stage of production. Having suitable glasshouses, however, is instrumental to the production of quality plants and can simplify production. A greater area of glasshouse would help the business to expand.

Removing the original wooden-sided block would not only give a more optimum environment for the plants and their production but would also allow a glasshouse twice its size to be built in its place. The old block backed onto a shed to the north. Removal of both began in July last year. At the end of August Ebtech Glasshouse Systems arrived to implement the replacement with previously used glass sourced from the Netherlands.

Adding in the challenges

Now we can add in the challenges. The shed that was removed butted onto a glasshouse lying to the north. The new glasshouse had to join that glasshouse, which sat at a higher level. On the western side of the old wooden-sided block there was a narrow gap and then a further timber-framed block of glass. Opposite, a wider gap separated the house from a newer, substantial and higher aluminium-framed block. Again the new block was to be joined. In the north-west corner, another shed juts out into the construction side, giving a difficult "L"' shape with which to contend.

Ebtech Glasshouse Systems set about the challenges of differing levels to the north and east, plus the corner of a shed that needed to be accommodated into the new block, and completed the task by late December 2015.

"It was a challenge because the new block had to be attached to the existing block to the east, then come round the corner of the shed and attach to the block lying to the north," explains Ebtech Glasshouse Systems director Tony Walker. With 28 years of experience in the horticultural industry, Walker has worked with big names including Van Vliet and Bridge Greenhouses before setting up his own business and then joining a partnership with Matt Blood in the formation of Ebtech Glasshouse Systems Ltd in 2014.

The company, based in Hull, East Yorkshire, specialises in the construction and equipping of commercial glasshouses, plus the recycling of buildings, but also undertakes refurbishments, alterations and repairs as well as turnkey projects. Also under the Ebtech umbrella, EBTech Biomass Energy Systems is an independent supplier and installer of biomass systems for horticulture, agriculture and commercial installations.

At PR & AJ Mercer, Walker shows me around the new block and points out the first challenge. "We had to join the new house to this older house. That meant attaching the trellis girder to the support of the existing house, which was lower, but needed to get the ridge height and gutter height exactly the same," he says. "Then the next challenge was to come round the corner of the shed and attach to the gable end of another block (to the north) that is lower and then seal it all in to make it weatherproof."

Now bear in mind the all-important bay width. This was selected to help with setting down the trays of blocks, all done by hand, and more importantly to accommodate a Jon Denton overhead boom irrigation system. The bays had to be 9.6m in width. Keeping to that measurement proved to be one of the trickiest points of the project.

"Coming round the corner of the shed, because the bays are 9.6m, we had to a do a slightly smaller one to make it fit, giving a smaller eave. So we've gone from attaching to a corrugated building, trying to keep the bay sizes the same for Paul to have his gantry irrigation," says Walker. "This was the most difficult bit because of the smaller eave and then all of a sudden we have to come down onto a lower gable end. But it has connected and it's tidy, neat and structurally sound. It works perfectly - a smaller trellis and a smaller bay. Then we attached some aluminium profile and expansion rubber to the existing shed."

Overcoming wet weather

With all the metalwork in place, glazing is normally a straightforward job. In this case, however, it took longer than expected thanks to the weather. "It was December and it was windy, but it was the rain that caused problems. It got very boggy. Paul had to dig trenches to let the water flow away and it was only when we got the roof in place that the soil could start to dry out," Walker explains.

Outside, the new block has been complemented with the same area of hardstanding for hardening off purposes. A narrow space between the shed and the northernmost house, perhaps some 50m in length, is at the moment unused. But what will happen in the future? In case access is required, Ebtech added a Perspex window with handles. Cheaper than a door, this access route is available should it ever be needed.

Inside the house, growing a first-class crop of lettuce seedlings when I visited, the boom irrigation system is in place. Basic ventilation is provided on both sides of the ridge by a Rail Mech mechanism with triple-pane vents. Heating, mainly for frost protection, is by direct-fired Priva air heaters. Environment monitoring and control is handled through a Priva computer.

Rainwater is harvested from the roofs via galvanised steel gutters with Bristol Mastic Coating. Water is collected at two points from the new block and passes through PVC pipes to be stored in a lagoon ready for pumping to two LS Systems' holding tanks prior to use via the booms.

At the end of the day it comes down to attention to detail, especially measuring it right, and then having good on-site guys who have experience and know what they are doing.

Amanda Mercer, one half of PR & AJ Mercer, is pleased with the work and praises all involved. "It was complicated but it has worked well. They were a great team," she says. But what does it mean from a production viewpoint?

Paul Mercer says: "It's easier to grow in the new house. The temperature doesn't peak like it tends to in the little ones and the dimensions mean it is a lot easier to put the plants down. It's a lot simpler and much easier to harvest."

Amanda adds: "The wider bays give more plants and it's easier to manage because there is more air circulation and the extra width can accommodate the irrigation." Referring to the older glass to the west of the new block, due for replacement in the near future, she says: "The remaining timber block does grow excellent plants but it is harder work. It is quite low, everything is in the way and we can't use gantry irrigation."

For Walker, the project means much. "Every project is different. Here I learnt a lot," he says. "To join together different orientations, different sizes and different materials is challenging but it has fitted together nicely and is giving benefits to the grower." He looks across at a live mains gas valve sticking up through the concrete pathway and adds: "It was challenging for the construction guys too. Hopefully the next section will be easier.

Key People

Amanda Mercer - Partner, PR & AJ Mercer, Moss Farm Nurseries, Banks, Lancashire

Paul Mercer - Partner, PR & AJ Mercer, Moss Farm Nurseries, Banks, Lancashire

Tony Walker - Director, Ebtech Glasshouse Systems (Hull-based glasshouse services company)


Previously used glasshouse, two acres in area, sourced from the Netherlands and installed at Moss Farm Nurseries in Lancashire for lettuce producer PR & AJ Mercer.

Width 57.6m comprising five bays of 9.6m and one of 4.2m

Length: 32m

Height: 3.5m from top of foundation to gutter

Vents: Three-pane with Rail-Mech mechanism

Irrigation: Jon Denton gantry irrigation

Heating: Priva direct-fired for frost protection only

Control: Priva computer

Materials used: Concrete dollies, Zeus prefab foundation panels, steel trellis, steel supports, steel wind bracing and purlins, steel gutters, aluminium roof and glazing bars, aluminium side and end gables, 4mm-thick glass

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