Greenhouse Construction Standards
Commercialare manufactured to a code of practice based on the minimum acceptable standards. This takes into account (1) the structure as a whole, (2) individual structural elements such as trusses, stanchions, purlins, joints, fixing, foundations, (3) cladding elements (glazing bars, etc).
Amateur greenhouse manufacturers have not perhaps given such detailed study to their designs as these products are not subject to government approval for grants, but minimum construction standards should ensure that all greenhouses are stable enough to withstand very strong winds, snow load and crop weight, this last being of particular importance; greenhouses can, for example, easily be damaged by a very heavy crop ofsuspended from the roof or from wires stretched between the ends of the greenhouse.
To lay down section sizes for the component parts of a greenhouse poses problems, owing to the differing bulk/weight ratios involved; astragal bars of Western red cedar 5 x 5cm (2 x 2in) will not be as strong as similar sections of Baltic redwood, for instance. Nor is it easy to give exact roof angles, height to ridge, height to eaves, size of doors, and all the other relevant measurements. A minimum eaves height of about 1.5m (5ft) gives adequate headroom but is not really sufficient; 1.65 to 1.8m (5ft 6in to 6ft) is better. The ridge height is commensurate with eaves height to give a roof pitch of from about 14-18° for Dutch light houses with no glass overlap, up to 35° for overlapped glass, with an average of 25.5°. Teething troubles can occur with a new model of motor car, and similarly with a new design of greenhouse, especially over mechanical items such as vents.
For the do-it-yourself enthusiast wishing to build a greenhouse there is a fairly wide margin of tolerance in design and, most amateur joiners tend to err on the side of safety by using heavier timber than is really necessary.
All nails used in timber greenhouses should be galvanized, and screws and bolts in all alloy structures should be of alloy or brass.
Constructional standards for greenhouses
There are accepted standards for commercial greenhouses in the UK. These vary according to regional location. Standards for amateur-sized greenhouses are less stringent but basic principles are the same.
This is to building design standards as shown in BS 6399 Part 3 1988. It varies from 0.3 KN/m2 (30 Kg/m2) in parts of southern England to 1.0 KN/m2 (98 Kg/m2) in parts of the Scottish highlands. Conversion from ‘snow on ground’ to ‘snow on roof depends on the shape factor but a good average for a single span greenhouse is 0.8 KN/m2 imposed load on roof.
Many ‘run of the mill’ greenhouses can only withstand 0.5 KN/m2 and ‘cheapies’ even less which shows their vulnerability in the wrong situation.
Depends on dynamic pressures and on height and shape. In parts of southern England 38 m/sec (82 mph) is acceptable, while in most of Scotland it is 52 m/sec (113 mph). An accepted standard for domestic greenhouses is a dynamic pressure range of 0.6 to 1.0 KN/m2 but this can be affected much by building shape.
Many ‘cheapies’ can only stand 25 m/sec (54 mph). Well-constructed greenhouses can stand up to 52 m/sec (113 mph).
BS 5502 requires that commercial greenhouses withstand a crop load of 0.150 KN/ m2 (15 Kg/m2). Amateur or domestic greenhouses are seldom likely to require this level of crop load. Very cheap greenhouses may, however, not stand much crop load at all.
Standards for overall stability of green- houses tend to be couched in broad terms as they much depend on shape, height and various other factors. It is, however, a relatively simple matter to check the general stability of a greenhouse by exerting sideways pressure at gutter height or in the middle of glazing bars or astragals. Excessive movement in any greenhouse will predispose the structure to glass slippage, glass breakage, ventilator damage etc. The need to consider these issues and pay accordingly has already been stressed elsewhere.
Standards and specifications for plastic greenhouses
There has, in recent years, been considerable upgrading of the standards required for plastic greenhouses. Reputable companies have carried out considerable research into plastic greenhouse design and have banks of information on computer when, by referring to various wind speeds across the country, a plastic greenhouse can be designed for virtually any conditions. The critical issues for the structure itself relate to tube diameter, tube wall thickness, spacing of hoops, roof and end stress bars etc.
Companies such as Clovis Lande Associates Ltd rigidly follow this procedure and their structures vary from 30 m/sec (67 mph) and a snow load of 460 N/m for the 14 ft wide model up to structures such as the crossbrace Highlande 21 which can stand a wind speed of 63 m/sec (141 mph) and a snow load of 745 N/m. It is important to note that these figures refer to the structure itself and not the coverings which themselves vary considerably in ability to withstand wind stress and snow load according to thickness, age and other factors. In extremely exposed areas such as the Western Isles of Scotland it may be necessary to resort to strengthened covers rather than standard grade polythenes.