Buying and Erecting Greenhouses

Buying and Erecting Greenhouses

Buying and Erecting Greenhouses

It is logical to think about the selection of a greenhouse before considering erection procedures, heating, ventilation, and other related matters, but it is imperative to understand the rudiments of these important matters before actually confirming the greenhouse order in precise detail. It happens far too often that people innocently buy greenhouses only to find out that they are insufficiently ventilated, not wide enough, or have other deficiencies.


Greenhouse buying can be approached in a variety of ways — you may send off for a catalogue you have seen advertised, or you may come across a greenhouse display at a flower show or garden centre. The information contained in greenhouse catalogues varies greatly: some are detailed in the extreme, some frustratingly imprecise. On the whole, however, they provide sufficient detail to enable one to make a selection even if technical issues such as stress and strain factors and general stability are seldom referred to.


Points to note when ordering:

1. Structural materials Select for stability, lack of maintenance and aesthetic appearance. Glazing system is most important.

2. Size and dimensions Largely a question of suiting the greenhouse to the site, its size to intensity of use, and cost to money available. A frequent mistake is to purchase too small a greenhouse and soon find it bursting at the seams. Comparatively speaking, a larger greenhouse is a better investment than a smaller one on a sq m (or ft) basis, this being readily ascertained by dividing the sq unit area obtained into the total price. There is not a great difference in price between some alloys and conventional wooden structures, especially if the latter are of superior wood.

3. Ventilation There is a distinct lack of reference in many catalogues to the actual ventilation area in relation to the actual area of the greenhouse, and this information is necessary in order to work out whether or not the design gives a satisfactory rate of air change. Methods of calculating ventilation requirements are given elsewhere and these should be studied before specifying the number of vents required. Very little reference is made in many amateur greenhouse catalogues to fan ventilation, although it is the most positive way of achieving air change, provided one does not mind paying for electricity.

4. Number of doors One door is standard in amateur greenhouses. Two doors would only be necessary where a greenhouse is large enough to merit them. Sliding doors are now almost standard, and these have great advantages.

5. Constructional base blocks These are frequently offered, particularly with wooden greenhouses, and they are worth purchasing as they not only ensure a tight seal between the sill plate and the foundations, but make sure that the greenhouse is firmly secured to the ground. They also assist in simple, quick erection. Base blocks can be a hazard around the door. Securing cleats are now popular.

6. Benching This is usually available from manufacturers if required, there being merit in the fact that their benching will fit precisely into the greenhouse purchased.

7. Other extras Autovents, heaters, gutters and downpipes (if not standard), shades and other items can usually be ordered along with the greenhouse, and there is much to be said for buying them at the outset.

IMPORTANT NOTE: Always double check the order, seeking the advice of a horticultural consultant or knowledgeable friend if in doubt on any important details. The cost of delivery should also be checked; some firms make a charge for delivery in certain areas, others include it in the basic price.



Greenhouses are delivered in various forms, from the fully complete sections of prefabricated wooden greenhouses, glazed or unglazed, to the alloy structures neatly packed in a container. Where bulky and heavy sections are involved, some care should be taken to ensure that they are offloaded from the delivery lorry relatively convenient to the erection site without impinging on it. These sections should be carefully stacked and adequately supported so that they are not likely to topple over in a gale; if in doubt lay them flat, but keep an eye on damage to a lawn!

Checking to see that everything is delivered undamaged and that no parts are missing is not always easy, but if at all possible it should be done, although this is not practical with alloy sections packed in boxes. The delivery note should be signed appropriately if there is any obvious damage, and the supplier informed immediately. Most reliable firms double check everything when it leaves the factory, as non-delivery of parts can cause a great deal of annoyance. Mistakes do occur however; new designs being particularly prone to missing parts, and in some instances sections do not fit properly together.


Plans and problems

Erection plans, in some cases very detailed indeed, and coded, should always be sent by the manufacturer, and they may in fact arrive before the greenhouse, which is a better arrangement as it allows the site and foundations to be prepared and the erection instructions and plans to be carefully studied. Note precisely whether centre to centre, outside to outside, or inside to inside measurements are quoted; outside to outside measurements are the most usual.

Consideration should be given to intended orientation for the greenhouse when making the actual purchase and most certainly if seeking planning permission. It can happen, however, that a different orientation seems preferable when the actual erection is in hand. It would then be better to delay work until the matter is corrected and I would imagine that most planning authorities would be fairly sympathetic to an alteration involving, in many cases, only a small distance one way or another with an amateur sized greenhouse. To ignore the tremendous implications of correct orientation would be folly.


Marking out the site

To peg out the position of the greenhouse on the ground, accurately cut pointed pegs 2.5 x 2.5cm (1 x 1in) and at least 30cm (12in) in length. Knock in the first peg lightly at the appropriate distance from some fixed line such as a fence, path or building, and by the use of a steel measuring tape tap in another peg to mark the length of the greenhouse, using centre to centre measurements for each peg. Check that the line of the pegs is in line with the fence or building making sure at the same time that it follows the desired orientation. Note that it is the length of the greenhouse, the line of the ridge, which must conform to east-west or north-south orientation. A few degrees off the intended orientation one way or another is not vital, it being better, however, to err to the north (for the U.K.) for an east-west ridge orientation and to the east for a north-south orientation.

With a reasonably square greenhouse it is, on balance, marginally better to have the ridge running east-west for transmission of winter and spring light. Where, by necessity, the greenhouse has to be erected with an orientation between east-west and north-south to maintain a tidy line, there is not much one can do about it: the full implications of light transmission must, however, be appreciated.

The levels of the two pegs should now be set accurately, knocking them into the ground sufficiently to make them stable. With the average amateur sized greenhouse up to about 3-3.6m (10-12ft) in length, a long straight unwarped board and a spirit-level should suffice for level checking. For a longer greenhouse an intermediate peg or pegs is advisable, checking their line by the use of a light piece of cord or string which should always be set tautly between the two end pegs. If a Cowley or similar level can be borrowed, set up and properly used, this helps to get the top of the pegs exactly level, although final checking with a spirit-level is always desirable.

Smaller greenhouses are usually erected dead level; a large type with a gutter may have a slight fall in one direction, as advised by the manufacturer, to facilitate the run of water. With commercial greenhouses, especially in multi-bay form, a run is essential to ensure that the water moves quickly along the gutter and does not flow back through the glass. A fall of about 15-30cm (6-12in) per 30m (100ft) length of greenhouse is usual.

Now tap a 5cm (2in) nail lightly into the centre of each peg. Forming the right angle necessary to measure up the width of the greenhouse can be tricky and should never be guessed. A triangle carefully made up with pieces of straight unwarped wood 90cm (3ft), 1.2m (4ft) and 1.5m (5ft) in length may be used, checking that the right angle formed is exact by using a joiner’s square. The triangle should be put exactly to the nail on one corner peg and one leg should be supported with bricks along the taut line between the two end pegs. After supporting the other right-angle leg, put a line along it and tie this to a peg set outside the width of the greenhouse. Then measure along the line and insert a peg at the exact width required. Repeat this procedure at the other corner and then check the two pegs longitudinally, also checking that the diagonals are equal. One usually finds it necessary to make slight adjustments so that everything tallies. (This task can also be undertaken with a site square, which is not perhaps the easiest instrument to use as it must itself be very carefully set up dead level over both of the first inserted corner pegs in turn.) Now level up the corner pegs and tap nails into their centres. Finally set taut good-quality lines on each corner peg to make the outline of the house by using short pieces of wood, or more pegs, 60-90cm (2-3ft) outside the house outline so that work can proceed on the foundations of the greenhouse without impediment by moving the corner pegs and lines out of the way as necessary.


20. March 2011 by Dave Pinkney
Categories: Greenhouse Equipment, Greenhouse Gardening | Tags: , , | Comments Off on Buying and Erecting Greenhouses


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