Greenhouse Building Procedures and Maintenance

Greenhouse Foundations

With certain types of Dutch light greenhouses it is merely necessary to set down base blocks on levelled soil, though this is not a very satisfactory long-term arrangement either in the interests of stability or to prevent lateral movement of soil moisture. It is a practice adopted frequently by those taking short cuts in the interests of economy, or where a greenhouse site is being treated as temporary. A foundation of one kind or another is highly desirable to keep the greenhouse completely level and entirely stable in spite of the destructive forces of such things as gales and hurricanes.

Greenhouse Building Procedures and Maintenance The precise nature of the foundation, if any, depends on a number of factors, and more specifically on the actual design of the greenhouse being erected. Study very closely the erection plans or booklet applicable to the greenhouse. Base blocks may be provided with the greenhouse if specially ordered; alternatively it may be necessary to build a brick base wall or run in concrete with fixing pegs. For most amateur greenhouses a half-brick 11.25cm (4-1/2in) wall will be adequate, and this is usually exactly the width of any special base blocks provided. In this case, after determining the exact centre line of the greenhouse base, take out a trench 20-23cm (8-9in) wide and 18 — 20cm (7-8in) deep, or deeper if the soil has recently been levelled in order to make contact with undisturbed soil. This may not always be practical. Using a mixture of 3 parts finely broken brick, 2 parts rough sand, and 1 part cement, run in a layer about 8-10cm (3-4in) deep and ‘dump’ this as level as possible by the use of a fairly heavy straight board, using a spirit-level on top of this to check levels regularly. Where a full brick foundation wall of 23cm (9in) is necessary, as could be the case on a sloping site, or for a large greenhouse, the trench should be 30-35cm (12-14in) wide and again 12.5-15cm (5-6in) deep, although in many cases a half-brick wall would suffice if suitably reinforced with piles.

Greenhouses can in certain circumstances be laid directly on the foundations without the base blocks, in which case any securing bolts will need to be inserted into the foundations at the appropriate points. Alternatively anchor cleats may be used. Special base blocks invariably have an inbuilt system of anchorage for the greenhouse. Always allow the foundations to harden sufficiently before starting building. In all cases it will be necessary to put the line back in position and keep this suitably taut when necessary to check that there is precise adherence to the stated outside measurement. Before building opposite walls, it is advisable to check the widths and length, as bricks tend to get out of alignment, especially if laid by someone who is not an experienced bricklayer.

Perusal of greenhouse catalogues frequently shows a complete perimeter wall with no allowance for doors, which involves an awkward step up unless there is a suitable ramp. Base blocks, when supplied, ideally should leave the door flush with ground-level, a much more sensible arrangement, provided there is a concrete step.

It may seem that these items are trivial, yet they make a great deal of difference over the years to the easy management of a greenhouse, especially when wheeling barrows full of soil or debris. It is advisable to have a slight rise up to the door to avoid rain running into the greenhouse, which may happen if the ground slopes the other way.

Skill in building even a low wall varies, but by using a 3:1 sand/cement mixture and checking the line and level of each brick or block carefully, it should be possible for even the rawest amateur builder to make a reasonably accurate job of it, failing which call in a builder or handyman. Note that if using faced bricks the faced side should be to the outside, otherwise they will quickly weather.


Greenhouse Building procedures

Where the type of greenhouse being built demands a low base wall of 75cm (30in) or thereabouts, this must be carefully built, otherwise there can be erection problems. Very few prefabricated greenhouses intended for base walls now seem to be about, but they can be obtained if required and may be necessary on very exposed sites. Do-it-yourself enthusiasts frequently use the base-wall method of building a greenhouse, following conventional designs of sill plates on top of the wall to bear the astragals. Second-hand ex-commercial structural material is bought for the purpose of cutting down to build an amateur greenhouse, which is not usually a difficult job, provided care is taken to clean all the wood properly, giving it a good coat of paint before building commences, especially as the ends of the old astragal bars are generally removed. Galvanized nails should be used throughout, as steel nails can quickly rust.

There are seldom any problems in building a prefabricated greenhouse, particularly if time is taken to study erection plans. Some of the instruction booklets are detailed in the extreme, every part being marked and identified by a diagram, there being explicit erection diagrams bearing part numbers in addition. With alloy structures it is normal to build up the gable ends and one side on the ground; then, after putting up one gable end, proceed first with one side and then the other before putting up the other gable end. The roof and ventilators are generally left until last.

The completely glazed sections of a prefabricated wooden greenhouse are erected in similar order, but it will be necessary to obtain help as the sections are often heavy. If building a greenhouse with wood on a do-it-yourself basis, it is a case of building the main framework first before putting in the astragals. When glazing, although procedures vary, it is often preferable to do the roof first, followed by the gable ends, leaving the sides until last so that if there should be a high wind it will blow through the structure. A very important point when building any type of greenhouse is to tie down loose vents at night, particularly if they are glazed, otherwise they can flap about and cause a lot of damage.

The need to anchor greenhouses securely cannot be emphasized too strongly, allowing cement to harden before fully erecting a valuable structure.

Glazing should take place only during dry weather, as neither putty nor strips of mastic seal properly if the glazing bars are wet, in addition to which wet glass is very dangerous to work with. With alloy greenhouses all joins in gutters or main members should be properly sealed with mastic or other material.

Where there is a gutter the downpipe should be taken to an appropriate outlet or soakaway, and where the greenhouse is built on a slope it may be necessary to install a drain along the higher side to stop rain seeping into the greenhouse.


Installation of water

The installation of water has already been mentioned in connection with site preparation. If a supply pipe at low level is taken into the greenhouse, care should be taken not to damage the foundations. Ideally the water pipe should be laid before the greenhouse is built. The precise arrangements for water will depend on a number of circumstances, but a screw type tap is more or less a standard fitment. The most convenient place for the tap will again vary, but in the majority of cases the end opposite the door is most satisfactory. For mist irrigation, capillary benches, spraylines, heating systems and other water-demanding equipment, special arrangements will be necessary, all readily accomplished with alkathene pipe and patent screw fittings.


Installation of electricity

Because greenhouses are damp places, and with alloy greenhouses in particular, plastic covered cable, preferably PVC conduit or PCP sheathed cable, must be used along with waterproof fittings for the installation of electricity. The complete installation should be supervised by a qualified electrician or someone with a sound knowledge of electrical installations, and properly earthed. An alloy greenhouse can frequently ‘go live’ because of the nearness of an open connection to the structure. With a wooden greenhouse, while the same care is not perhaps as necessary, the installation should be carried out to rigid standards of safety. Whether to bring power in overhead or by underground cable is a matter best decided by a qualified electrician who will also ensure that the supply cable is adequate for the load demanded by the equipment, and taken right back to a suitably placed junction box and not a convenient plug.


Main path and greenhouse flooring

There should be a good slabbed path, or one of concrete or asphalt, to the greenhouse door or doors, there being nothing worse than a dirty wet path which, apart from being dangerous, constantly carries dirt and possible disease into the greenhouse.

The nature of the flooring in a greenhouse will usually be dependent on the cropping arrangements. With a permanently benched greenhouse with a bench on each side, a slab path at least 60cm (2ft) wide is the general rule. The area under each bench is usually left ‘rough’, covered with a larger of gravel, although the fastidious gardener may well wish to slab the whole floor, Slabs can be laid on a bed of sand, ashes or rubble, this last allowing the slabs to be held at the corners by cement.

Alternatively a concrete path can be run in using a 5:3:2 mixture and shuttering boards, making the concrete at least 5cm (2in) thick.

When the borders are used for cropping purposes, a wooden ‘cat walk’ may be used instead of a slabbed centre path as it can be removed to allow complete soil sterilization and cultivation. Where ring culture of tomatoes is practised, slabs are simply laid down the centre of the aggregate, unless some more permanent arrangement is preferred.


Maintenance of greenhouses

The painting procedure for greenhouses has already been referred to, as also have types of paint and frequency of painting. The current theme of erecting a greenhouse of materials and of a design which require little or no maintenance is founded on sound common sense, as there can be nothing more troublesome than constantly attending to the upkeep of buildings.

One of the main chores with any type of greenhouse is the cleaning of the glass, especially in industrial areas. Dust adheres to sticky deposits and in time a skin of dust develops on the glass, restricting light transmission very considerably. Moss and algae grow at any overlaps and on the glazing bars unless a carefully sealed barcap glazing system is used. Even then moss and algae can develop under the bar-cap if moisture gains access, which it usually does. The build-up of moss and algae, especially the former, can be extremely destructive and will literally push the glass out of position, causing leaks.

Glass cleaning can be carried out by using a proprietary glass cleaner or oxalic acid crystals dissolved in water, 100g to 1 litre (1lb to 1 gallon) sprayed on and then washed off, the latter being especially necessary when a metal greenhouse is involved. The oxalic acid also burns off a fair proportion of the moss, the remainder being freely dislodged by a good strong jet from a hose pipe. Moss and algae also grow inside a greenhouse and the removal of these is better undertaken when the greenhouse is empty. The cleaning operation usually demands the wearing of protective clothing, including a waterproof hat. Cresylic or carbolic acid (or Jeyes Fluid) used in a solution of 1 part to 39 parts of water is useful for cleaning down the inside as it burns off moss and algae very effectively. Moss and algae in glass overlaps may have to be removed with a piece of metal and a jet of water.

Base walls should be cleaned annually with a wire brush; if necessary paint them with whitewash or emulsion paint.


20. March 2011 by Dave Pinkney
Categories: Greenhouse Equipment, Greenhouse Gardening | Tags: | Comments Off on Greenhouse Building Procedures and Maintenance


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