There is more to planning a greenhouse than its construction. Choice of location, size and orientation involve some horticultural forethought: will it be used as a cool house for over-wintering plants or a hothouse for plants that appreciate tropical heat? In the UK, it is usually best to think about catching the winter sun; if your options give you a choice between morning and evening sun, morning will promote more growth.
There are many free greenhouse plans online that you can adapt and elaborate, and Gabriel Ash will provide advice about design, venting and materials. Choose how much you want to do yourself; for example, many people choose to prepare the ground, services and base but call in experts to deliver and erect a pre-fabricated frame.
Greenhouses can be made from bricks, timber and glass, or from lightweight PVC tubing covered in polythene; however, one thing they have in common is sound foundations.
Level the ground accurately and consider whether you require drainage, mains water or electricity. Electricity makes a big difference, powering winter heating and summer ventilation. Cables should be sheathed and sunk to a safe depth. Drainage pipes must be laid with steady camber to the nearest drain or soak-away.
Cedar is popular for its rot resistance. When buying timber, remember to allow for planing and sanding; for example, timber that is 4”x4” when rough-hewn is equivalent to dressed timber of approximately 3½”x3½” (90x90mm). In most constructions, timber begins as 2×4 (1½”x2½”). An 8’x6’ greenhouse will use around 100 metres. Wood treatments are usually applied before you assemble, but paint after.
If you are glazing, a common mistake is ordering glass the same size as the frames; instead, panes need a couple of millimetres gap each side to comfortably fit. Double-walled polycarbonate plastic, acrylic sheets, fibreglass panels and polythene sheeting are alternatives. An 8’x6’ greenhouse covered by polythene will use about 30 square metres.
You will also need miscellaneous galvanised nails, plates, and hinges.
A few good tools are better than many of dubious quality. Use mitre blocks and set squares or a T-bevel gauge to set your angles.
If you are seating your frame on a dwarf wall, estimate the quantity of bricks. Engineering bricks repel damp better and last longer.
Even if your design does not require brick walls, a single course sitting on concrete offers a stable base for your timber clear of the wet ground. If your project dispenses with a brick or concrete base, a rectangle of seasoned 4×4 timber can be anchored to the ground by attached stakes. Galvanised plates or brackets save you the trouble of constructing corner joints.
Flooring is optional, but bricks also make an attractive and durable floor and laying gravel will assist drainage.
Follow your plans to pre-assemble each wall and roof section. As a general guide, walls will have vertical studs every 2½’ to 3’. Diagonal planking or metal straps will brace the corner angles and endow rigidity.
Erection of the structure is a two-person job. Temporary wooden props will help, while brackets provide the quickest and easiest way to secure the sections together. When you are happy with its rigidity, hang your doors and other fittings; finally, fit your glazing or panelling.