How to... construct a greenhouse
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A greenhouse will bring new life to your garden, providing a controlled environment in which to propagate plants, bring on hardy ones early, protect semi-hardy specimens from frost, and grow exotic species in year-round warmth. Choose an open sunny situation, with the longest side of the greenhouse facing south for maximum light penetration.
Securing the base to the ground
You can buy a separate purpose-designed galvanised steel base, also available as a flatpack, on which the greenhouse frame sits, secured by frame fixings. A base will usefully raise the height of the greenhouse, and make construction easier, though it is not essential. However, the frame or base must be firmly secured onto a level surface. There are different methods of doing this:
- Push specially designed metal hooks into isolated pockets of wet concrete. These are then attached to the base, securing it to the ground.
- Or you could install a concrete strip footing 200mm wide and 100mm deep. The base can then be anchored by drilling and bolting it into the concrete. Drill holes through the base, or directly through the bottom sill of the greenhouse frame in the centre of each bay (being careful to leave room for the glazing).
- Better still is to lay a single course of bricks on top of the concrete footing and fix the base – or the greenhouse frame without a base – to the bricks. This is the most attractive and sturdiest method, and it raises the height of the greenhouse by around 125mm.
If you are not using a base, the manufacturer may recommend wooden battens as a buffer between the brick/cement and the bottom of the greenhouse frame.
Installing the base
This is the procedure to follow if you want to attach your base and frame to a course of bricks on a cement footing. Check the kit before you begin to make sure all the base and frame fixings are included.
1 Construct the galvanised steel base, following the instructions supplied. It is best to work on a flat, level surface such as plywood or hardboard sheets. Make sure the site is firm and level, and lay down the base to mark out the position of the footing. The footing needs to be 200mm wide. Mark its edges by running two builder’s lines parallel to each side of the base, one 50mm from the outside face, the other 150mm from its inside face.
2 Dig out the footing trench to a depth of 100mm and fill it with concrete. The surface must be absolutely level. When dry, rest the base on the concrete and pencil a line around its outside edge. Lay one course of bricks in the centre of the footing. The outer faces of the bricks should be flush with the outside of the frame. Leave the mortar to dry for 24 hours.
3 Lift the base onto the prepared brick foundation.
Building the greenhouse
Make sure you have the manufacturer’s instructions and all the pieces to hand before you start. You need to assemble the whole frame and make sure it is straight and square before fully tightening the bolts and screws.
1 Lay the pieces for one gable end of your greenhouse on a flat surface. Arrange them carefully into position, with metal sections and bolts the right way round and angled pieces leaning in the correct direction.
2 Bolt the pieces loosely together to assemble the first side. Follow the same procedure for the other end and then for the sides.
3 Now bolt the sides and gable ends of the frame together and screw them to the base. Leave all bolts and screws loose at this stage.
4 Attach the roof ridge bar, screwing it loosely between the the two gable ends. Slide on the fittings for the roof vent then screw in the roof glazing bars.
5 Now use a builder’s square and spirit level to check that the whole frame is straight and the corners are square.
6 When you are confident that everything is level and true, tighten all screws and bolts, including those attaching the frame to the base.
7 Secure the metal base to the brick plinth, using 25mm frame fixings. These must be drilled through the base and into the brickwork. Take care to drill only into brick, not cement.
8 Assemble the frame of the roof vent. Once it is tightly screwed together, slide it into place along the roof ridge.
9 Push and slide the thin flexible rubber line into the tiny slots all around the frame.
10 Starting with the roof, place each pane of glass into the frame and secure it with clips. Be sure to wear gloves and safety goggles. Once you have the knack this process is quick and easy. Take care, be patient, and make sure you are working with the rightsized glass for the section of the frame you are glazing.
11 Position the glazing clips according to the manufacturer’s instructions. Keep them evenly spaced.
12 Assemble the door, fit the glass and slide it into place.
