How to build a greenhouse

A greenhouse will bring new life to your garden. It provides a controlled environment in which you can propagate plants - bringing some on early, protecting others from frost and growing exotic species in year-round warmth.

You can buy a separate, purpose-designed galvanised steel base (which is also available as a flatpack) on which your greenhouse frame can sit, secured by frame fixings. The base raises the height of your greenhouse and makes it easier to build, although it's not essential - but you must secure the frame or base firmly onto a level surface. There are three different ways you can do this:

Push specially designed metal hooks into isolated pockets of wet concrete. You then attach these to the base, which secures the frame to the ground.
You could lay a concrete strip footing that's 200mm wide and 100mm deep. Then you can anchor your base 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 (taking care to leave room for the glazing).
Better still, you can 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 increases the height of your greenhouse by around 125mm. If you're not using a base, the manufacturer might recommend using wooden battens to act as a buffer between the brick/cement and the bottom of your greenhouse frame.

These are the steps you need to take if you want to attach your base and frame to a course of bricks on a cement footing. It's worth checking the kit before you start to make sure all the base and frame fixings have been included.

Person fitting two sections of a steel greenhouse base together in the corner.

1. Build the steel base

Put together the galvanised steel base, following the instructions you've been supplied. It's 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 your footing. This 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.

Two people lifting a steel greenhouse base onto a brick base which has already been laid.

2. Lay the brick foundation

Dig out the footing trench to a depth of 100mm and fill it with concrete. Make sure the surface is absolutely level. When it's dry, rest the base on the concrete and draw a line around its outside edge with a pencil. Next, lay one course of bricks in the centre of the footing. Check the outer faces of the bricks are flush with the outside of the frame, and leave the mortar to dry for 24 hours.

Once the base is dry, lift the steel base onto the brick foundation you've prepared.

Make sure you have the manufacturer's instructions and all the pieces to hand before you start. You'll need to assemble the whole frame - and make sure it's straight and square - before you fully tighten the bolts and screws.

Safety first

If you have children (or they sometimes play in your garden), it's wise to choose the safer option of polycarbonate glazing panels in your greenhouse. Ordinary horticultural glass is a potential hazard.

Person fitting two sections of a metal greenhouse frame together on the floor.

1. Assemble the frame

Start by laying 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 around and angled pieces leaning in the correct direction.

Bolt the pieces loosely together to assemble the first side. Follow the same procedure for the other end and then for the sides.

Person fitting a roof ridge bar to a greenhouse frame which has been secured to the metal base.

2. Fit the frame to the base

Now bolt the sides and gable ends of the frame together and screw them to the base. Leave all the bolts and screws loose at this stage.

Attach the roof ridge bar, screwing it loosely between the two gable ends. Slide on the fittings for the roof vent, then screw in the roof glazing bars.

Using a spanner to tighten a nut that secures a greenhouse frame to the base.

3. Tighten the fixings

Next, use a builder's square and spirit level to check the whole frame is straight and the corners are square.

When you're confident that everything is level and true, tighten all the screws and bolts (including the ones that attach the frame to the base).

Secure the metal base to the brick plinth using 25mm frame fixings. Make sure you drill these through the base and into the brickwork - and take care to drill only into brick, rather than cement.

Person securing a greenhouse roof vent to a completed greenhouse frame.

4. Fit the roof vent

Assemble the frame of the roof vent. When you've screwed this together tightly, slide it into place along the roof ridge.

Person wearing gloves and eye protection slotting a section of glass into a greenhouse frame.

5. Fit the glass

Push and slide the thin flexible rubber line into the tiny slots all around the frame.

Starting with the roof, fit each pane of glass into the frame and secure it with clips - making sure you wear gloves and safety goggles while you're doing this. Once you've got the knack, you should find this process quick and easy. Take care, be patient and double-check that you're working with the right-sized glass for the section of the frame you're glazing.

Position the glazing clips, following the manufacturer's instructions. Try to keep them evenly spaced.