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.

Greenhouses come in lots of shapes and sizes, with frames in timber or (more often) aluminium, which has the advantage of being low maintenance. Try to choose the biggest greenhouse you can afford or have room for, as you'll find that plants fill the space very quickly.

Glazing

You have three main choices here. Horticultural glass is the most common and is inexpensive. Toughened glass is a stronger alternative - and a safer one if you have children in the garden. And tougher still are polycarbonate panels, which reduce scorching by diffusing the sunlight.

Access

Make sure you can get all the way around your greenhouse from the outside, so you can clean the glass each year.

Ventilation

The right amount of ventilation is very important to control the temperature and keep the air circulating. If you don't have enough, it increases the level of humidity and the likelihood of plant disease. Most greenhouse frames include adjustable air vents, or you could install thermostatically-controlled automatic vents that don't need a power supply. Louvre windows are another option - giving you adjustable, draught-free ventilation.

Shading

You'll need to provide shade for your greenhouse in the summer months to stop it overheating and your plants getting scorched. There are three main options:

You can put special greenhouse shading paint on the exterior surface of the glass. This is weatherproof, but you'll still be able to remove it with a duster in late autumn.
You can clip shading panels or woven plastic netting to the inside of the frame.
You can put up internal shelving, which provides shade for the most delicate plants.

It's not a good idea to put your greenhouse under a large tree, as a falling branch could shatter the glass.

Heating

In cold seasons and to grow certain plants, you might need to heat your greenhouse. Again, a number of options are open to you:

Paraffin heaters are cheap to buy - but they use a lot of fuel, are difficult to regulate and encourage condensation, which can increase humidity and the chance of plant diseases.
Bottled gas heaters are thermostatically controlled, but can also cause condensation.
Electric heating is probably your best option, but you'll need a connection to a power supply. You can choose between heating pipes, a fan heater or even a warming cable laid under the soil in a bed of coarse grit.

Irrigation

Capillary matting, which retains water and releases it according to a plant's requirements, is enough for most greenhouses. Or you could run a pipe along the soil from an outside water tap. You could even add a timer switch for automatic watering - which is particularly useful if you go away regularly.

Laying paths

It's easiest to create paths around and inside your greenhouse before you build it. Watering and trampling will make soil paths in a greenhouse turn to mud, so build a path from concrete slabs or paving stones, or simply lay down gravel or coarse wood chip between retaining boards nailed to pegs hammered into the ground.

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.

Step 1

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.

Step 2

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.

Step 3

Lift the 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.

Step 1

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.

Step 2

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

Step 3

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.

Step 4

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.

Step 5

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

Step 6

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).

Step 7

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.

Step 8

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

Step 9

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

Step 10

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.