How to Build a Timber Greenhouse

Building a timber greenhouse is a low-cost alternative to buying a pre-made one. You can build the greenhouse to any size so it fits your plot perfectly.

The simple yet effective timber frame design gives you a sturdy structure, providing you with the perfect growing conditions at a fraction of the cost.

Timber Selection

The first thing you need to do is select your timber. We chose Rough-sawn tanalised 3"x2" (75mm x 47mm) for our structure as it can be used for the walls and for the roof.

Timber Cost

Fencing suppliers offer the cheapest supply of timber because they make their money predominately selling it in bulk to fencing contractors.

The timber is a lot cheaper than you would find in the timber merchants as it is not planned up and kiln-dried.

The timber is however pressure treated with preservatives (tanalised) making it perfect for external use, which is exactly what we need!

We used 3"x2" (75mm x 47mm) for the frame and roof, and 3"x1" (75mm x 47mm) for the diagonal braces

Choice of Cladding for the sides

When it comes to cladding the greenhouse you have two main choices - Plastic or Glass. We chose to use ¼" (4mm) twin-walled polycarbonate sheeting as it is cheap, especially if you order it in standard-sized 2ft x 4ft (610mm x 1220mm) sheets.

It is also easier to work with as it can be cut to size on-site alot easier than glass.

This is the website we used for our polycarbonate cladding, they seemed to be the cheapest around and they will deliver it straight to your door.

https://www.thepolycarbonatestore.co.uk/4-mm-clear-polycarbonate-multiwall

The twinned wall design gives a little bit of insulation which can be useful at both ends of the growing season to take the chill off.

Making the frames

Building a large structure can be daunting at first but when you break it down into stages it gets easier.

The simplest and easiest way to build any self-supporting structure is to make 4 frames and screw them together in the corners.

The frames can be easily assembled on the ground and then lifted up into position, Once all four frames are secured together the structure will be stable so you can add your roof.

Cut the top and bottom rails to the desired length of the greenhouse first, Then cut the studs to go in between to the same length. This will determine the height of your Greenhouse.

We recommend building your greenhouse 8ft high so you can fully stand up in it. lt also works really well for the polycarbonate sheets as two 4ft high sheets will give you the desired 8ft with no cutting to do.

Setting the Stud position

Use 4" (100mm) screws to fix the top and bottom rails to the studs.

The spacing of your studs will be determined by the size of the polycarbonate sheets.

Space your studs out evenly so that they catch the polycarbonate cladding on the joins.

Adding the lateral Braces

Once constructed, the timber frames will have no lateral support. We can do this by adding braces, we have used 3"x1" (75mm x 22mm) timber but any type of batten will do.

These will need to be diagonally fixed to the studs to lock the frame into a square position.

Once this is done the frames will be completely solid and square. You should get the same measurement if you measure them diagonally from corner to corner, and then repeat on the opposite corner to corner.

Fixing the Cladding

Always remember to check if the polycarbonate cladding has an external facing side. Usually, it will tell you on the protective film you peel off.

Once you have made your frames you can then start to fix your cladding. We chose to use Clear Silicone as it is an incredibly versatile adhesive.

It will have no problem sticking the polycarbonate sheets to the rough-sawn 3"x2" (75mm x 47mm) timber frame, providing the timber is dry.

We then used 3"x1" (75mm x 47mm ) and 2"x1" (50mm x 22mm) battens over the joints and screwed them to the 3x2 frame behind to sandwich the polycarbonate into position whilst the silicone set.

Finished Frame Side

This is one of our completed frame sides you can see the polycarbonate cladding allows maximum light into the greenhouse.

We used battens that were the same thickness as the frame to maximize the light gains.

All the joints in the polycarbonate sheet are covered with battens and you can see the diagonal brace at the back keeping the whole frame in shape.

Joining the frames

Once all the frames have been assembled you can start to put them together. Getting the first two frames joined together is the hardest part of the whole process.

Once the two frames are fixed together they form a stable, self-supporting L-shape.

Getting the next two frames into position is even easier as the structure will hold itself together and become increasingly stronger as you fix all four corners together.

Fitting the Roof Timbers

To keep things simple we have used a single-pitch roof. We created the desired fall by adding an extra 3"x2" (75mm x 47mm) timber to one of the sides.

Making one frame slightly taller than the one opposite, gives us our fall, so any water will run away downhill.

When constructing the roof always try to have the roof timbers span the shortest distance if the greenhouse is rectangular.

This reduces the chances of them sagging over time or due to any imposed load such as snow.

Fitting the Corrugated Roof Battens

We need our corrugated roofing sheets to run in the same direction as our roof rafters so we were forced to fit counter battens.

These have been fitted the full length of the roof at 90 degrees to the roof trusses. These battens then provide support and fixing points for the corrugated roofing sheets.

Fitting the Roof Sheets

The corrugated roofing sheets are always fixed down through the peaks of the corrugations. You never fix them in the valleys as this is where the majority of the water will run off.

It would essentially be like drilling holes in the bottom of your gutters!

Building Raised Beds

Another benefit to building your greenhouse out of wood is that you can always fix to it and build off the structure. We have fixed some 6"x1" rails to the sides of the greenhouse around the perimeter and formed some raised beds perfect for growing tomatoes.

Using timber also means you can easily mount hooks and fit shelves to hold all your tools and equipment.

Materials

• 3"x2" Rough Sawn tanalised treated timber - To make the 4 Frames and the Roof
• 3"x1" Rough Sawn tanalised treated timber - To make the braces for the frames and the roof counter battens
• 2"x1" battens - To secure the polycarbonate cladding
• ¼" 4mm thick Twin Walled Polycarbonate sheets 2ft x 4ft - To make the sides
• Upvc corrugated roofing sheets
• 4"x10 Wood Screws (5.0 x 100mm) - To fix the 3x2 framework together
• 3" x 10 Wood Screws (5.0 x 80mm) - To fix the polycarbonate roof sheets
• 2" x 8 Wood Screws (4.0 x 50mm) - To fix the battens to hold the polycarbonate sheets to the frames.

Equipment

• Circular Saw
• Speed Square
• Cordless Drill + Driver
• Spirit Level
• Hammer
• Tape Measure
• Pencil

We hope this article has been useful and given you a few insights into the stages involved in building your own Timber Greenhouse. Let us know in the comments if you are thinking of building your own!

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