How To Build A Mini Wooden Greenhouse

Building this mini greenhouse is a simple task that most DIY enthusiasts can complete over the course of a weekend. Sometimes called a cold frame greenhouse, these warm box frames with opening lids are perfect to start off the growing season from seed. With this step-by-step guide, I’ll show you how I made my own mini greenhouse using some 25x50mm timber, 4mm polycarbonate sheets and a selection of screws, hinges and a gas piston stay. I’ve used a selection of basic tools for this process, with the exception of my trusty table saw. If you don’t have access to a table saw I’ll show you another way to achieve the same result.

You can make your own mini greenhouse to the specific size and dimensions you want. Mine measures 1050mm tall, and 1000mm wide, with a depth of 500mm. This was designed to fit within a specific space we had allocated. It fits perfectly between our rear patio door and our outdoor kitchen.

Why Build a Mini Greenhouse?

Growing from seed can be a tricky process. Too often we found that the sprouting plants would get eaten by slugs and snails and many of our seeds never sprouted. Perhaps the seeds were being eaten by birds, or the temperature just wasn’t right. Either way, we were really struggling to get our crops going. We decided that we needed a greenhouse. A warm and safe environment to nurture our young plants.

We looked into buying a flat-pack mini greenhouse but weren’t overwhelmed by the quality and styling of the products out there. So I decided to build my own.

We decided to build our own mini greenhouse due to these factors:

• We didn’t like the quality and build of the ‘off the shelf’ products.
• We were limited on space.
• We wanted to fill a very specific area in our garden.
• I love building new things and sharing my experiences with the wider community.
• Woodworking is a hobby and passion of mine.

Materials Required to Build This Mini Greenhouse

You’ll only need a few basic materials for this cold frame, three-door mini greenhouse.

• 50x25mm treated timber batten
• 4mm dual wall polycarbonate sheets
• 50mm hinges
• Gas piston hinge stay
• 25, 45, 60 & 70mm screws (4mm diameter)
• Clear silicone sealant
• Paint or wood stain

DIY Mini Greenhouse – Step-by-Step Build Guide

Before you get started, measure your space and sketch your design on a piece of paper. This will help you understand and remember your dimensions as well as visualise the finished product. Next, purchase your materials and prepare your workspace. A workbench is ideal for securing wood whilst cutting and drilling.

Step One – The Frame

Floor Frame Construction

Begin by measuring and cutting the pieces of timber for the floor frame. These can then be secured together using 70mm screws as indicated below. Drill some countersunk pilot holes to prevent the timber from splitting.

Rear Frame

Now it’s time to cut and secure the rear frame into place. Cut the timber pieces and secure them into place using 70mm screws. You will need to avoid hitting the other screws where they cross.

Top Rail

It’s time to create a top rail. This will be used to support the lifting lid and hinges. Cut the timber and secure in place using 70mm screws. Remember to drill countersunk pilot holes before securing your screws.

Side Frame

Here, we need to cut some angles to provide run-off for the top opening door. I’ve gone with an 8-degree angle but you can increase this slightly to 15-degrees if desired. The front supports are about 100mm shorter than the rear.

The arrows indicated in blue require pocket holes to be drilled. I use a Kreg Jig to do this but you can use an alternative pocket hole drill set. This is the one I use:

Securing the Frame

The final stage of the frame which adds structural integrity is the front top support timber. This can simply be cut to size and secured in place with 45mm screws.

Now you should be left with something that looks like this.

Completing the Frame With Shelf Supports

Before I moved onto the doors, I added bracing supports at two equal heights. These provide a secure location for the shelves to be fixed to.

I’ve set the side shelf supports 6mm in from the outer frame so that I can mount the polycarbonate snug within the frame. See the below image.

Step Two – Constructing the Doors

The doors are relatively straightforward to build. We will require three in total. One for the top lid and two for the front.

Measure and Cut all the Lengths of Timber

The door frames will house the polycarbonate sheets, therefore we need a groove cut into the timber so that the sheets can slot into them. Here is what it will look like.

The first task was to cut all the timber to the correct lengths for all three door frames. That’s 12 pieces in total. The top door should overhang the top frame by about 5mm on each side. Measure the top of the frame and add 5mm to each side.

The two front doors will fit within the frame. Allow a 3mm gap around each side to ensure the door fits within the space and can open freely. Measure the frame opening and reduce each side by 3mm for your doors.

Cut Grooves into the Timber

I then used my table saw, set at 10mm depth to cut the grooves into each piece of timber. The groove was inset by about 6mm from one edge.

If you don’t have access to a table saw there are few alternative methods for achieving this groove.

1. Use a palm router with a 4mm bit and guide fence. Secure the timber into a bench clamp and run the router down the length of the wood using the guide set to 6mm.
2. Use a router table set to 10mm depth and 6mm from the fence. Again, use a 4mm router bit.
3. Use a circular saw set to a 10mm depth. Use the saw’s fence guide set to 6mm.

To join the door frame pieces together I drilled pocket holes and started by securing 3 pieces together. Apply a small amount of wood glue to hold these pieces together.

It’s now a good idea to sand all your timber as this is much more difficult when the frame is built. I used 120 grit sandpaper on my orbital sander for the best results.

Cut and Insert the Polycarbonate Sheets

Now, lay the incomplete frame over your polycarbonate sheet and mark out the size to cut.

The 4mm double walled polycarbonate sheets can be cut using a sharp utility knife. Use a metal ruler to help guide the cut and watch your fingers. The size of the sheet you need to cut should be:

The inner frame size + 7mm + 7mm

Although our groove cut depth was 10mm I’ve only added 7mm to each side as I want a little wiggle room. Silicone sealant will be added into the groove to seal the door and prevent water ingress.

With the polycarbonate sheet cut to size, this can now slide into the frame. Firstly, apply a bead of silicone sealant into the groove and slide the sheet into place. Wipe off any excess sealant with a piece of tissue. The final piece of the frame can be glued and screwed into place. Place the frame onto a flat surface and allow to dry.

With each frame built it’s time to secure them to the frame with hinges. Hinges should be inset into the door frame to avoid large gaps down the side. If you are unfamiliar with how to install a door hinge, this video is great:

Fitting the Doors to the Frame

Before we fit the top door, we need a piece of timber across the top rear section of the frame. This will house the hinges. It does need to be cut at an angle so that the door can close neatly. This cut can be completed with a table saw or circular saw.

The top door can be secured to this rail using 25mm screws in the hinges.

The front doors can also be added using the same screws. The hinges should be mounted about 70mm from the top and 70mm from the bottom of the door. You should now have something that looks like this. (Minus the side panel and shelves which I will cover next.)

Fitting the Shelves

For this part of the build, I used the same timber as the frame, placed in a slat formation. I left a 8mm gap in between each piece to allow for airflow and light penetration. I cut each piece to length and attached it to the frame with 45mm screws.

Step Three – Fitting the Side Panels

The two side panels were cut to the exact aperture of the frame. I used thin pieces of wood cut from the 25x50mm timber to help secure the polycarbonate sheets into the frame.

These thin pieces were cut to length and secured 6mm in from the edge using 25mm screws.

I then laid the whole greenhouse frame on top of the poly sheets and drew around the inner frame using a marker pen. I then cut the sheets using a utility knife.

To make all the panels watertight I applied a bead of clear silicone sealant.

Step Four – Finishing Touches

To finish the DIY mini greenhouse I applied magnetic door catches, black metal handles, a piston stay to keep the top door open and stained all the wood. The top opening door also had a simple frame added to ensure a good overlap over the frame below.

The piston stay is great for a soft open action on the lid. It can be a little fiddly to fit but once you find the right location it’s a great addition to the overall finish of the greenhouse.

I also added a thin strip of wood between the two doors at the front to help prevent any little bugs from accessing the lovely plants inside.

Now it’s all ready to stain and add the next generation of fruits and vegetables.

Conclusion

A garden cold frame or mini greenhouse is an easy DIY project for those who want a bespoke design and fit for their garden. This project can be completed over a weekend with a small selection of tools and materials. A DIY mini greenhouse for seedlings and winter growing makes a great addition to any garden, whether you want to extend your growing season or just want to grow some more varieties.

Happy building and happy growing!

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Tools Required for this Build

To build this DIY mini greenhouse I used the following tools:

• Combi Drill
• Impact Driver
• Tape Measure
• Mitre Saw
• Table Saw
• Metal Ruler
• Orbital Sander
• Pocket Hole Jig Set (Kreg Jig)
• Set Square

Accessories

• Pencil
• Countersunk drill bit
• 80 tooth mitre saw cross cut blade

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Why Not Add Some Colour

Find the perfect matching colour for your garden decor and spice up your life with a colourful mini greenhouse. We’ve recently painted ours to match the rest of the garden.

FAQ?

How much does a mini greenhouse like this cost?

In total, this greenhouse cost me roughly £108.

• £32 – timber
• £46 – 4mm double wall Polycarbonate sheets
• £3 – 3x hinges
• £5.20 – gas piston stay
• £14 – Assorted screws
• £8 – Wood Stain

You could make this cheaper by swapping the polycarbonate sheets for a polythene sheet and ignoring the gas piston stay.

Do mini greenhouses need air holes?

It’s a good idea to have small gaps in your mini greenhouse for air to circulate. With this DIY mini greenhouse, small gaps between the doors allow for this air movement. You can also place spacers beneath the top lid to allow further airflow on hot days. I made a couple of different sized wooden blocks to place under the lid for this reason.

How do I keep a small greenhouse warm in the winter?

Adding a layer of bubble wrap to the interior walls and blocking any gaps will help retain heat and prevent heat loss. It’s also beneficial to ensure your mini greenhouse is located in full sun view throughout the day.

Horticultural bubble wrap insulation can be the best for this job but normal bubble wrap can be equally effective.

Thanks for reading. Please feel free to leave a comment below should you have anything to share or would like to know more information about a specific part of this build.