How to build a greenhouse base with paving slabs

Last updated on July 16th, 2021

Just recently, I needed to install a new greenhouse, nothing fancy just a small 6ft x 4ft greenhouse to give me a little more indoor growing space under glass for propagating seedlings and growing tomatoes. As a result, I found the perfect position for it in an unused flowerbed that doesn’t get much love.

Anyway, I thought this would be the perfect opportunity to show you how to build a greenhouse base with paving slabs suitable for a small greenhouse. This guide will be broken down step by step to help you do the same. It is very easy, takes a little bit of work preparing the ground and getting it level but it is rather straightforward. Now I have built a solid base which is great for pots and having greenhouse staging in but you obviously will no longer be able to plant anything directly in the ground.

Before

After

What you will need to lay a new patio for your greenhouse

• Paving flags/slabs (I used 44cm x 44cm concrete flags)
• Cement
• Sand (grit sand will do)
• Edging tool if you have one and need to put into your lawn
• Spade
• 2″ x 3″ piece if timber or similar for tamping
• Spirit level

How to build a solid greenhouse base from flags as a beginner

Step 1 – Decide how big your base needs to be

The first thing you need to do is decide is how large you need your greenhouse base to be so you can plan on how many flags you will need. In my case, my new greenhouse was only 6ft (180cm) x 4ft (120cm) and the area I decided to build the greenhouse was a little large than the greenhouse. With this in mind, I chose to cover the whole area as it was only marginally larger than the greenhouse. This would give me some harder surface at the front and at the side of the greenhouse too.

So for my base, I decided I would like the dimensions to be around 220cm wide by 176cm. The patio flags I used were 44cm x 44cm from B&Q for just over £4 each, from the size of the individual flags, my base ended up being 5 flags wide by 4 flags deep, so in total, I used 20 flags.

Step 2 – Preparing the ground

Next, I needed to prepare the ground with pegs and string to clearly outline where the patio will be. For small patios like this, you could just lay the patio flags out as you would have them (once fully fitted) and mark them out around them. Once I had it marked out I then dug out the foundation until it was completely level.

I then used a piece of 5ft piece (3″ x 2″) piece of timber to tamp it down and drag it from back to front to get the ground as level as possible. I then used a spirit level to make sure it was completely level. This is a very important step, so spend some time getting the levels correct.

The next thing I did was lay down the patio flags on the foundation to double-check it was the right size and everything looked good and level before I filled it with the sand and cement mix.

Optional step I didn’t need to do but you may need to

What I will say is that my soil was compact and heavy, and being that I was only laying a small base for a small greenhouse, I decided I didn’t need to lay a sub-base first. If you have loose sandy soil or you are making a base for a much larger greenhouse, you may want to consider laying a sub base first with MOT Type 1 (hardcore) which is crushed carboniferous limestone, used to create a hardcore base under patios. This is available from most garden centres or builder’s merchants. You would simply layer this in the base to form a more solid surface.

Step 3 – Mix the concrete and sand to create the dry mix base to sit the flags on

I got a large sheet of wood to mix the dry mix concrete and sand on. Basically, the ratio you use can differ from 1:10 (1 cement, 10 sharp sand) to as high as 1:3, depending on the amount of foot traffic you expect. I decided to go for 6 parts sand to 1 part cement.

So all I needed to do is put 6 shovels of sharp sand on a board, then mix 1 shovel of cement and mix it together.

After making my dry mix, I shovelled it into the foundation I dug out, ideally, it needs to be around 30-40mm deep. I had to do about 3 mixes to get enough to fill the foundations with a good bed of dry mix.

I then used a rake to make sure the foundation was still level, although this does not need to be perfect at the stage because I will be using a rubber mallet and spirit level to get the level exact when fitting the flags in place.

Step 4 – Laying the flags on top of the dry mix

Now I carefully laid the flags on top of the sand and cement mix. It is important to use a spirit level during this step to make sure every flag is level, and use a rubber mallet to gently tap them down.

Now the moisture in the sand and from the ground will help the cement and sand set which will hold the flags in place. I also gave the flags a quick rinse off with the hosepipe as this will also help the cement set.

Once I finished laying the patio flags I left them for 24 hours before I walked on them and started building my greenhouse.

Now learn how I secured my greenhouse to my new patio base

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