We’ve finally finished the foundation of the plinth for the pizza oven! (Essentially a big concrete square).
The plinth base is 154cm x 154cm with a height of about 10cm at the front and 15 – 20cm at the back. (The garden is on a hill). The Boyfriend almost broke his back digging the garden to level the foundations, squaring them and making them level.
In a way, this is the most important part of the oven build, as without a stable, level base that doesn’t move over time the oven would crack worse than is usual for this sort of oven, and it could even slide off the plinth and smash! The key parts of the foundation build is ensuring it’s square and ensuring it’s level. In my opinion it’s best to ensure it’s square, then ensure it’s level, then double check that it’s square and take it from there; checking and double checking as you go.
Tools we used:
– Spirit Level
– Wheelbarrow and / or cement mixer
– Scrap pieces of wood, one small one long
– Spade / Shovel
– Tarp to cover
– 4 pieces of wood 4 inches high and 154 cm long
– 8 nails
– 31 bags ballast
– 10 and 1/3 bags cement
Building the Shuttering:
The first thing you need to do when building the foundations for your oven plinth is build the shuttering ‘form’ into which you’ll pour your concrete:
• Decide how large it needs to be. (About 5cm either side larger than the outer edge of your plinth – presumably square – and about 4 inches thick at its lowest point to hold the weight of the oven.)
• Buy rough sawn / rough cut wood x 4 of the appropriate height and cut to length.
• Nail the wood into a square using two nails per corner. (You hammer the nails into the flat ‘side’ of the wood through to the blunt ‘end’ of the neighbouring piece.)
• Lay the square out where you want to put the oven and check that it’s square by measuring diagonally in an X shape from corner to corner of the square, ensuring that both figures are the same.
• Next check that the lengths are level by placing a spirit level along each length. Check the ‘bubble’ is centred on the level.
• If the lengths are not level then you have a few options: either dig the higher end into the ground and settle it in (this may require a hammer), or build up the lower end with something (we used soil sods and spare compost I had to hand.
• Check it’s level again and check it’s square. Keep adjusting till it’s correct.
• Make yourself a cup of tea, have a biscuit and stretch your back out!
If I’m honest I didn’t make our shuttering entirely square, there was a 2cm difference between lengths of the X. Not being too enamoured with the idea of re-adjusting EVERYTHING again, I took my railway sleepers, placed them in a square inside the shuttering and, happy that they fit with plenty of space to spare, I left the squaring be.
We’re complete novices and this took about 8 hours in total + lots of frustration and a bad back.
On the plus side I’m sure that my foundation will be level and therefore the oven isn’t going to slide off the back end of the plinth any time soon. Hopefully.
Laying the concrete:
Once you’re happy that the shuttering is square and level you can make your concrete and pour it in. Making concrete is (in my father’s words) a doddle. Or at least, it is if you have a cement mixer or a very small amount of cement to make. We mixed our cement by hand and I would highly recommend that if you are requiring the same dimensions of slab you hire a cement mixer. It took us three days to concrete the area within the shuttering, and a few weeks of very sore backs. The cost of hiring a cement mixer will repay itself in your non-aching back many times.
How to make concrete:
• Mix ballast and cement in a 3:1 ratio either by hand in a wheelbarrow with a shovel or in a cement mixer.
• Once the mixture is an even colour throughout (a dull mid-grey), add water a little at a time and mix it in until the mixture resembles a thickish sloppy cake mix.
• Taking care not to disturb the shuttering, pour the concrete into one corner of the form. (We found it easiest to work from side to side, building up in layers as we went.)
• With a scrap piece of wood, tamp down the poured concrete to knock out any air bubbles. The surface should form a ‘butter’, (a water that rises to the top and makes it look shiny).
• If you’re building it up slowly, it’s advisable to ‘rough up’ the surface of the concrete, so subsequent layers have something to bind to.
• When building the final layer, cover the whole of the top to just above the lip of the shuttering. Tamp the entire thing with a length of wood longer then the width of the shuttering (this will take two of you). Next, use the same length of wood to scrape across the surface of the concrete to level it in line with the top of the shuttering using a ‘to me, to you’ see-saw motion. (You can use the excess as rubble once it’s dry for the inside of the oven).
• You can now re-check the levels if you particularly like. (I did).
• Leave the concrete to dry for a minimum of 3 days.
I have since been told that it is possible to part fill the shuttering with rubble (hardcore) and pour a sloppy cement mixture over this to save money on cement. Live and learn, eh?