Gather your supplies
- Formwork sides (2) 434 x 467 x 17mm formply
- Formwork back 400 x 467 x 17mm formply
- Formwork base 400 x 400 x 17mm formply
- Formwork front 400 x 377 x 17mm formply
- Shutter 90 x 45 x 400mm pine
- Concrete (six or seven 20kg bags per block, depending on size of polystyrene blocks, if used)
- Seat pieces (2) 200 x 100 x 2400mm hardwood timber
- Cleats 50 x 50 x 1800mm hardwood timber
You’ll also need Wheelbarrow; waste polystyrene blocks (optional); 45 and 74mm wood screws; steel finishing trowel or float; power saw; belt sander; electric planer; cordless drill; tape measure; spirit level; 10mm blue metal; exterior furniture oil; safety gear; selection of plants (we used agave)
1. To cast a solid concrete block of a certain size, you need to build formwork with an internal size that is the same as the required block size. Here, we created blocks that are 400 x 400mm square with a height of 450mm and a 140 x 90mm rebate across the top to support the hardwood seat.
2. To make multiple blocks, you simply re-use the same formwork, but you will only be able to make 1 block a day. You will also need to clean the formwork after each use, so it is smooth.
3. All screws must be driven from the outside of the formwork, otherwise you will not be able to strip it from the cement without destroying it. Then, you can keep using it many times until it wears out.
Predrill, then, using 45mm wood screws, attach formwork sides to formwork back and base with joints flush. Attach formwork front so top edge is 90mm down from top of sides. Predrill, then screw shutter between sides with back edge 140mm from front edge (this creates the rebate in block).
Pour contents of 3 bags of concrete into wheelbarrow and slowly Photography Brent Wilson add water to make a workable mix. You do not want it too stiff as it will not flow into corners of formwork. To lessen final weight of blocks, you can add waste polystyrene blocks to sit inside concrete. Just make sure there is at least 50mm of concrete all around polystyrene. Continue mixing and adding concrete to fill up to level of bottom of rebate. Using a spade or a short length of timber, tap formwork to vibrate concrete so it settles properly and to eliminate excess air bubbles.
Finish filling formwork to top. Use steel finishing trowel or float to smooth surface of rebate and top of block. Make sure concrete rebate finishes flush with underside of shutter. Let concrete set overnight, leaving formwork in place as long as possible after concrete has set and keeping block moist by hosing gently.
Remove all screws from formwork and gently tap with a hammer to release it from block. Carefully remove formwork piece by piece (you will need to tip block over to remove base).
Repeat Steps 1-4 to make a second concrete block.
Find desired positions for concrete blocks, allowing for desired length of timber sleepers (here, ours measured 2100mm after they were cut), then spread a bed of 10mm blue metal for each block.
Position both concrete blocks on blue metal, then check level.
Using power saw, cut both seat pieces to desired length (here, 2100mm). Using electric planer, smooth upper surfaces of sleepers and chamfer edges to prevent splinters forming. Using belt sander, smooth all surfaces, starting with a coarse-grade sandpaper and finishing with a medium grade.
Using power saw, cut three 50 x 50 x 360mm cleats with a 45° angle at each end.
Put seat pieces together and position 1 cleat at each end and 1 in centre. Predrill, then, using 75mm wood screws, attach cleats to seat pieces. Finish with a coat of exterior furniture oil
Position finished seat on rebates of concrete blocks.
Add plants, then cover area around and under bench with blue metal.
Looking to make more outdoor furniture? Give these DIY's a go: