How to make a concrete bench
Materials you will need
• 11ft (3.3m) of 2" x 4" (100mm x 50mm) wood, allowing for a little bit of waste.
• 4ft (1200mm) of 2½" x 5½" (140mm x 62mm) wood.
• 13ft (3.9m) of ¾" x 5½" (140mm x 20mm) wood. * see Note 1 below.
* Note 1: 2½" (62mm) thick wood is less common than ¾" (20mm) thick wood, and harder to cut the shape with a jig-saw (although it can be done). The ¾" (20mm) thick wood is more readily available and of course a lot easier to cut, but you will need three times as many pieces, as you will need to glue and screw three pieces together to make up for every one piece of the thicker wood. The thicker option is the best way to go if you can manage to acquire the wood (or close to it) and have the tools to cut it.
• 8ft (2400mm) of ½" (12mm) angle fillet, allowing for a little bit of waste. * see Note 2 below.
* Note 2: Angle fillet (in this context) is a triangular shaped strip of wood. Its purpose is to make a beveled-type edge around the top of the bench. You can use a different profiled piece of wood if you want something a bit more fanciful, or you can disregard it altogether for a plainer look.
• Half a standard sheet of ⅝" (15mm) thick plywood. A standard sheet is 4ft x 8ft (1200mm x 2400mm) so you will need half of that, allowing for a little bit of waste.
• I piece of ⅛" (3mm) thick plastic sheet, 40" x 22" (1000mm x 550mm). * see Note 3 below.
* Note 3: The plastic sheet is to make a smooth finish on the top of the bench. The end product of a concrete cast will be as smooth as the form. The plastic sheet can be any thickness, or you can disregard it altogether, in which case the finished product will be as smooth as the plywood form. That can still be a reasonable finish.
• 12 of 4⅜" (110mm) long x ⅜" (10mm) diameter hex bolts.
• 4 of 3½" (90mm) long x ⅜" (10mm) diameter coach/carriage bolts.
• 16 of ⅜" (10mm) wing nuts.
• 4 of ⅜" (10mm) standard nuts.
• 32 of ⅜" (10mm) round washers.
• 30 of 1½" (38mm) long wood screws and 12 of 1" (25mm) long wood screws.
• 4 of 4" x 2" x 2"wide (100mm x 50mm x 50mm wide) galvanized angle brackets. * see Note 4 below
* Note 4: The angle brackets are to hold the leg form to the bench form. They make for easy alignment when it comes to assembling the form for casting. The bracket size given above is not absolute. Anything similar will do as long as it does the trick. You may have to make your own by cutting, bending, and drilling a piece of sheet metal. On the other hand, you can do away with them altogether and use clamps to hold the leg forms to the bench form. This latter method will require a bit of extra measuring and aligning when it comes to assembling the forms for casting.
• 5 sq ft (0.5 sq m) of 5/32" (4mm) or slightly thicker reinforcing wire mesh. * see Note 5 below
* Note 5: Home-mixed concrete is not always as strong as it could be, so reinforcing steel or mesh is necessary. The mesh and method used in this project is but one way of adding adequate reinforcing. You can even make up your own grid using the same size or a thicker rod, and join the intersections with tie wire. The most important point is that the reinforcing covers the area of the bench and is also tied to other reinforcing that bends into the leg area.
• About 3/4 of the average heavy-duty wheelbarrow full of concrete mix (gravel, sand, and cement).
• And... some release agent so the concrete doesn't stick to the form. * see Note 6 below
* Note 6:
A release agent is a substance that is applied to surfaces to prevent them from sticking, in this case so that the concrete
does not stick to the form.
You can purchase form release agents from some building supply stores, often in handy little spray bottles.
You can also make your own for a fraction of the price out of an even mixture of used car oil and diesel, and apply it with a paintbrush or a rag. That's what we do.