Make a Concrete Garden Bench

Building the plywood form to cast the bench legs

The bolt block and bridge are made from standard framing lumber. When assembled, concrete is poured in to fill the form.

Cutting fir plywood for the concrete forms
Ripping Fir plywood for the base

I’ll start by cutting plywood on the table saw and checking that it’s square. For these forms I used plywood as it’s what I already had around the shop. The legs are identical so one form is enough but, as I was filming and prototyping, I built two.

Cutting birch plywood for the concrete forms
And some Birch plywood too

One from recycled fir plywood, and one from birch.
You don’t need a lot of power tools to built the forms or the bench. You can get by just fine with a circular saw, drill, and driver. You could use a circular saw, a home-made guide, and clamps for these cuts as well.
If you’ve built the garden box forms then this should be pretty straightforward.

angle cuts on the table saw
Angled cuts for the inset

Attach the Inset

Like the base, the inset is plywood but beveled at 30 degrees. I make one inset from birch, and the other from fir.

angled plywood inset for the garden bench concrete legs
Concrete Garden Bench – Inset attaches to base with screws

I lay out and mark the corner locations on the base and drill pilot holes for the screws that will attach the inset. I apply a little glue then attach the inset with screws from the underside of the base.

angle cuts on the table saw
Concrete Garden Bench – Ripping side walls for leg form

Next I’ll cut the three plywood sides for the form. I rip them to width then cut them to length with a mitre saw. You could use a speed square and circular saw to make these cuts instead of a miter saw.

standard framing lumber for concrete form parts
Bolt block is in two halves

The bolt block is made from two pieces of framing lumber. I rip the upper and lower pieces from a 2×6. I rough cut these an inch longer than needed as I will trim it to final length after the pieces are screwed together. It’s easier to align this way. I mark the location for the three bolt holes and mark the screw locations between them. I use four screws to attach these two pieces together. I clamp them, drill pilot holes, then run the screws in.

forstner bit in the drill press - making the concrete forms
Counterboring bolt block on drill press

I’ll trim it to it’s final length then take this bolt block to the drill press to cut three counterbores on the outside face. I swap bits and drill three holes on the inside face to hold the bolts. I’ll talk more about the reason for the counterbore later.
The thru hole is 3/8’s diameter and the bolts fit snug with little or no play. I want the bolts to be held firmly and to not move once they are set in place.

drill press
Drilling Bolt Block

I mark and drill pilot holes for the screws that will hold the bolt block to the base and sides. I use the drill press for this but you can freehand these.
The bridge is made from a 2×4 and holds the bolt for the stretcher.

Assembling the form parts

assembling the bridge of the form with screws and impact driver
Assembling bridge

I cut the span and two supports to length and mark the center for the bolt. I counterbore and drill on the drill press.
All holes and counterbores could also be done using a drill guide instead of a drill press.
I mark and drill pilot holes in the supports. Then assemble the bridge with 3″ screws.

using self drilling screws for some of the concrete form assembly
Screws for assembling concrete form

With all the parts done I’ll do a test assembly of the form to see if everything fits together as planned. I’ll use self drilling, washer head style cabinet screws for most of this. Along with some two inch and three inch wood screws.
The pilot holes really help speed this up.

adding the bridge to the form
Attaching bridge to form

Now I’ll check the fit of the bridge and attach it to the walls of the form with 2″ screws.
It doesn’t always go this well but everything fit the first time for both forms. I mark each part with a sharpie, so I can reassemble it the same way.

the bolt block is made in two pieces
Bolt block taken apart
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