Make a Wood and Concrete Outdoor Garden Bench

Background for the Concrete Garden Bench ONE

Hi everybody this is Kent from man about tools and today we’re making this wood and concrete garden bench.

Concrete Garden Bench

I’ve been wanting to make some benches to go along with the concrete garden boxes I’ve been casting. I like the blend of concrete and wood with simple, clean lines. This design has square concrete legs and a flat planked wooden seat. Here’s how the bench goes together.

Concrete Garden Bench – Square Legs to match Garden Boxes

The concrete legs have three protruding seat bolts at the top and one bolt on the inside to attach a stretcher.

Garden Bench ONE – digital design

Three planks make up the seat.

Nut on Stretcher Bolt

Nuts with washers are threaded on the stretcher bolts through a hole in the side.

Nuts hold seat planks to legs

The seat planks are also secured with washers and a nut. And two ties are screwed to the bottom of the planks.

Oh she’s a beauty right?

I wanted to make strong connections between the concrete and the wood. I know there’s a number of ways of doing this with drilled anchors and brackets. But, for this first bench in this series I wanted to try casting anchor bolts in the form during the pour. In the similar fashion as a house foundation with anchor bolts for the sill plate.

Stretcher gives stability

And so nothing else would be needed once you stripped the form from the casting. Now I know this would mean a bit more work in the construction of the form, but building the bench in the end should be faster and should make it very sturdy.

So here’s the form for casting the legs.

Form Design for pouring legs

The form has a 3/4″ plywood base with an angled inset attached, just like the the garden boxes.

Inset attached to base

A two part block holds the seat bolts in place and can be taken apart after casting.

Bolt block

The end wall and side walls are made of plywood. The anchors are 3/8th’s galvanized carriage bolts 3 1/2″ long.

Carriage bolts sit in bolt block

The form has a bridge that holds the stretcher bolt in place.

The bridge holds the stretcher bolt while pouring concrete

This stretcher helps to prevent side to side racking, as the seat boards have only a single attachment point on each end.

Form Bridge Assembly

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

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.

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.

Angled cuts for 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.

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.

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.

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.

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.

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 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.

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.

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.

Bolt block taken apart

I take the forms apart before I coat them with oil. I also disassemble the bolt block to get oil on all surfaces and seams.

I had some mineral oil left over from making the garden box forms.

Applying mineral oil to seal wood

This oil is sold as Butcher’s Block or Cutting Board Oil. It’s fast and easy to apply and seems to work well. Mineral oil can be bough from Animal Feed suppliers or in drug stores or pharmacies.

I apply it to all faces then leave overnight to soak in and dry to the touch.

I reassemble the main box of the form and run a bead of latex caulk on the inside edges.

Latex calking to form

This makes the form watertight, gives a small radius on the outside corners, and it’s easy to remove after casting. Latex works well as it’s not overly strong and will allow you to disassemble the mold once the concrete sets up.

When the caulking is dry I spray the inside of the form with vegetable cooking oil to keep the concrete from sticking.

Cooking spray as form releasant

Now I’ll insert the three bolts. I line them up with the outside face of the bolt block. I don’t want the ends to stick out past the outside edge as this will make the bolts stick up on the bench seat. I add a nut to the ends of these bolts and snug it finger tight. The counterbore allows room for my fingers or a socket.

As this is a prototype, I wasn’t sure how well the bolt block would hold the bolts in place during assembly, pouring, and vibrating the form. The nut is just a bit of added insurance.

Carriage bolts used

Now I secure the bolt to the bridge with a washer and nut then set the bridge in place and screw the supports to the form walls.

Form is now ready for concrete

The form is now ready for concrete.

I set out saw horses and use some shim strips to level them front to back and side to side. This is an important step as the form has to be level.

Levelling form on saw horses

So here’s the ingredients for the concrete. CSA Mortar Mix, glass fibre, plasticizer, colour, and Citric Acid powder.

Ingredients for Concrete Mix

I use 55 lbs of CSA Mortar Mix to 5 quarts of cold water, a packet of plasticizer, 1.3 ounces of citric acid, and 4 and a half ounces of colour. I also will add a bit of glass fibre for extra strength. One 55lb bag of this Mortar Mix is required per leg.

Mix recipe I used

I measure the water then add it to a pail. I then add the other ingredients and dissolve them in the water.

Mixing concrete in a pail

I have an egg beater style mixer on my drill that works well for mixing this concrete.
I use a plasticizer to make the concrete flow and pour like water.
And I add the citric acid to give more working time.

I then slowly add the mortar mix, about a quarter of a bag at a time. In North America, the Rapid Set brand can be found at Home Depot. I add a third of an ounce of glass fibre for more strength. This is the manufacturers recommended amount.

The CSA mortar mix has a light beige colour. And that’s fine but I wanted to make the legs grey so I added black concrete dye to the mix. I was thinking that it would blend better if I added it to the water first. I’ve been asked a lot about colour additives for concrete so I added some for these castings and will try other colours in the future.

I used a length of wooden dowel to stir the mix to check for any clumps on the sides or the bottom of the pail.

Pouring concrete in form

When it’s blended smooth I pour it into the form, filling it right to the top.

Almost full

I really like this CSA mortar mix and I used it in Part 4 of the garden box series so I decided to use it again for these forms. It’s really easy to mix, pours like water, sets up fast, and requires little or no troweling. After about 10 minutes I vibrated the form to bring any bubbles to the surface. For this I use a reciprocating saw without the blade.

Vibrating form with recip saw

While the first pour was setting up I mixed another bucket of concrete. And filled the second form. Then vibrated it after about 10 minutes.

In about 45 minutes the concrete began to solidify. Then shortly after that it began to dry and show a white haze.

Concrete drying – showing haze

I sprinkled water on the surface every 10 minutes or so for about an hour to keep it wet and allow it to cure.

Water curing Mortar Mix

After an hour of this, the concrete can be removed from the forms. It was the end of the day so I covered the forms with plastic and left them overnight.

Now for what I think is the best part of all of this: stripping the forms to reveal the casting.

Stripping forms

I wasn’t sure how well the split bolt block would work or if it would be difficult to remove but it went just fine.

The base is freed from the casting

The newly cast legs should now be at around half their full strength. They will reach their full strength if you keep wet to cure for four weeks. For this project I’m not going to worry about that here and I’m going to build the bench right away.

Painters tape applied to exposed bolts for safety

For safety, I put a few wraps of tape around the exposed bolts while I was handling the leg castings.

Cutting cedar seat planks to length

I used a small block of concrete from a previous pour to round off any sharp corners on the casting while it was still relatively soft.

I’ll make the seat for the bench from red cedar 2×6’s. I’ll cut them to length on the mitre saw then lay out the hole locations on each end.

Marking hole locations in seat planks

I’ll use the drill guide to first counterbore for the nut and washers, then drill through for the bolt.

Counterbore seat planks

I use a square as a gauge to get a consistent bore depth.

Using a drill guide ant forstner bit

The stretcher is made from a 2×4. I had some cedar 2×6 left from the seat planks so I ripped it down on the table saw.

Drilling stretcher

I lay out and mark the location for the holes on each end and drill them from both sides with a forstner bit. I drilled a pilot hole first to help guide the larger bit. And this worked pretty well.

To make drilling into end grain for the stretcher bolt easier I first make a drill guide from a scrap piece of 2×2.

Drilling into end grain of stretcher with a drill guide

Then I clamp the stretcher and guide to my bench.

Stretcher holes intersect

The drill will follow the guide and give a centered and true hole.

I’ll attach ties under the seat to connect the planks and prevent any side to side racking .

Seat ties

These are made from a 1×4. And they have a 45 degree bevel on each end. I’ll drill pilot holes now to make assembly of the bench easier.

I sand the ends of the planks to round over the corners. And sand off any rough spots on the other parts.

Then apply two coats of oil-based stain and leave everything to dry overnight.

Staining red cedar

To help with assembling the bench I mark the corner locations of the legs on a flat surface.

Prepping  to assemble bench

I use a long straight edge and framing square for this. I set the legs in place while I slide the stretcher onto the bolts. Then add the seat planks and check everything for square. I add washers and nuts to the seat bolts just finger tight for now.

Sliding stretcher onto bolts in legs

I slide the flat washer and lock washer on the stretcher bolts.

Placing seat planks

Then thread the nut on the bolt and tighten with a wrench. Not too tight, just enough to pull the end of the stretcher against the concrete leg. And I do the same on the other end. I can now tighten the nuts on the seat planks with a socket wrench .

Clamping ties to underside of seat

The ties are screwed to the seat planks from underneath. I use a scrap strip of wood and clamps to hold it in place while I run in screws. With the seat planks having only two attachment points, the ties are required to counter any twisting forces.

I realized that my center two pilot holes should have been offset as the driver was blocked by the stretcher. So I just drilled two more holes.

Rubber gaskets

I forgot to add rubber gaskets to keep any wood from contact with concrete. Wood will wick moisture from concrete and cause rot. So a rubber gasket will help prevent this. I used an old tire inner tube and cut the gaskets with scissors.

Gaskets laid on legs

When I move this bench out to the garden I’ll add the rubber gaskets then.

Please leave a comment with your thoughts or suggestions. I am amazed by the wealth of knowledge out there and I so appreciate everyone who has shared it.

Concrete Garden Bench – DONE!!

So I hope you have enjoyed this post, and thank you so much for reading. If I make any further improvements or changes I’ll post them here.

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