DIY Faux-Concrete Bench with Planter Boxes (with Wood Seat)

Create a modern outdoor bench with integrated planter boxes using cement board and thinset for a realistic “concrete” look, and a wood slat seat for warmth and contrast. This blog covers tools, cut list, framing, cement board wrap, thinset finishing, sealing, and seating—plus tips to avoid cracking and get a smooth result.

Project Specs

• Overall concept: Twin planter boxes with a connecting bench; cement-board + thinset “concrete” shell; wood slat seat

• Seat height: 18 in from ground to top of seat frame

• Seat span: ~4 ft between planters (adjust to fit space)

• Planter features: Slim outer wall; thicker inner wall to form an armrest next to the bench

• Finish: White thinset mortar (multiple coats) sanded smooth, then sealed clear; thermally treated ash seat boards (or similar exterior hardwood)

Tools & Supplies

Framing & Fastening

• Circular saw + speed square, miter saw or table saw (optional for ripping)

• Drill/driver; pocket-hole jig (e.g., Kreg) for cross-ties

• Clamps

• Exterior wood screws; pocket-hole screws

• Wood glue (optional but recommended on structural joints)

Board Wrap & “Concrete” Finish

• 1/2 in cement board sheets (3×5 ft is common)

• 1-5/8 in cement-board screws

• Utility knife + straightedge (preferred for cutting cement board)Avoid spinning blades to reduce silica dust; use a jigsaw with a cement-board blade only for specialty cuts.

• 4 in alkali-resistant cement-board mesh tape (for seams/corners)

• White thinset mortar (modified)

• 1/2 in square-notch trowel + flat trowel

• 150-grit sanding blocks/sheets (hand sanding recommended between coats)

• Clear exterior concrete sealer

Seat & Finishing

• 1×6 thermally treated ash (or exterior-rated hardwood/modified wood)

• Exterior trim screws (pre-drill; countersink helps)

• Paste wax or exterior wood finish of choice

Safety

• Respirator rated for silica dust, safety glasses, hearing protection, gloves

Cut List

Planter Frames (two planters; totals below)

• 2×4 @ 23½ in — 8 pcs

• 2×4 @ 15 in — 8 pcs

• 2×4 @ 13½ in — 8 pcs

• Additional 2×4 cross-supports for bench platform and seat slats (cut to fit on site)

Cement Board

• 1/2 in sheets, cut to wrap all faces (14 pieces were used in the build). Leave ~1/8 in gap at bottom to avoid ground contact.

Seat Boards

• 1×6 ash, length just over 53¼ in (four boards ripped to equal width for a uniform seat)

Build Steps

1) Frame the Planter Boxes

1. Assemble two styles of frames per planter:

   • One with 2×4s on edge (thicker wall/armrest side).

   • One with 2×4s flat (thinner outside wall).

2. Pre-drill to prevent splitting; glue optional.

3. Connect the two frames with horizontal 2×4 supports; use pocket-holes on 13½ in pieces for clean assembly.

4. Build a second planter to match.

   • Layout places the thin wall outward, thicker wall inward beside the bench to form the armrest.

2) Tie the Planters Together with the Bench Frame

1. Target 18 in seat height. Mark 5½ in down from the top of each planter leg to place the top of the 2×4 stretchers.

2. Install front and rear stretchers (glue + screws).

3. Add cross supports along the span for seat slats and rigidity.

4. Shim the bench face flush with planter faces using additional 2×4s so the cement board wraps cleanly.

3) Wrap with Cement Board

1. Score and snap 1/2 in cement board using a utility knife + straightedge.

2. Maintain a consistent overhang and ~1/8 in gap off the ground.

3. Fasten with 1-5/8 in cement-board screws, spaced ~9–10 in, snug but not over-driven.

4. Cut planter openings in the top panels:

   • Measure 2½ in borders on three outer edges

   • 4½ in on the inner edge (creates a comfortable armrest)

   • Drill a starter hole; use a jigsaw with a cement-board blade to follow the lines.

4) Tape Seams & Corners

• Apply alkali-resistant mesh tape to every seam and corner. This reinforces transitions and reduces crack risk in the thinset.

5) Apply Thinset (3-Coat System)

Mixing: Combine to a peanut-butter consistency; let slake ~5 minutes; remix.

• Coat 1 (Build Coat):

  • Lay large faces horizontally when possible to fight gravity.

  • Trowel on full coverage; then use a 1/2 in square-notch at ~45° to gauge thickness.

  • Flatten ridges with the flat side (medium pressure).

  • Aim for ~1/4 in build, not perfection. Let dry overnight.

• Sand (150-grit):

  • Knock down highs; even surfaces. Hand sanding gives control and avoids coloring issues from certain sanding discs.

• Coat 2 (Refine):

  • Repeat application; smoother passes; focus on corners and transitions.

  • Dry ~8 hours; sand smooth.

• Coat 3 (Final):

  • Best practice: skim the entire surface for uniform sheen and texture (spot-patching can telegraph after sanding).

  • Final sand by hand to a uniform, stone-like finish.

Tip: Thicker mixes behave better on vertical faces. If thinset sags, remix slightly stiffer.

6) Seal the Faux Concrete

• Apply two coats of clear exterior concrete sealer.

  • First coat soaks in; second builds a protective film and water resistance.

  • Follow label cure times.

7) Build and Install the Wood Seat

1. Cut four 1×6 ash boards just over 53¼ in; rip to equal widths for a tight, even seat.

2. Sand progressively; finish with paste wax or a UV-resistant outdoor finish (for sun-exposed installs, consider a marine-grade or penetrating oil finish).

3. Pre-drill and fasten with exterior trim screws (countersink recommended—dense species benefit from it).

4. Space boards evenly; keep consistent overhangs.

8) Planting & Finishing Touches

• Add planters (example: compact fig trees). Shim pot height for a clean reveal.

• For a built-in look: create a removable plywood false bottom topped with Mexican beach pebbles around the plant pot.

Durability Notes

• Cement board provides a stable substrate vs. plywood/MDF (which move with humidity and can cause cracking).

• Mesh tape + multiple thinset coats limits hairline cracks at seams.

• Maintain rigid framing (cross-bracing, flush faces) to minimize flex.

• Re-apply sealer per manufacturer guidance, especially in harsh climates.

Layout & Dimension Tips

• Keep the seat 18 in high for comfort.

• The planter opening with 2½ in borders and a 4½ in inner armrest yields a comfortable resting surface.

• Leave ~1/8 in gap from ground to cement board to avoid wicking moisture.

• The wood seat span of ~4 ft works well visually and structurally; add cross-supports to prevent deflection.

Cost & Time

• Materials: 2×4 lumber, cement board, fasteners, thinset, tape, sealer, 1×6 exterior wood, finish, pebbles, plants

• Tools: Mostly common DIY carpentry tools + trowels

• Time: A weekend for framing/wrap; additional days for thinset coats, cure, sanding, and sealing (allow overnight dry times between coats)

Common Mistakes to Avoid

• Over-driving cement-board screws (weakens the face).

• Skipping mesh tape on seams.

• Using thinset too wet on vertical faces (sagging).

• Spot-only third coat—skim whole surface for uniformity.

• Forgetting to pre-drill/countersink seat screws in dense hardwoods.

Material Checklist

• 2×4 lumber (frames, stretchers, cross supports)

• 1/2 in cement board sheets (14 pcs cut in example)

• Cement-board screws, 1-5/8 in

• Cement-board mesh tape (alkali-resistant), 4 in

• White modified thinset mortar

• 1/2 in square-notch trowel + flat trowel

• Utility knife, jigsaw w/ cement-board blade

• 150-grit sanding sheets/blocks

• Clear exterior concrete sealer

• 1×6 thermally treated ash (or exterior-rated wood)

• Exterior trim screws (for seat)

• Paste wax or exterior wood finish

• Mexican beach pebbles (optional)

• Plants/pots, shims/blocks for pot height