Making concrete molds from wood is an easy and cost-effective way to create customized concrete structures and objects. With some basic carpentry skills and the right materials, you can make durable wooden molds for concrete planters, statue bases, stepping stones, walls, and much more. Here is a step-by-step guide to making concrete molds from wood.
Choose the Right Wood
The type of wood you select for your concrete form is important, as it needs to withstand the pressure of poured concrete without warping or decaying. Here are some good options:
- Plywood – Plywood like birch or lauan works well for many concrete applications. Go with at least 3/4″ thickness for sturdiness. The layers help prevent warping.
- MDO Plywood – Medium density overlay plywood has a resin-treated surface that resists moisture absorption. It holds up to repeated concrete pours.
- Melamine – Melamine covered particleboard is very smooth, non-absorbent, and durable. The slick surface results in attractive concrete finishes.
- Plastic Lumber – For complex forms, plastic lumber’s flexibility and waterproofness can be useful. Look for types made with recycled content.
For large or heavy objects, 3/4″ to 1″ thick plywood is best. Smaller projects can use 1/2″ thickness. Avoid thin plywood or wood paneling prone to distortion.
Construct the Mold
Building the form involves basic carpentry techniques. Follow these tips:
- Design forms following the shape and dimensions of the finished concrete object. Leave room for concrete thickness.
- For curves and angles, use several narrower pieces instead of a single wide board to create the form.
- Use wood screws, nails, or glue to connect form parts together securely. Seal edges with caulk.
- Coat forms with form release agent so concrete won’t stick to the wood.
- Include openings or tubes for pouring concrete and allowing air bubbles to escape.
- Brace molds sturdily underneath and on the sides. Temporary cross braces keep sides from bulging.
- Level and position mold properly before pouring. Elevate on blocks to allow concrete flow.
Add Mold Features
Wood forms can be adapted to give concrete interesting shapes, textures, and accents:
- Attach wood, metal, plastic, foam, or rubber profile strips and textures to create imprinted designs.
- Use pegs, tubes, and wood blocks to form holes and voids for drainage or design effects.
- Incorporate ties and anchors for attaching features later like metal rails or stone veneers.
- Coat areas with form release or plastic sheeting to produce a smooth, untextured finish.
- Line the form with fiberglass or polyethylene for easy demolding.
Prepare the Concrete
The right concrete mix and viscosity ensures the wood form is filled completely:
- Use a high performance 4000+ psi concrete mix for durability and strength.
- Self-consolidating concrete flows well into detailed molds without vibrating. Or, add plasticizer.
- Make sure the mix isn’t too runny. Aim for a consistency that won’t cause segregation when poured.
- Let concrete cure at least 48 hours or as directed before removing wood forms.
Remove the Form
Once concrete has hardened sufficiently, it’s safe to disassemble and detach the mold:
- Remove screws, clamps, and braces. Carefully separate the form pieces.
- Use pry bars and hammers to loosen if stuck. Take care not to crack concrete.
- Clean wood thoroughly after use to remove concrete and reuse forms repeatedly.
- Allow concrete to continue curing for several additional days before excessive handling.
Now that you know the basics of building wood forms for concrete, you can create all kinds of useful and decorative objects from this versatile material!
Frequently Asked Questions
What are the advantages of wooden concrete forms?
Wood offers many benefits for concrete forming:
- Easy to work with using ordinary carpentry tools
- Readily available and low cost
- Can create a wide range of shapes and textures
- Durable for repeated use
- Won’t bend, dent or warp like metal forms
- Easier to remove than forming with clay
What sealing should be used on wood concrete forms?
Polyurethane sealants and form release agents help keep moisture from being absorbed and prevent concrete from sticking. Wax, varnish, plastic sheeting, and foam tapes can also assist demolding. Avoid sealers that react with concrete such as creosote or oil-based products.
Can MDF or OSB be used instead of plywood for concrete forms?
Plywood is generally a better choice than MDF or OSB. The many layers of veneer in plywood provide strength to withstand concrete pressure without warping. MDF and OSB are more prone to swelling and deterioration from moisture over time.
How thick should plywood be for concrete forms?
Most concrete forming applications require a minimum 3/4″ thickness plywood. Very large vertical pours may benefit from 1″ thick boards. Use multiple layers of thinner plywood for unusual shapes. For simpler projects like landscape borders, 1/2″ plywood can be sufficient.
How many times can you reuse wooden concrete forms?
Wood forms can be reused many times if properly built and cared for. Use sturdy plywood, seal surfaces, and detach concrete carefully without damage. With proper use and maintenance, wood forms often last through dozens of pours before needing replacement.
Can you embed rebar or wires into wooden concrete molds?
Yes, it is common to include anchoring points in wood forms for attaching rebar, mats, and meshes before pouring concrete. The steel reinforcements become embedded within the finished concrete for greater strength and stability.
What glue works best on wood concrete forms?
Polyurethane glues bond wood firmly to create watertight concrete forms. Epoxy adhesives also work very well. Avoid standard wood glues which can fail under concrete’s moisture and pressure. Screws and nails provide plenty of strength in most cases without glue.
How do you align wood planks for a concrete form?
Use level lines marked on the form boards to align edges precisely. Cut tapered strips to smooth transitions between misaligned planks. Shims can adjust small gaps as needed for flush alignment. Avoid cracks by sealing joints with caulk, foam, or epoxy filler.
How do you prevent air bubbles in wood concrete forms?
Vent holes and channels allow air and excess moisture to escape as concrete flows into the form. Plastic tubes, drinking straws, and half-inch pipes create effective vents. Tap on forms with a mallet while pouring to dislodge trapped air pockets.
Can wood concrete forms be painted?
Yes, painting wood forms can hide imperfections, reduce absorption, and prevent shapes transferring through the concrete surface. Avoid latex paint which may peel. Alkyd or epoxy paints provide good results. Remember to coat forms with form oil or release agent after painting.
Professional Tips and Tricks
- Use form ties that snap-off below the surface rather than leave metal holes to patch
- Line forms with 6mil plastic sheeting for a smooth, modern concrete look
- Use radii cut from plywood to create curved corners instead of sharp 90 degree edges
- Add waterproofing admixtures or acrylic sealers to protect outdoor concrete projects
- Incorporate decorative accents like tiles, stones and embeds into wood mold designs
- Build sturdy support frames allowing forms to withstand concrete weight and vibration
- Check for level and plumb orientatation before pouring to prevent flaws in finished products
- Let freshly poured concretepieces cure adequately before handling to prevent surface damage
Constructing your own wooden concrete forms gives you the flexibility to produce precisely the custom shapes and textures you want cost effectively. With some practice at building forms following the tips above, you can create beautiful planters, countertops, statues, and anything else you can imagine from concrete. The results will look professional and built to last when you make concrete molds properly from wood.