A set of cast concrete house numbers costs a few pounds to make and will still be on the wall long after a shop-bought plaque has faded. Steve from Proper DIY casts three signs in his workshop - a house number, a mile marker and a catchphrase block - and the number plaque is the one most people will want to copy for their own front door or gatepost.
The whole thing comes down to a reversed mould. You cut your digits out of plywood, glue them face-up in a shallow melamine box, then pour concrete over the top. When it cures and you strip the box away, the numbers are left as clean recessed figures in the face of the slab. If you have not batched a craft-sized load before, read how to mix concrete for crafts first, since the 4-to-1 ratio and the water you add decide how sharp those numbers turn out.
Two details lift the finish: a bevelled edge cast straight into the mould, and a proper vibrate to chase the air bubbles out from around the numbers. Once it is cured you can seal it for the weather - see how to seal concrete crafts for the indoor-versus-outdoor sealer choice. The same box-mould method scales to plenty of other pours, like concrete bookends or a concrete catchall tray.
Which concrete mix to use
Skip bagged post mix or anything with gravel in it. Coarse stones sit against the mould face and break up the edge of each digit. You want fine material: 4 parts sharp sand to 1 part cement, a sand-and-cement mortar mix, or a countertop mix for a smoother face. A handful of fibres helps a thin plaque resist cracking. For a charcoal or warm grey sign, read up on colouring concrete for crafts before you add water.
Getting crisp numbers
The 10-degree taper on each plywood digit does most of the work. Straight sides grip the concrete and chip the rim on the way out. Sand every edge smooth, because concrete copies whatever grain or fuzz is on the wood.
Craft foam or foam board is faster if you have no band saw. Two rules apply. The digits go in back-to-front, since you are casting face down. And foam tears more easily than plywood, so give it an extra coat of oil.
Mould release options
- Mineral oil or butcher-block oil, brushed thin (what Steve uses)
- Cooking spray, fine for a one-off
- Packing tape over any raw plywood edge, so it does not soak up water
Curing and demoulding
Give a 25-40mm plaque at least 48 hours before you strip it, and longer in a cold garage. Cover it with plastic so it cures damp rather than drying out. Concrete keeps gaining strength for about four weeks, so let it sit a week or two before you drill or mount it.
Sealing for outdoor use
An unsealed slab soaks up rain, and in winter that water freezes and flakes the surface. A penetrating silane or siloxane sealer is the safest outdoor choice. It soaks in, does not peel, and keeps the natural look. Paint the recesses first, let them dry, then seal over everything.
Mounting on brick vs siding
On brick or render, drill with a masonry bit and use wall plugs and stainless screws, with a bead of construction adhesive behind for extra hold. On wood or vinyl siding, screw into a stud or a backing board, never into the cladding alone. A slab this size is heavy, so weigh it before you choose fixings.
Common mistakes
- Pits around the digits. Not enough vibration. Keep going until bubbles stop rising. More fixes in fixing air bubbles in concrete crafts.
- Chipped corners. Demoulded too soon, or no chamfer on the edge.
- White chalky bloom. That is efflorescence. Scrub it off with a stiff brush, let it dry, then seal.
- Numbers that read backwards. Check the layout in a mirror before the silicone sets.
