Invisible Mitered Joint - Worktops

This job depends on accuracy. An error nobody would notice elsewhere shows up here as a visible gap or a small step between the boards. The process usually looks like this:

1. Check that the base units are fitted, level and at the right height. Also check whether the walls in the corner really meet at a right angle.
2. Measure the worktops and decide where the joint will go. Keep it away from the sink and not right next to the hob.
3. Clamp a worktop jig to the board and rout the joining edge. If the worktop has a rounded front edge (postformed), the jig also follows this profile so the edges meet cleanly at the front.
4. Rout recesses on the underside for the worktop connector bolts. On thicker boards, biscuits or dowels are added as well to hold both boards at the same height.
5. Dry-fit the boards, check the gap and the height, and fine-tune if needed.
6. Coat the cut edges with the sealant or adhesive the worktop manufacturer recommends, then gradually tighten the boards together with the bolts.
7. Wipe off any squeezed-out adhesive or sealant straight away and leave the joint to cure as the manufacturer instructs.

Why is the joint only made once the units are finished? The worktop follows whatever it sits on. If the units are adjusted after the worktop goes on, the joint takes the strain and can open up.

The joint is made a little differently for each material. So talk the material through with your fitter before you order the worktop.

Laminate worktops with a chipboard core

These are the most common type in kitchens, and a routed joint works well on them. The key is sealing. Chipboard swells when it gets wet, and that damage cannot be repaired. The cut edge must be sealed thoroughly along its whole length.

Solid laminate (compact) worktops

These are thin, dense and not affected by water. The joint is usually very clean. They do need a jointing system made for compact materials and tools that can cope with such a hard material.

Solid wood and stave worktops

Wood moves as humidity and temperature change: it expands and shrinks slightly. The joint has to allow for this, and the surface, including the cut edges, must be treated with oil or lacquer. This work calls for a joiner with experience of solid timber.

Budget worktops with a thin or brittle surface layer, and hollow-core boards, are risky to rout. The edges can chip, and the bolts may not hold well in the core.

How demanding the joint is, and so how long it takes and what it costs, varies from kitchen to kitchen. These things make the job harder:

• A postformed worktop with a rounded front edge, where the edge profile must also line up exactly
• A U-shaped kitchen with two joints, where an error in one corner carries over to the other
• Walls that are not square and an uneven floor under the units
• A dark gloss finish, which shows every inaccuracy more than a light matt surface
• Solid wood and compact worktops, which need a different method and different tools
• Working in a tight space where the boards are hard to turn and clamp

A straight joint on a standard square-edged laminate worktop, on well-levelled units, is one of the simpler cases. It also helps if the joint is routed in a workshop or on a solid bench rather than balanced on trestles on site.

In practice a joint often looks fine at handover, and the problem only appears a few months later. The usual causes are:

• A joint near the sink. This is where water sits most often. If the joint cannot go anywhere else, it must be sealed especially carefully.
• An unsealed cut edge. Water gets into the core, and the board swells and lifts at the joint.
• A height difference. This happens when biscuits are left out, the units are not level, or the bolts are tightened unevenly.
• A chipped edge. This comes from a blunt cutter or routing in the wrong direction.
• Strain from appliances. A heavy built-in oven, or a badly fitted sink or hob, can load the units so much that the joint slowly opens.

Connecting the oven, hob and cooker hood to the mains is work for a qualified electrician. A gas appliance may only be connected by a qualified, registered gas engineer. If you smell gas while work is going on in the kitchen, turn off the supply at the valve, open the windows, do not use any electrical switches and call 112 or your local gas emergency number.

A well-made joint lasts for years, but a little care in everyday use helps. Wipe up water and spills on the joint straight away rather than leaving them to sit. Keep harsh chemical cleaners off the joint. Do not put hot pans directly on the joint, especially on laminate worktops. Treat a wooden worktop regularly as the manufacturer recommends, so the surface stays sealed at the joint too.

At handover, look at the joint closely and in daylight. Check:

• That the line is evenly fine along its whole length and does not open anywhere
• That both boards are flush, so you feel no step when you run a finger across
• That the front edges and any rounded edge meet exactly
• That the edges are free of chips and the surface has no scratches
• That all squeezed-out adhesive or sealant has been removed
• That the bolts underneath are tight and can be reached for later adjustment
• That the joint is away from the sink, or is sealed especially carefully if it is close to it
• That you have received the worktop manufacturer's care instructions