Guides · Technical

A stone worktop over a dishwasher or an oven: steam and heat from below

In short. Stone is one of the few materials for which dishwasher manufacturers write, in their own instructions, that the steam deflector is not required. That is true, and it is the half-answer that does the damage: steam does not ruin the slab, it ruins the cabinet holding it up and the joint nobody sealed. Below is what the appliance manuals say, in their own words, and the lines worth insisting on before the slab is cut.

Does dishwasher steam damage a stone worktop?

Not the slab. The steam released from the door at the end of a cycle is clean water at low pressure: on polished or honed marble it leaves at most a dark shadow while the stone takes it up, and the shadow goes when it evaporates. It neither etches nor stains, because marble is etched by acids and alkalis, not by water: the difference between a shadow that passes and a dullness that stays is in marble stains and etching.

One qualification the manuals do not make: marble is not waterproof the way Corian is, it is porous. But its porosity is not that of chipboard, and a marble edge exposed to steam for ten years is still a marble edge. That is not where steam does its work: it goes into the cabinet, where it finds chipboard, glued edges and screws.

So why do appliances come with a steam deflector?

Because the worktop they have to protect, in most kitchens, is not stone. Miele’s installation instructions for integrated dishwashers are explicit: a stainless steel deflector is supplied «to protect the countertops from steam and condensation», but — their words — «if the countertop is made of Corian®, granite, marble or other solid, waterproof material, the steam deflector is not required».

A few lines below is the instruction almost nobody reads: if the counter is made of different materials, the deflector should be positioned so that it covers the edge where the materials are joined. Translated for site: the stone needs no protection, the joint between the stone and everything else does.

So what actually gets ruined?

The manual of a Bosch built-in dishwasher puts it plainly: at the end of the programme there is steam escaping which «in the long term can damage fitted units». Not the worktop: the units. And it is what you see in five-year-old kitchens — doors swollen along the top edge, a cabinet side delaminating, the rail under the worktop bowing.

If that rail is the worktop’s support, the failure becomes the stone’s problem indirectly: a bearing that deforms stops supporting where it matters, and stone gives way where it was weakened, which is at the corners of the cutouts (why a stone top cracks at the cutouts) — and the dishwasher sits almost always right under that area.

Three measures that solve it at the root:

• a vapour barrier — foil tape or a metal plate — on the underside front edge of the worktop and on the side panel of the adjoining cabinet, stone worktop or not: it is not for the stone, it is for the wood around it;
• the joint between worktop and cabinet sealed: a two-millimetre gap carries steam into the unit for ten years;
• the extended drying option on recent appliances, which Miele names as a further measure to protect the counter from condensation.

How is a dishwasher fixed to a stone worktop?

This is the stone-specific question, and the manufacturer answers it: it is not fixed to the stone. The same Miele instructions that exempt marble from the deflector add that the mounting brackets must not be installed «if the counter is made of an extremely hard material», because «the securing screws for the mounting brackets are not suitable for such materials».

The other requirement does not fall away, and both manuals word it the same: install built-under and integrated appliances only beneath continuous worktops screwed to the adjoining units. The appliance must therefore sit under a worktop made one with the structure, and at the same time cannot be screwed to it: stability comes from side fixing to the cabinet panels, not from two screws in the slab.

A marble worktop merely resting and unrestrained, above a dishwasher that cannot be screwed to anything, is the kitchen where sooner or later somebody drills the stone from below with a plug: it is avoided by deciding at design stage how the worktop is held.

Can a dishwasher go under a hob?

No, and the manuals agree bluntly. Bosch: above all, do not fit it under a hob. Miele: do not install the dishwasher beneath a cooking surface or oven. It is a requirement about the appliance, not about the stone, but it has a deadline: settle it before the slab is cut, because afterwards the hob cutout is where it is.

An oven below: how much heat reaches the stone?

Far less than feared, on one condition: that air circulates as the manual asks. Bosch installation instructions for built-in ovens state how much heat that housing has to take: the housing unit must withstand 90 °C and adjacent units 70 °C. They also require 5 mm of free space towards neighbouring fronts and a ventilation slot in the false bottom.

None of these figures mentions stone, and that is the point: marble and onyx take 90 °C without altering. The enemy of stone, with heat, is direct and localised contact — a hot pan set down — not the diffuse warmth of a well-ventilated oven. On composite surfaces direct heat does leave a mark: can a burn mark be removed from a quartz worktop?.

If there is an induction hob above the oven, Bosch instructions for those hobs call for at least 5 mm between hob and oven below, a worktop at least 20 mm thick and 65 mm of clearance to a drawer: these are the figures that decide the thickness of the slab. And the oven manual asks for something that often goes wrong on site: complete all cutting work on the units and the worktop before the appliances are inserted.

Can stone crack because of heat from below?

On its own almost never; together with a restraint, yes. A slab that warms expands a little, and if it is forced into place and sealed with no clearance that little discharges into the inside corner of the cutout: a crack that looks caused by heat is almost always caused by two missing millimetres. Where a seam can fall is in how long a stone top can be without seams.

The lines to put in the order

• Which appliances sit under the worktop, with make and model: the figures change from model to model and the manual prevails.
• That no appliance sits under the hob cutout.
• The vapour barrier on the underside of the worktop and on the cabinet side panel, and who fits it.
• How the worktop is restrained and how the dishwasher is fixed: no screws into the stone.
• The expansion clearance around the appliances and the sealant the manufacturer specifies.
• The ventilation of the oven housing, with the slot in the false bottom.
• The order of works: cutting finished before the appliances go in.

How we work

Stoneform makes tables, tops and works in marble, onyx and natural stone. We buy blocks directly at the quarry and cut the slabs ourselves, in our own factory. We do not use resin to reinforce slabs: on large surfaces a support helps, and the mesh on the back is already good backing. When a top has to host appliances we ask for make and model before cutting: it is the appliance manual, not our habit, that sets the figures. On contract jobs installation is part of the work (Contract).

On price: dealers apply a 35-50% mark-up on the maker’s price, those who import the stone directly save a further 20% or so, and a 240 cm marble table costs around 3,800 euro — the price depends on size and finish, and smaller does not mean cheaper (the detail, and the edge profile). The pieces are among the tables and the works. Stoneform and Brianform are two divisions of the same company: write to us.

Frequently asked questions

Does dishwasher steam stain marble?

No. The steam released at the end of a cycle is clean water at low pressure: on polished or honed marble it leaves at most a dark shadow while the stone takes it up, and the shadow goes when it evaporates. Marble is etched by acids and alkalis, not by water. What steam ruins, over the years, is the cabinet: chipboard, glued edges and screws.

Is a steam deflector needed if the worktop is stone?

Not for the stone. Miele’s installation instructions for integrated dishwashers state that if the countertop is made of Corian®, granite, marble or another solid, waterproof material the deflector is not required. It is needed where stone meets another material: the same instructions ask, on worktops of different materials, that the deflector be positioned so that it covers the joint line. A vapour barrier on the side panel of the adjoining cabinet, which is wood, remains worthwhile.

Can a dishwasher be screwed to a stone worktop?

No, and the manufacturer says so. Miele’s instructions require that the mounting brackets not be installed if the counter is made of an extremely hard material, because the securing screws supplied are not suitable for such materials. Stability must come from side fixing to the cabinet panels and from a continuous worktop secured to the structure, not from two screws in the slab.

Will an oven below ruin a marble worktop?

No, provided the housing is ventilated as the manual requires. Bosch installation instructions for built-in ovens call for a housing unit able to withstand 90 °C, adjacent units able to withstand 70 °C, 5 mm of free space towards neighbouring fronts and a ventilation slot in the false bottom. Marble takes those temperatures without altering: the enemy of stone is direct, localised contact, not the diffuse warmth of an oven that breathes.

Can a dishwasher go under a hob?

No. The Bosch manual says above all not to fit it under a hob, and Miele’s instructions say not to install the dishwasher beneath a cooking surface or oven. It is a requirement about the appliance, not about the stone, but it has a deadline: it must be settled before the slab is cut, because afterwards the hob cutout is where it is.

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The quoted sentences come from Miele’s installation instructions for integrated dishwashers (steam deflector, exemption for Corian®, granite and marble counters, positioning over the joint between different materials, prohibition on using the mounting brackets with extremely hard materials, requirement for a continuous countertop secured to adjacent cabinetry, prohibition on installing beneath a cooking surface or oven, extended drying against condensation), from the use and installation manual of a Bosch built-in dishwasher (built-under and integrated appliances only beneath continuous worktops screwed to the adjoining units, no fitting under a hob, escaping steam that in the long term can damage fitted units) and from Bosch installation instructions for built-in ovens (housing unit resistant to 90 °C and adjacent units to 70 °C, 5 mm towards the fronts, ventilation slot in the false bottom, cutting completed before insertion, worktop fixed to the housing unit). Minimum worktop thickness, clearance to a drawer and distance from an oven below are the figures Bosch gives for induction hobs. Figures change from model to model: in all cases the instructions of the appliance and slab manufacturer prevail, and they are read before cutting. These manuals are written for the European market; appliances sold elsewhere may carry different requirements. This page describes general practice and technical documents and does not replace a project. The figures on price are orders of magnitude and are not a quotation.