Timber frame kitchen extension

Wide openings to the garden

Sliding or folding doors of 3 m and more are the feature most people want, and they are the part of a timber frame that needs most thought. The panel above the doors cannot carry the roof on its own, so the maker designs in a beam: laminated timber (glulam or LVL) for spans up to about 4 m, a steel beam or a welded steel goalpost frame beyond that.

The posts either side of the opening bring the load down to two small areas of foundation, which the engineer thickens. Deflection matters more than strength here. A beam that sags 10 mm will jam a sliding door, so the calculation is done to a tight limit and the door supplier should be told what it is.

Decide the door size before the frame is drawn. Changing a 3 m opening to 5 m after manufacture means a new beam, new posts and often new footings.

Hanging wall units on a stud wall

A row of full wall cupboards loaded with crockery can weigh 50 kg or more per metre. Plasterboard alone will not hold that. The fix is cheap if it is done at the right time: a band of 18 mm plywood or OSB fixed across the studs behind the plasterboard, wherever units, a cooker hood, shelves or a television will hang.

Mark those zones on the kitchen layout and give it to the frame maker or the first-fix carpenter. Factory-made closed panels can have the backing built in. If the kitchen design is not final, line the whole run of wall; a few sheets of board cost far less than opening a finished wall later.

The same goes for a wall-hung boiler, radiators and tall larder units, which should be screwed back to solid timber for safety.

Heat, steam and ventilation

Building regulations ask for an extractor in any new kitchen: 30 litres a second if the hood sits over the hob and ducts outside, 60 litres a second for a fan elsewhere in the room. A recirculating hood does not count. Plan the duct route while the frame is being drawn so it runs between joists to an outside wall in the shortest line.

Kitchens make a lot of water vapour, and timber frame walls rely on a vapour control layer behind the plasterboard to keep it out of the insulation. Every socket box, pipe and downlight that pierces that layer should be sealed. A 25 mm battened service void on the room side of the membrane makes this easy, because the cables never go through it.

Where a gas hob sits near a timber-clad or boarded internal wall, follow the appliance maker's clearances and use a non-combustible splashback.

Floors, islands and drains

Kitchen extension floors are nearly always a ground-bearing concrete slab or a beam-and-block floor with 100 to 150 mm of insulation and a screed, the same as under a masonry build. Stone worktops and an island are no trouble on either. Underfloor heating pipes go in the screed.

An island with a sink or a hob needs its waste pipe, water and power set in the floor before the screed is poured, so its position has to be fixed weeks ahead of the kitchen fitter arriving. Get the kitchen designer's plan onto the builder's drawings.

Many rear extensions cross a drain. If it is a public sewer within 3 m, you need a build-over agreement from the water company before work starts, and manholes cannot be left inside the new room without sealed, bolted covers.

How long you are without a kitchen

With some planning, two to three weeks. The extension is built as a sealed box against the back of the house while the old kitchen carries on working. Once the shell is plastered, the opening is cut, the steel goes in, and the old kitchen comes out.

The gap is then the time to make good the floor and walls where the two rooms join, fit the new units, template and fit the worktops (stone takes seven to ten days from template) and connect the appliances. Set up a kettle, a microwave and a washing-up bowl somewhere else in the house for that stretch.