Installing a timber or WPC deck

The deck installation item covers setting out the area, placing and levelling the subframe on a prepared base, fixing the deck boards with screws or hidden clips, cutting boards to size and fitting edge trims or fascia boards. Checking the fall and the gaps is part of the job.

The item normally does not include excavation, the gravel bed and its compaction, concrete footings, the boards and subframe as materials, steps, balustrades, a pergola or oiling the timber. These are priced separately. During the site visit it is worth writing down exactly what the quote includes and what it leaves out.

Decks are usually built once a house is finished, as part of a garden makeover, or to replace an old concrete patio that has cracked and become uneven. Before ordering the work, it helps to be clear about the following:

• the finished height of the deck relative to the door threshold and the ground, so that no step forms at the entrance,
• where water from the deck and from the roof above it will drain,
• what the subframe will rest on: gravel, footings, pedestals or concrete,
• whether anything will be fixed to the deck later, such as a pergola, an awning or a balustrade,
• whether the boards will run along the house or across it, and from where the deck will mostly be seen.

A pergola, balustrade or roof cannot be anchored only to the deck boards or joists. Such elements need their own footings or fixings into a load-bearing structure, and this has to be known before the subframe goes in. Check with your local authority whether your deck needs a permit or notification, especially if it is raised, roofed or attached to the house.

Softwoods and European timber

Siberian larch is widely used across Europe. It is fairly hard, contains resin and lasts for years with regular care. It turns silver-grey in the sun, which is natural, and oil only slows this down. Pressure-treated pine is the budget option and is suitable as long as it carries a use class rating for outdoor exposure and the cut ends are treated. Untreated spruce is not suitable for decking because it rots quickly. Oak is durable but more expensive and can release tannins that leave stains on concrete or stone beneath it.

Thermally modified timber

This is wood, typically pine or ash, treated at high temperature. The process reduces water absorption and improves dimensional stability. The downside is greater brittleness, so the boards are pre-drilled and must not be used as load-bearing members.

Tropical hardwoods

Bangkirai, ipe, massaranduba and cumaru are very hard and durable, but more expensive and harder to work. They almost always need pre-drilling and high-grade stainless screws. When buying, it makes sense to ask for proof of legal and responsible origin, such as FSC or PEFC certification.

WPC

WPC is a composite of wood flour and plastic. It does not splinter, needs no oiling and keeps its colour better. On the other hand, it expands significantly with heat, gets hotter in the sun and in dark shades can be uncomfortable for bare feet in summer. Solid profiles are tougher and flex less. Hollow profiles are lighter and cheaper but more sensitive to impact and need a denser subframe. With WPC, the subframe, clips and screws should come from the manufacturer's system, otherwise the warranty may be lost.

A deck is only as good as the base beneath it. On soil, the topsoil is usually removed, a weed membrane is laid and a compacted gravel bed is spread. Concrete slabs, pads or adjustable plastic pedestals are placed on top, and the subframe rests on these. The subframe should never lie directly on soil or in wet gravel. Air has to circulate underneath, otherwise timber stays damp and WPC deforms.

Subframes are made of timber, WPC or aluminium. Timber joists must be durable, ideally the same species as the boards, and treated. An aluminium subframe costs more but does not rot and stays straight. The joist spacing is set by the board manufacturer. For WPC it is often around 30 to 35 cm for domestic use, and less for public areas. For timber it depends on the board thickness and is commonly around 40 to 50 cm.

The whole deck is laid with a fall of roughly 1 to 2 % away from the house so that water does not stand on the boards. Grooved boards are laid so that water runs along the grooves, not across them.

1. Set out the deck area, fix the finished level from the door threshold and decide the direction of fall.
2. Remove the topsoil, compact the formation, lay a geotextile, then spread and compact the gravel bed.
3. Place slabs, pads or adjustable pedestals in a grid that matches the joist spacing.
4. Lay the timber joists or aluminium profiles, level them to the planned fall and connect them.
5. Keep the subframe clear of the house wall, WPC manufacturers typically specify at least 10 mm.
6. Double the joists along the edges and at board joints so that every board end has its own support.
7. Let WPC boards acclimatise on site before fitting, as the manufacturer instructs.
8. Fit the starter profile or the first board and work outwards from the house.
9. Fix the boards with clips or stainless screws, pre-drilling timber and hard species.
10. Keep the gaps between boards and at the ends according to the material, board length and temperature at the time of fitting.
11. Trim the edges straight and cover the ends with trims so that ventilation under the deck stays open.
12. For a timber deck, apply oil, ideally also to the underside and cut ends before fitting.

Timber moves with moisture, WPC with temperature. The gap between boards is therefore not cosmetic. It is the space the material expands into and the route by which water drains. With timber, side gaps of around 5 to 8 mm are common, depending on board width and moisture content. With WPC the gap is often set by the clips and must not be squeezed. At board ends and next to walls, expansion gaps follow the manufacturer's table, often around 20 mm next to the facade.

Screws should be stainless steel. Zinc-plated screws corrode in oak and some tropical species and leave dark stains. Each board normally takes two screws at every joist, kept a set distance from the board edge and end so that the timber does not split. With WPC, holes are drilled larger than the screw diameter and screws are not over-tightened. The head should finish flush with the surface. Hidden clips give a clean look, but replacing a single board later takes more work.

• subframe directly on soil or grass: timber stays damp and rots from below, WPC starts to ripple,
• no fall: water stands on the boards, timber greys unevenly and ice forms in winter,
• joists too far apart: boards flex and crack, hollow profiles can break,
• missing gaps: in summer the boards buckle and push against the house wall,
• blocked ventilation: fascia boards down to the ground seal the space and moisture stays under the deck,
• untreated cut ends: the end grain absorbs water fastest and starts to split and rot,
• ordinary zinc-plated screws: they corrode, snap and leave stains,
• deck right below the threshold with no drainage: in heavy rain water reaches the door and the masonry,
• board joints without double support: the ends lift and become a trip hazard.

It often happens that a deck looks perfect in its first year and problems only show after the second winter. Most often the subframe should have been higher above the ground and ventilation is missing. That is why it pays to look underneath the deck at handover, not just at the surface.

Most problems arise where the deck meets the house. A gap is left between the subframe and the facade so that neither timber nor WPC touches the insulation or render. If the house has external wall insulation, the plinth area behind the deck should be made of a water-resistant material, for example extruded polystyrene with a plinth render. Water from the boards must never run towards the plinth, which is why the fall always leads away from the house.

If there is no gutter above the deck, rain from the roof falls in a single line and the boards in that strip age much faster. The solution is a gutter or a gravel drainage strip next to the house. At doors, the deck height is set so that a few centimetres remain between the board and the threshold and water cannot reach the entrance in heavy rain. On the balcony or terrace of a block of flats, consent from the building manager or owners' association is needed, as these are shared parts of the building.

Deck edges are finished with a fascia board, trim or a small board overhang. The overhang beyond the last joist should be small, WPC systems often allow at most around 5 cm, otherwise the edge bends when stepped on. Steps are built on their own structure with equal riser heights, never on cantilevered loose boards.

Decks are usually built by carpentry firms, landscaping companies or specialist decking installers. Because there is no single qualification for decking across Europe, it is sensible to look at finished references, ask whether the installer has fitted the chosen system before and check that the firm is properly registered and insured. For WPC, some manufacturers keep a list of trained installers, and installation by such a firm can matter for the warranty. A written quote should state the subframe type, joist spacing, fixing method and what happens with the base.

The job is easier with:

• a level, compacted base prepared in advance,
• a simple rectangular shape without corners,
• boards whose length suits the deck size with a minimum of cuts,
• a system subframe and clips from a single manufacturer,
• good access to the site and space for cutting.

The job takes longer with:

• removal of an old deck or paving,
• a large difference in ground level and the need for a tall subframe,
• working around posts, drains, light wells and trees,
• diagonal laying and patterns with alternating directions,
• hard tropical timber that has to be pre-drilled at every fixing,
• integrated lighting and made-to-measure steps.

Walk across the whole surface and notice whether any board flexes or creaks. Place a spirit level across and along the deck, the fall should lead away from the house. Pour a bucket of water near the door and watch where it runs. Look at the gaps, they should be even and straight. Screws should be in line and flush with the surface. Look under the deck from the side to check that the subframe does not lie on the ground and that air can circulate.

Handover checklist

• boards do not flex or creak, including at the edges,
• the fall leads away from the house and no water stands,
• gaps between boards are even,
• expansion gaps at the facade and board ends follow the manufacturer,
• every board joint has its own support in the subframe,
• screws are stainless, flush and in line,
• the subframe does not touch the ground and the space below is ventilated,
• timber cut ends are treated with oil or wax,
• the installer has handed over information on the material, warranty and care.

A timber deck is cleaned with water and a timber cleaner, brushing along the grain, and once dry it is oiled in thin coats. Oil should not be applied in direct sun. This is usually done once or twice a year, most often in spring and autumn. WPC only needs washing with water and a mild detergent, and grease stains from a barbecue are best removed straight away. Leaves and dirt are cleared from the gaps regularly so that water can drain.

In the NUMIQ price list, timber or WPC deck installation is listed as labour per m² including minor consumables, without the boards, subframe and other main materials. The reference labour price is EUR 32.50 per m². Actual prices vary by region and city, and are usually higher in large cities.

The price is affected by the shape of the deck, the number of cuts and corners, working around posts and drains, the hardness of the timber and the need for pre-drilling, the fixing method and the height of the subframe. A small deck with steps and corners costs more per square metre than a large rectangle. Excavation, the gravel bed, footings and compaction are priced separately. You can find them in the articles on sub-base preparation and the levelling course and sub-base compaction with a plate compactor. If you are considering paving instead of boards, see block paving and edging.