How to Build a Decking Frame: The Complete UK Guide

In England and Wales, most domestic decking falls under permitted development, meaning no planning permission is needed. However, there are specific conditions. The deck must not exceed 30 square metres in area. It must not be elevated more than 300mm above natural ground level on any side adjacent to a boundary. It must not be forward of the principal elevation of the house (the front). Listed buildings and properties within conservation areas have stricter rules.

In Scotland and Northern Ireland the rules differ slightly, and some local authorities have removed permitted development rights in specific areas. Check with your local planning department if you are in any doubt. A call or an email takes ten minutes and is worth every second.

For a deck that leads from a house door, the finished deck surface should sit 12 to 25mm below the internal floor level or the bottom of the door threshold. This keeps rain from running into the house. If the deck is freestanding or leads from a step down, you have more flexibility, but the surface must always be at least 150mm above the finished ground level to allow air circulation beneath the frame and prevent timber sitting in standing water.

Build in a 2% fall away from the house across the full deck width. On a 3-metre wide deck, this means the far edge sits 60mm lower than the house edge. Set this fall into the frame itself, not by packing under the boards later. A deck laid perfectly level will pool water and accelerate timber degradation significantly.

Using pegs and string line, mark out the full footprint of the deck on the ground. For a deck attached to the house, run a string line along the house wall to establish the back edge, then measure out to the full deck depth and set a parallel string. Mark the two side edges with more string, using a builder's square to check the corners are at 90 degrees.

Verify square using the 3-4-5 method: measure 3 units along one string from a corner, 4 units along the adjacent string, and confirm the diagonal between those two points measures 5 units. Scaling up (for example, 1800mm, 2400mm, and 3000mm) gives enough accuracy for a domestic deck. Both diagonals across the full footprint should also be equal.

Remove any turf, vegetation, and topsoil from within the deck footprint to a depth of approximately 50 to 75mm. Compact the exposed subsoil with a plate compactor or by hand tamping if the area is small. Lay a weed-suppressing membrane over the entire area, pegged down at the edges. Cover the membrane with 50mm of pea gravel or compacted crushed stone to improve drainage and prevent the membrane from pooling water.

This preparation is often skipped on domestic builds and consistently leads to accelerated joist degradation. Vegetation growing under a deck creates a permanently damp microclimate against the timber, regardless of the preservative treatment applied. The two hours spent on ground preparation repay themselves many times over in timber lifespan.

Posts should be positioned at the outer corners of the frame and at intermediate points along the bearers to keep bearer spans within the capacity of your chosen timber section. For a 47x150mm bearer, maximum unsupported span is approximately 1.8 metres. For a 47x200mm bearer, the maximum increases to around 2.4 metres. For spans beyond this, either use a larger section or add an intermediate post.

For ground-level decks with adjustable post bases (also called deck feet), mark the base positions, fix them to a concrete pad or directly to compacted hardcore with anchor bolts, and adjust the height using the threaded spindle before fixing the timber post or bearer directly to the base. This method keeps all timber above ground level, is significantly faster than setting posts in concrete, and simplifies future maintenance.

For elevated decks requiring posts in concrete, dig holes to a minimum depth of 600mm in stable ground (deeper in clay soils that move with frost). Use at least C20 concrete, set the post plumb, and brace it while the concrete cures for a minimum of 48 hours. Do not force-fit posts by partially burying them without concrete — this method fails in most UK soil types within five years.

The ledger board runs horizontally along the house wall and carries one end of every joist in the frame. It must be fixed into the structural masonry or the floor joist band of the house, not just into render, cladding, or brickwork mortar joints. In a cavity-wall house, the fixings must penetrate through the outer leaf and into the inner structural block, or through a timber frame member at intervals specified for the load.

Set the ledger board at the correct height for your finished deck surface, accounting for the joist depth plus decking board thickness. For a door threshold 25mm above internal floor level, with a 28mm board and a 150mm joist, the top of the ledger board should sit at 175mm below the threshold. Fix with M10 or M12 coach screws or resin anchors at 600mm centres, staggered slightly to avoid splitting the timber. Pre-drill the masonry with an SDS drill at the correct diameter for your anchor type.

Before fixing, fit a 10mm continuous strip of closed-cell foam or proprietary DPC tape between the ledger board and the house wall. This creates a drainage gap, prevents moisture bridging, and stops water tracking along the wall behind the board. Do not omit this step. Water sitting between a ledger board and a masonry wall is the primary cause of wall dampness and premature ledger rot on house-attached decks.

The outer frame consists of the two side rim joists (also called rim boards or end joists) and the front rim joist, which together form three sides of the deck's perimeter. The ledger board forms the fourth side on a house-attached deck. On a freestanding deck, a fourth rim joist closes the back of the frame.

Fit the rim joists to the outer faces of the corner posts using post cap brackets or by direct face-fixing with structural screws. Where rim joists meet at corners, use a double-lap joint secured with at least four 100mm structural screws driven at staggered angles through the face of one joist into the end of the other. This joint carries significant shear load and must be made with structural screws, not standard wood screws or nails.

Bearers sit between posts below the level of the rim joists and carry the joists across their full span. Fix bearers to posts using proprietary post cap brackets, which provide the correct bearing area and prevent twisting. Avoid toe-nailing or angled screwing as the sole bearer-to-post connection, as this joint can work loose with seasonal timber movement.

Joists run perpendicular to the decking boards. If your boards will run parallel to the house, the joists run out from the house. If your boards will run at 45 degrees, the joists should still run at 400mm centres measured perpendicular to their length, not parallel to the boards, and the board ends will need to be supported by blocking between joists at the diagonal cut line.

Standard joist spacing for UK domestic decking is 400mm centre to centre. For composite decking boards, some manufacturers specify 300mm centres, and a small number allow 600mm centres for their thicker profiles. Always check the composite board specification before setting out your joist positions. Using the wrong centres for composite boards can void the board warranty and will cause visible deflection under foot load.

Hang each joist into a joist hanger bracket fixed to the inner face of the rim joist or ledger board. Position the bracket so the top of the joist sits flush with or just below the top of the rim joist. Drive the specified joist hanger nails or screws through every hole in the bracket. Leaving fixing holes empty in a structural hanger reduces the rated capacity of the connection and is not acceptable practice on any load-bearing structure.

Noggins are short pieces of the same timber section as the joists, fitted between joists at mid-span and at the ends of any joist run that does not terminate in a hanger. They serve three functions: they prevent the joists from rotating on their axis (called joist roll), they transfer lateral loads across the frame, and they provide a solid fixing point for the rim board on any free end.

For joists spanning up to 2.4 metres, one row of noggins at mid-span is sufficient. For joists spanning between 2.4 and 4 metres, fit two rows of noggins at the third points. For spans beyond 4 metres, fit noggins at the quarter points and consult a structural engineer for the joist sizing.

Noggins can be fixed in a straight run (all in line) or staggered by 50mm, which makes face-fixing significantly easier. If fixing in a straight run, you will need to angle two screws from each side through the face of the joist into each noggin end. If staggered, face-fix with two 100mm structural screws through the joist face into each noggin end.

Run a long spirit level or a string line across all the joist tops in multiple directions to identify any that are sitting high or low. A small variation of 1 to 2mm across a joist span is acceptable. Anything beyond 3mm should be corrected before boarding by packing the low joist slightly (using preservative-treated shim timber), or by planing the top of a high joist. Never force a board down onto a high joist with screws alone, as this distorts the board and creates stress cracks over time.

Confirm the 2% drainage fall is present across the full deck width by measuring the height difference between the ledger board side and the outer rim joist. On a 3-metre wide deck this should be approximately 60mm. If the fall is insufficient, this is the last practical opportunity to adjust it by raising or lowering the outer rim joist slightly at the post connections before any boards restrict access.

Finally, apply a brush coat of end-grain preservative to any cut ends of treated timber where the saw has exposed untreated wood. This is particularly important for joist ends hanging in joist hangers, as these are fully enclosed and will not benefit from surface weathering and drying. Most cut ends can be treated with a quality preservative such as Cuprinol or Barrettine End Grain Solution in two minutes per cut, and the benefit in timber lifespan is significant.

Screws used in the structural frame must be rated for structural use, which means they will have a declared shear and withdrawal value and will be made from material appropriate for outdoor use in treated timber. For UK domestic decking frames, the minimum acceptable specification is hot-dip galvanised structural timber screws. Stainless steel (A4 grade) is preferred for coastal locations or high-exposure sites.

Standard bright zinc-plated wood screws should not be used in any structural connection in the frame. They have neither the shear resistance of a structural screw nor the corrosion resistance required for outdoor use in CCA-free preserved timber, which contains copper compounds that react with zinc plating and accelerate its deterioration.

Board-fixing screws perform a different job from structural screws. They must pull the board flat to the joist, resist the upward forces created by board movement in wet and dry cycles, and sit flush or just below the board surface to prevent a trip hazard. The correct board-fixing screw depends on the board material and the exposure of the site.

For pressure-treated softwood decking boards, a 4.5x60mm or 4.5x75mm hot-dip galvanised or A2 stainless decking screw is the standard specification. Use 60mm for boards up to 25mm thick and 75mm for 28mm to 32mm boards. Two screws per board per joist are required for boards wider than 120mm, to prevent cupping. For hardwood boards, use A2 stainless steel as a minimum and pre-drill with a pilot hole at every fixing point without exception.

Even UC4-treated timber in ground contact will degrade faster than elevated timber because the soil holds moisture against the wood surface continuously, prevents air circulation, and supports fungal and bacterial activity that dry conditions would suppress. The correct approach is to elevate all frame timber at least 150mm above the finished ground surface using adjustable post bases, concrete pads, or a raised base frame. Ground screws are an increasingly popular alternative for solid ground conditions.

A frame built perfectly level will shed water only through the board gaps, which is insufficient in UK rainfall conditions. The 2% fall away from the house is not optional. Build it into the frame height by setting the outer posts or outer rim joist 60mm lower than the ledger end on a 3-metre deck. Never try to achieve fall by packing under boards, as the packing compresses and fails over time.

A joist that spans further than its section can carry without excessive deflection will feel springy underfoot and will creep over time. The limits in this guide are conservative and are based on C16 timber at 400mm centres. If you have any doubt about your timber grade or if your spans are near the upper limit of a section, use the next section up. The difference in material cost between 47x150mm and 47x200mm across a standard deck is modest.

The ledger board carries half the deck's entire load. It must be anchored into structural masonry at regular intervals with fixings that are correctly sized for the load. A ledger board fixed only into render, external cladding, or the mortar joints between bricks will pull away under the weight of the deck and the people on it. If in doubt about what is behind your wall, open up a pilot hole with a small drill and investigate before committing to the ledger position.

Treated timber contains copper-based biocides that react with the zinc coating on BZP screws and accelerate corrosion substantially. A BZP screw in treated outdoor timber may fail within 2 to 3 years in UK conditions. Hot-dip galvanised screws have a coating thickness that significantly outperforms BZP in the same environment. For any structural frame connection or board fixing that will be in outdoor use for more than a couple of seasons, use hot-dip galvanised as the minimum and stainless steel where the budget allows.