How to Build a Decking Frame: The Complete UK Guide
Planning, ground preparation, posts, ledger boards, joist sizing and spacing, plus the right fixings for every stage. Everything you need to build a solid, long-lasting decking subframe from scratch, explained the friendly way.
Here is a small truth that catches out a lot of first-time deck builders. Your beautiful boards are only ever as good as the frame hiding underneath them. You can spend a fortune on gorgeous hardwood or premium composite, but if the subframe below is wobbly, badly spaced, or slowly rotting, the whole deck will bounce, creak, and sag long before its time. The frame is the unsung hero of the job, and it deserves your best attention.
The good news is that a decking frame is not complicated to build. It is essentially a big, level, well-supported timber grid. Once you understand the handful of rules that keep it strong and stable, the rest is careful measuring and steady work. This guide walks you through the whole thing, from that first tape measure across the lawn to the final level check before the boards go down, whether you are laying a low patio-style deck or raising a frame up onto posts.
If your deck feels bouncy underfoot, the usual culprit is joists set too far apart for the boards on top. Most timber boards want joists at 400mm centres, and many composite boards want them closer still. Widen that gap to save a few lengths of timber and you trade a small saving for a lifetime of flex, movement, and loose boards. Get the spacing right from the start and everything above it behaves.
A decking frame, also called a subframe or substructure, is the structural skeleton that your decking boards are fixed to. Think of it as the floor of a room built outdoors. It carries every footstep, every piece of furniture, and every party guest, then passes that load safely down to the ground or to the supporting posts. A frame usually has three main parts: the perimeter or outer frame that forms the shape of the deck, the inner joists that run across the middle at regular spacing to hold the boards flat, and the supports underneath, which might be a solid base, a run of concrete pads, or a set of posts for a raised deck.
Getting the frame right matters for three reasons: strength, safety, and lifespan. A well-built frame stops the boards flexing and sagging, keeps the surface level and pleasant to walk on, and holds everything together through years of British weather. A poorly built one gives you a spongy deck, gaps that open and close with the seasons, and fixings that work loose. The frame is genuinely the part of the deck where care pays off the most, so it is well worth slowing down and doing it properly.
| Board type and layout | Recommended joist centres | Why |
|---|---|---|
| Timber boards, laid straight | 400mm | The standard for most softwood and hardwood decking |
| Timber boards, laid diagonally | 300mm | The board spans a longer distance across each joist |
| Composite boards, laid straight | 300 to 400mm | Composite flexes more, so follow the maker's spec |
| Composite boards, laid diagonally | 250 to 300mm | Tighter support prevents dips between joists |
| Heavy use or hot tub area | 300mm or less | Extra support for concentrated loads |
Always check the joist spacing that your board manufacturer specifies before you build the frame, not after. Composite brands in particular publish their own required centres, and fitting a frame at the wrong spacing is one of the few mistakes you cannot fix once the boards are down. Two minutes with the installation sheet saves a rebuild.
Every good deck starts on paper. Sketch the shape you want, note the dimensions, and work out where it will sit in relation to the house, any doors, drains, gullies, and boundary lines. A quick plan tells you how much timber to order, where your posts or supports need to go, and roughly how many boards and fixings you will need. It also flushes out the awkward bits early, such as a door threshold you need to sit below or a manhole cover you must keep accessible.
Height decides how your frame is supported. A low, ground-level deck can often sit on a bed of concrete pads or paving slabs, which is the simplest build. A raised or elevated deck needs posts set into the ground on concrete footings, along with bracing to keep everything rigid. As a rule, keep the finished deck surface at least 150mm below the damp-proof course of your house, and keep air moving underneath so the timber can breathe and dry out. A deck that traps damp against the wall or against its own frame will not last.
In much of the UK, garden decking falls under permitted development when it sits no more than 300mm above ground level and, together with other extensions, covers no more than half of your garden. Higher decks, properties in conservation areas, listed buildings, and some flats can have different rules. It is always worth a quick call to your local planning authority to confirm before you start, so a finished deck never becomes a problem later.
Order about 10 percent more timber and fixings than your plan says you need. Boards get miscut, the odd joist arrives with a bow you would rather not use in a critical spot, and a spare length is always cheaper than a second trip to the merchant halfway through a Saturday build.
Your frame should be built from pressure-treated structural softwood. C16 is the standard grade and is perfectly fine for many garden decks. C24 is graded higher, which means it is stronger and tends to arrive straighter with fewer twists, so it is well worth the small extra cost on longer spans and anything raised off the ground. Always confirm the timber is pressure-treated for outdoor use, since untreated softwood will rot far too quickly in contact with damp air and ground.
The size you need depends on how far each joist has to span between supports. As a rough guide, use the table below and always err on the deeper joist if you are unsure. A deeper joist adds a little cost but removes any question of bounce.
| Timber size | Typical use |
|---|---|
| 47 x 100mm | Ground-level frames, short spans, and joists close to a solid base |
| 47 x 150mm | The everyday choice for most garden decks and general spanning joists |
| 47 x 200mm | Longer spans, wider deck areas, and elevated structures |
| 100 x 100mm | Support posts for raised and elevated decks |
A frame is only as strong as the fixings joining it. For building the outer frame and joining joists end to end, use proper structural timber screws, which drive quickly and grip hard without pre-drilling in most softwood. For heavy connections, such as fixing a ledger board to the wall or bolting bearers to posts, use coach screws or structural bolts, which are much stronger than a standard screw in a loaded joint. For joining inner joists neatly into the outer frame, galvanised joist hangers give a clean, strong connection and keep the tops of the joists perfectly level.
Whatever you choose, corrosion protection matters as much as strength on an outdoor frame. Stick to hot-dip galvanised or stainless steel fixings throughout, and never use bright zinc-plated indoor screws, which will rust within a season or two. Once the frame is built and you move on to the boards, you will want the correct decking screws for the job as well, and our guide to timber decking screws covers exactly what to pick for softwood and hardwood boards.
- Tape measure, pencil, and a long straight edge
- Spirit level, ideally a long one, plus a string line and line level
- Circular saw or a good hand saw for cutting timber to length
- Cordless drill or impact driver with the correct bits for your fixings
- Spade or post-hole digger, and a mixing tub if you are using post concrete
- Set square or the 3-4-5 method for checking corners are true
With a plan in hand, the first physical job is to mark out the exact footprint of the deck on the ground and prepare the surface beneath it. Time spent here makes every later step easier, because a clean, level, weed-free base gives you a proper reference to build from.
Knock a peg into each corner of the planned deck and run a string line between them to mark the outline. Check that the shape is square by measuring the two diagonals, which should be equal, and adjust the pegs until they match. Once you are happy, clear everything inside the lines. Lift any turf, remove roots and large stones, and roughly level the ground so it is not wildly uneven.
Lay a weed-control membrane across the whole area to stop plants pushing up through the finished deck. A layer of gravel over the membrane helps with drainage and keeps the ground from turning to mud, which also helps the underside of your frame stay dry. Keeping air and water moving under the deck is one of the simplest things you can do to help it last.
To check a corner is a true right angle without a large set square, use the 3-4-5 method. Measure 3 units along one side and 4 units along the other. When the straight line between those two points measures exactly 5 units, the corner is square. Use metres for a big deck and the corner will be spot on.
How you support the frame depends entirely on its height. A ground-level deck usually sits on concrete pads or slabs spread across the base, which is quick and straightforward. A raised deck needs posts, and posts need firm footings, so this is the stage where a raised build asks for the most care.
For a low deck, position concrete pads or paving slabs at regular intervals across the base, roughly following where your bearers and joists will run. Aim to support the frame every metre or so in both directions, so no section of timber has to span too far without something solid beneath it. Bed each slab firmly and check it is level, then check the whole set of slabs sits in the same plane using a long spirit level or a straight edge laid across them.
This method keeps the timber up off the wet ground, which greatly reduces the risk of rot, while giving the frame plenty of even support. It is the easiest way to build a patio-style deck and a great first project for a confident DIYer.
For a raised deck, dig a hole at each post position, drop in the post, and set it in post concrete. Keep each post perfectly plumb using a spirit level on two adjacent faces, and brace it with temporary battens while the concrete cures so it cannot lean overnight. Deeper, taller, or more exposed posts need deeper footings, so match the hole to the job rather than guessing.
Once the posts are set and cured, you can fix the bearers, which are the horizontal timbers that carry the joists, to the posts using coach screws or bolts. This is a heavily loaded joint, so it is one place where proper structural fixings really earn their keep. Add diagonal bracing between posts on any tall deck to stop the structure swaying, a movement builders call racking.
If your deck attaches to the house rather than standing free, the ledger board is the length of timber that fixes the frame to the wall. It carries the inner edge of the whole deck, so it is one of the most important connections on the build and one that must be done carefully. A freestanding deck does not use a ledger, and instead relies on posts or bearers on all sides plus good bracing.
Set the ledger board so the finished deck surface will sit at least 150mm below the damp-proof course, which keeps rain off the wall above the deck and protects the house from damp. Fix the ledger into sound, solid masonry using appropriate structural bolts or coach screws and wall plugs rated for the load. Space the fixings out along the board, and use two staggered rows so the ledger cannot rotate away from the wall under load.
Just as important is what sits between the ledger and the wall. Fit spacers behind the board and use a suitable strip of damp-proof material so water can drain freely and air can circulate, rather than being trapped against the timber and the brickwork. A ledger bolted tight against a damp wall is a fast route to rot, so give it room to breathe.
Now for the part that finally looks like a deck. You build the outer frame first, get it square and level, then fill it with the inner joists at your chosen spacing. Work methodically and this stage is genuinely satisfying, as the shape of your deck appears in front of you.
Cut the four sides of the perimeter to length and join them at the corners with structural timber screws, driving at least two fixings into each joint for strength. On a house-attached deck, one side of this perimeter is the ledger board you have already fixed. Before you go any further, measure the diagonals of the assembled frame. When both diagonals are equal, the frame is square, and everything you add from here will line up neatly.
Rest the frame on your supports and check it is level across its whole length and width. A gentle fall of around 10mm for every metre, running away from the house, helps rainwater drain off the finished surface, which is especially worth doing for composite decks. Adjust the supports until you are happy, then fix the frame down.
Mark your joist positions along the top of the frame at the spacing your boards require, usually 400mm centres for straight-laid timber and often closer for composite. Fit each joist into the frame using galvanised joist hangers, which hold the joist tops flush and level, or by screwing through the outer frame into the joist ends where the design allows. Keep every joist straight and check the run stays true as you go.
Add noggins, which are short lengths of joist fitted between the main joists, to stiffen the frame and stop the joists twisting over time. They are particularly useful under the joints where two boards meet end to end, and under any area that will take a heavy or concentrated load. A frame with good noggins feels reassuringly solid the moment you step onto it.
A short roll of joist protection tape along the top edge of every joist is one of the cheapest ways to add years to a deck. It stops water sitting in the screw holes on the joist tops, which is where frames tend to start rotting first. It takes minutes to fit and the frame will thank you for it.
Before a single board goes down, give the whole frame a proper inspection. This last check is quick, and it is far easier to correct a problem now than to lift finished boards later to get at it.
Walk slowly across the whole frame and pay attention to your feet. If you feel any bounce, sponginess, or movement, that section needs more support, an extra noggin, or a tighter fixing before the boards go on. A frame that feels dead solid under your weight will make a dead solid deck. Trust your feet, they are a surprisingly good level.
Run through the essentials one more time. Confirm the frame is level across every direction, with your intended fall for drainage. Recheck the diagonals to be sure nothing shifted while you were fixing joists. Make sure every joint has the right fixings fully driven home, that joist hangers are properly nailed or screwed off, and that the frame does not move when you push against it from the side. On a raised deck, confirm your bracing is holding everything rigid against racking.
Once the frame passes all of this, you are ready for the fun part: laying the boards. Your careful frame will reward you with a flat, quiet, long-lasting deck, and the right decking screws will keep those boards firmly in place for years. If you are fitting thicker or hardwood boards, our guide to 100mm decking screws explains when you need the extra length, and for composite boards our composite decking screw guide covers a clean, secure finish.
Most decking frame problems come from a small number of avoidable errors. Know them in advance and you sidestep the vast majority of callbacks, wobbles, and early rebuilds.
Stretching the joist spacing to save timber is the most common frame mistake of all. The boards flex between the widely spaced joists, the deck feels springy underfoot, and over time boards cup and fixings loosen. The fix is simple: follow the joist centres your board manufacturer specifies, and if in doubt, go tighter rather than wider. A little more timber now buys a rock-solid deck for the life of the build.
If the frame is out of square, your board gaps will run wonky and your cuts will fight you all the way across the deck. If it is out of level, water pools, furniture rocks, and the deck simply looks off. Both are easy to prevent with the diagonal check for square and a long spirit level for level, done before you commit the inner joists. Five minutes of checking saves hours of frustration later.
Timber that stays wet rots, and a frame that sits directly on soil, traps water on its joist tops, or is bolted tight against a damp wall will not reach its potential lifespan. Keep the frame up off the ground on pads or posts, protect joist tops with tape, give a ledger board room to breathe with spacers, and keep air moving underneath the whole deck. Good drainage is invisible when you get it right and painfully obvious when you do not.
Using cheap bright zinc-plated screws, or under-sized fixings in heavily loaded joints, leaves you with corrosion and connections that work loose. Always fix an outdoor frame with hot-dip galvanised or stainless steel structural fixings, use coach screws or bolts on the heavy connections such as the ledger and post joints, and match the fixing to the load. When it comes to the boards on top, the same principle applies, and our full 75mm decking screw guide walks through choosing a rust-free option for the deck surface.
- Deck shape, size, and height sketched out with dimensions
- Finished surface planned at least 150mm below the damp-proof course
- Local planning rules checked and confirmed before starting
- Timber and fixings ordered with roughly 10 percent spare
- Pressure-treated structural timber chosen, C24 for longer spans
- Joist size selected to suit the span, deeper if in doubt
- Hot-dip galvanised or stainless fixings only, no bright zinc-plated
- Coach screws or bolts on hand for ledger and post connections
- Footprint marked with pegs and string, corners checked square
- Area cleared, roughly levelled, and weed membrane laid
- Pads or slabs level, or posts set plumb in concrete and braced
- Footing depth matched to post height and ground conditions
- Outer frame squared with equal diagonals before joists added
- Joists fitted at the correct centres for the chosen boards
- Noggins added, joist tops taped, drainage fall confirmed
- Whole frame walked, level rechecked, and no movement felt
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