The Junction in One Drawing
Everything on this page comes back to this section. Six things are decided at the wall, and five of them cannot be corrected once the boards are fixed.
Indicative section through a ground-level terrace laid against a dwelling. Dimensions shown for the 140 × 23 mm four-sided co-extrusion board; bearer depth shown is an example, use your own section.
Why a Terrace Against the House Is Its Own Problem
Most guidance on composite decking assumes a free-standing platform in the middle of a garden. A terrace deck built against a dwelling has four constraints that a free-standing deck never faces, and all four are decided before the first board is cut.
The door threshold is your real height budget
Every terrace against a building starts from one number you cannot move: the underside of the door. Board thickness, bearer depth and any levelling pedestal all have to fit beneath it, with the finished surface low enough that water cannot run back over the sill. A 23 mm board on a 45 mm bearer — a common section, not a universal one — has already consumed 68 mm before you level anything. Measure the sill first and design downward from it. Where the available height is too tight for a board-and-bearer build-up, deck tiles laid directly onto the existing surface are usually the only way to keep the door working.
Never bridge the damp-proof course
The damp-proof course at the base of the wall only works if it stays clear of the finished surface. In England and Wales, Part C of the Building Regulations requires the ground-level DPC to sit at least 150 mm above the final finished level — and that level is measured from your finished deck, not from the ground you started with. Fixing a deck against the wall is one of the classic causes of what surveyors call DPC bridging. Record the DPC line before you set levels and treat 150 mm below it as a ceiling you cannot exceed. Where level access is required, that needs a purpose-designed threshold and drainage detail rather than simply raising the deck. Requirements outside the UK differ. [待确认: 德法荷波的等效要求]
The ground below must fall away — the deck surface need not
This is widely got wrong in both directions. Boards laid with open gaps drain through those gaps, so the deck surface itself does not need a fall. What must fall away from the building is the ground or slab underneath, so the water passing through does not collect against the wall. Paving behaves the opposite way: a paved terrace needs a surface fall, commonly a minimum of 1:80, about 12.5 mm per metre. If you are replacing paving with decking, check what the existing ground actually does rather than assuming the old fall still serves you — on retrofits it very often runs the wrong way.
Keep the gap at the wall open
Composite boards move with temperature, and the cavity beneath needs air. Leave a clear gap along the wall — 15 to 20 mm is the usual minimum for ventilation alone, before any allowance for thermal movement — and do not fill it with sealant or mortar to tidy the edge. Where the gap needs to be visually closed, use a removable trim rather than a fixed seal; see the decking accessories for edge and trim options. The perimeter against a building is never a fixing point; support comes from the bearers, not the wall.
The Same Three Details, Built
Terraces laid against a dwelling. Each photograph shows one of the junction decisions described above.
Boards run away from the wall
Deck finished tight to the sliding doors, boards laid perpendicular to the facade so the gaps between them carry water away from the building rather than along it.
The threshold sets the level
The existing tiled area on the left sits higher than the new deck. Finished level was taken from the door, not from the ground, which is what keeps the sill usable.
Perimeter closed, not sealed
Fascia and step closing the open edge. The line stays removable, so the cavity underneath can still be inspected and cleared.
What a Terrace Against a House Demands of the Board
A wall on one side changes the environment the deck lives in. These six conditions do not apply to a free-standing garden deck.
Six conditions created by the wall
Splash-back zone
The first 300 mm out from the wall stays wettest. Rain off the facade lands there and dries slowest. It is also why the DPC rule exists.
Reflected heat
Light-coloured render bounces sun onto the deck. Surface temperature near the wall runs above the open field.
Trapped debris
The wall gap collects leaves and grit. It has to stay reachable, which rules out a sealed edge.
Thermal movement
A long run against a facade moves most at its ends. Expansion has to be planned at the corners, not the middle.
Restricted airflow
One side is closed, so the cavity ventilates in three directions instead of four. Bearer spacing and edge trim decide whether it breathes.
Freeze-thaw at the junction
Standing water at the wall is the first thing to freeze and the last to thaw. The fall of the ground underneath matters more here than anywhere else.
Figures to Fix Before Setting Out
These are the numbers that decide whether a terrace build-up fits under an existing door.
| Item | Figure | Why it matters at the wall |
|---|---|---|
| Board section | 140 × 23 mm | Sets the minimum build-up before bearers are added |
| Cap | Four-sided co-extrusion | Back and edges capped — not a three-sided economy board |
| Board weight | 3.25 kg/m | Rarely critical at ground level; becomes the limit on balconies and roof terraces |
| Maximum joist spacing | 350 mm | Industry range is 300–400 mm; 350 mm is the conservative end. Tighten further near the threshold |
| DPC clearance (UK, Part C) | 150 mm above finished deck level | Hard ceiling on your finished level. Check the local equivalent outside the UK |
| Ventilation gap at the wall | 15–20 mm minimum | For airflow alone, before any thermal movement allowance |
| Bending resistance | approx. 5,200 N at 350 mm span | Design figure for the 140 × 23 mm section |
| Board lengths | 2.9 / 3.6 / 4.0 / 5.8 m | 5.8 m cuts the number of butt joints on a long wall run |
| Warranty | 10 years, stepped | Cover reduces from year 6 — see FAQ before quoting it |
| Production lead time | 20 days | Plan the site sequence around it |
Build Sequence at the Wall
The order matters. Three of these six steps are impossible to correct once the boards are down.
Measure the door sill and set finished floor level
Work downward from the underside of the door, not upward from the ground. Record the figure before ordering anything.
Record the damp-proof course line
Mark it on the wall. In the UK the finished deck must sit at least 150 mm below it under Part C. Outside the UK, check the local equivalent before you set levels.
Set the fall of the ground, not the deck
Boards with open gaps drain through the gaps. What needs a fall away from the building is the ground or slab underneath. Establish it at the bearer stage and check at both ends, not only in the middle.
Lay bearers at 350 mm centres or tighter
350 mm is the maximum for the 140 × 23 mm board. Reduce centres in the strip nearest the threshold, where foot traffic concentrates.
Leave the ventilation gap along the wall
15 to 20 mm minimum. Keep it clear and keep it open. Do not seal it to tidy the line.
Close the edge with removable trim
Finish the perimeter so it can be lifted later for inspection, rather than fixing or sealing boards to the wall.
Terrace Surfaces Compared at the Wall Junction
The choice is usually between composite boards, timber and a tiled or paved terrace. The wall junction is where they behave most differently.
| Composite boards | Timber decking | Porcelain / paving | |
|---|---|---|---|
| Build-up above existing surface | Board + bearer, or tiles direct | Board + bearer | Bed + slab, usually thickest |
| How water leaves the surface | Through the open board gaps | Through the open board gaps | Surface fall, commonly 1:80 minimum |
| Behaviour at a damp wall | Four-sided cap; cut ends sealed on site | End grain absorbs unless sealed and resealed | Inert, but bedding can wick |
| Movement to allow for | Thermal, predictable, planned at ends | Moisture-driven, seasonal, less predictable | Minimal in the unit; joints crack instead |
| Lifting one unit for inspection | Straightforward if edge trim is removable | Straightforward | Usually destructive |
| Routine upkeep | Wash down; no coating cycle | Clean, sand and recoat on a cycle | Wash down; re-point joints |
| Where the cost sits | Material and freight | Material plus lifetime coating | Material plus labour-heavy bedding |
Board Sections and Tiles for Ground-Level Terraces
Six build-ups for the same situation. Which one you can use is decided by the height left under the door.
Co-extrusion Terrace Board
23 × 138 mm
Capped on four sides. The default where a normal board-and-bearer build-up fits under the sill.
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Co-extrusion Wide Board
23 × 148 mm
Wider face, fewer joint lines across an open terrace. Same build-up height as the 138 mm board.
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Classic Square-Pattern Board
140 × 25 mm
Uncapped classic section. Lower cost per metre where the terrace is sheltered and budget-led.
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Classic Solid Board
140 × 21.5 mm
Solid section, 1.5 mm shallower than the 23 mm board. Useful when the threshold is marginal.
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Classic Deck Tile
Direct-lay, no bearers
For terraces where the door sill leaves too little height for a framed deck. Lays onto the existing surface.
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Co-extrusion Deck Tile
Direct-lay, capped
Capped tile for the same tight-threshold case, where the terrace sees heavier weather.
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What Determines the Delivered Cost of a Terrace Deck
Area alone does not price a terrace. Three other variables move the figure more than most buyers expect.
Run length decides how much you waste
A terrace against a house is usually one long rectangle. Whether your longest run divides cleanly into the board lengths on offer — 2.9, 3.6, 4.0 and 5.8 m — decides how much of every board ends up as offcut. A 4.2 m run cut from 5.8 m boards wastes more than a quarter of the material. Measure the longest run before you estimate quantity, not just the area.
Coverage width, not nominal width, sets the metre count
Boards are ordered by linear metre but laid by covered width. Calculating from the nominal section rather than the actual coverage is the most common way a terrace order comes up short, and a shortfall on a container shipment is expensive to correct. Confirm the covered width for the exact section you specify before converting area into linear metres.
What we need before we can quote accurately
Terrace area and the longest single run; the door sill height and the existing surface level; whether the substrate is soil, existing paving or a slab; and the destination port. With those five figures a quotation is firm. Without them any number is a guess.
Terrace Decking: Common Questions
Heights and build-up
How much height does a composite terrace deck need above the existing surface?
Allow for the board, plus the bearer or pedestal, plus a levelling tolerance. A 23 mm board on a 45 mm bearer is 68 mm before levelling, though bearer sections vary. Check that figure against the door sill before ordering, because the sill cannot be moved. Where less than roughly 70 mm is available, deck tiles laid on the existing surface are the usual alternative.
How far below the damp-proof course must the finished deck sit?
In England and Wales, Part C of the Building Regulations requires the ground-level DPC to be at least 150 mm above the final finished level, and the finished deck counts as that level. Raising a deck into that 150 mm zone is called DPC bridging and is a common cause of damp at the base of the wall. Requirements outside the UK differ and should be checked against local rules before you set levels.
Can composite terrace decking be laid straight onto an existing patio?
Yes, provided the existing surface is sound and water that passes through the board gaps can still drain away from the building. Deck tiles are designed for this and need no bearers. A framed deck over an existing patio still needs bearers, which adds height you may not have.
Drainage and detailing
Does a composite deck surface need to be laid to a fall?
Not when boards are laid with open gaps, because the water drains through the gaps rather than across the surface. What does need a fall away from the building is the ground or slab underneath. Paving is the opposite case and does need a surface fall, commonly a minimum of 1:80.
What if the ground already falls toward the house?
Correct it at the bearer stage by packing the bearers, or regrade the substrate. This is straightforward while the frame is open and impossible once the boards are fixed. On retrofits over old paving the existing fall very often runs the wrong way, so check it rather than assuming.
How wide should the gap between the deck and the wall be?
15 to 20 mm is the usual minimum for ventilation alone, before any allowance for thermal movement on a long run. Keep it open. Sealing it with mortar or silicone to tidy the line traps moisture in exactly the place that dries slowest.
What joist spacing should be used for terrace decking?
350 mm is the maximum centre for the 140 × 23 mm board. The industry range across manufacturers is 300 to 400 mm, so 350 mm sits at the conservative end. Near a door threshold, where people step in and out most often, tighter centres reduce deflection.
Should the boards be fixed to the house wall?
No. The perimeter against a building stays free so the damp-proof course remains clear and the cavity stays ventilated. All support comes from the bearers.
Cost and supply
Do board lengths matter on a long terrace?
Yes. Boards are supplied in 2.9, 3.6, 4.0 and 5.8 m. On a long run against a house, the 5.8 m board reduces the number of butt joints, which is where movement and staining show first. It also changes how much of each board becomes offcut.
What information do you need to quote a terrace?
Terrace area and the longest single run, the door sill height and existing surface level, the substrate type, and the destination port. With those we can give a fixed quantity take-off rather than a rate per square metre.
What does the warranty actually cover after year five?
The cover is stepped, not flat. It reduces to 50% in year 6, 40% in year 7, 30% in year 8, 20% in year 9 and 10% in year 10, and ends after year 10. Quote it that way in a contract rather than as a flat ten years.
Which certification applies for EU projects?
CE marking under the Construction Products Regulation, EU 305/2011 Annex V. Certificate M.2024.206.C97619, issued by UDEM International Certification, a notified body, valid to 11 March 2029. Quality and environmental systems are certified to ISO 9001 and ISO 14001, valid to 29 April 2027.
Can we see the boards before committing?
Samples are free — you cover the courier, and we credit it against your first order.
Check the threshold before you order
Send the door sill height, the existing surface level and the run length. We will confirm the build-up, the board section that fits and a fixed quantity take-off.