Deck sizes get quoted as standard numbers — 3 by 4, 4 by 5 — which is backwards. A deck is a floor, and a floor is sized by the furniture that goes on it and the space around that furniture for bodies. Below are the sizes worked out from real footprints, then the fixed numbers: how far below the floor a deck sits, what a step riser and going should be, how tall a railing is, and how decking boards space out. Every figure here is the one Gridsnap actually draws with.
How big: size it by what stands on it
The arithmetic is the same as sizing a room. Take the footprint of the furniture, add the space a person occupies while using it, and that is your clear deck area — before steps, before railing posts, before the barbecue nobody drew.
| What is on it | Clear size | Area | Where the number comes from |
|---|---|---|---|
| Dining, four people | 3.2 × 2.5 m | ~8 m² | A 1600 × 900 table plus about 800 mm behind each chair to stand up. |
| Dining, six to eight | 4.0 × 3.0 m | ~12 m² | A longer table, same chair zone, and room to walk behind someone seated. |
| Lounge seating | 3.0 × 2.6 m | ~8 m² | A 2000 × 900 sofa, a 1200 × 600 low table, 400 mm between them. |
| Dining and lounge | 6.0 × 3.0 m | ~18 m² | The two zones side by side with a walking route between them. |
| A door and a way past | — × 1.5 m | — | The minimum that is not just a landing: someone stands at the door, someone else gets past. |
A deck is not a room: it has no walls to push furniture against, so the useful area is the middle, and the perimeter is circulation. Losing 600 mm all round to walking is normal and worth allowing for.
The fixed numbers
These are the defaults Gridsnap draws with, and they are the ones worth carrying in your head whether you draw it here or on paper.
| Dimension | Default | Why |
|---|---|---|
| Deck below the floor inside | 150 mm | So rain and snowmelt drain away from the threshold rather than into it. |
| Step riser, target | 150 mm | The drop is divided into equal risers as near this as it will go. |
| Step going (tread depth) | 320 mm | Deeper than an indoor stair — outdoor treads are walked, not climbed. |
| Risers in one flight, max | 12 | 1.8 m of drop at 150 mm each. Past that the risers grow instead of the flight. |
| Railing height | 1100 mm | Above the deck surface. Whether you need one at all is your local code. |
| Railing posts | 1200 mm c/c | 90 mm square. |
| Railing opening, default | 1000 mm | The gap you walk through onto the lawn. |
| Decking board | 145 mm c/c | The common board. Anything from 60 to 400 mm. |
| Paving flag, stone terrace | 600 mm | For a masonry terrace instead of a boarded deck. 200–1200 mm. |
Height, and the steps that follow from it
A deck's height is not a choice you make on its own — it follows from the building. The floor inside sits at a known level, the deck sits 150 mm below it so water runs away from the threshold, and the ground is wherever the ground is. What is left is the drop, and the drop decides the steps.
Divide the drop by a target riser of about 150 mm and round to whole risers, then share the drop equally between them — equal risers matter more than hitting 150 exactly, because an odd step out of a flight is what people trip on. A flight tops out at 12 risers, which at 150 mm each is 1.8 m of drop. Past that the risers get taller rather than more numerous, and that is the signal you need a proper staircase with a handrail, not a band of steps.
The worked example
Say the finished floor inside is 600 mm above the ground:
- The deck sits 150 mm below the floor, so its surface is 450 mm above the ground.
- 450 ÷ 150 = 3 risers, each of exactly 150 mm.
- Three risers means two treads before you reach the ground — the deck edge itself is the top one.
- Two treads at 320 mm going = 640 mm of step band.
And that 640 mm comes out of the deck. The step flight is cut into the platform, not added beyond it, so a 2800 mm-deep deck with steps down one long edge has 2160 mm left to stand on. Draw the area you want to use, then add the step band outside it — and if you raise the house on its plinth later, the flight gains a riser and takes another 320 mm.
Railings, and where they belong
A railing is drawn at 1100 mm above the deck on posts about 1200 mm apart. Where it goes is not a decoration decision: it belongs on the edges that are open and high, which means it should not run along the house, and it should not run across the edge you step off. Both of those are handled by the geometry rather than by you remembering — railings skip the edges that meet the building and stop where a step flight begins.
Where you want to walk straight off onto grass, cut an opening into the run — 1000 mm by default, which is a comfortable width to carry something through. A leftover piece of railing shorter than 150 mm is not drawn at all, because it is not something anybody would build.
Whether your deck legally needs a guard is a code question, and codes differ: many require one above roughly 600 mm of drop, with heights between 900 and 1100 mm. Check yours. The drawing gives you the geometry; it does not give you permission.
Boards, flags and direction
Decking is drawn at 145 mm centre to centre — the common board — and accepts anything from 60 to 400 mm. The direction matters more than people expect: boards running away from the house make a narrow deck look longer, boards running along it make a long deck look wider, and either way the run should be the direction water needs to leave in.
A masonry terrace is drawn the same way with flags instead of boards, 600 mm by default (200–1200). That is the real difference between a deck and a terrace in construction terms — boards on a frame versus a surface laid on a made-up level. The words are sorted out in the patio vs terrace vs deck glossary.
When the ground slopes
Most gardens are not flat, and this is the honest limit of any drawing of this kind. Gridsnap models the site as one ground level — a single datum the building stands on — not as a surface with a fall across it. Sloped terrain is not modelled.
What that means in practice is less limiting than it sounds, because a deck is a level platform anyway: it does not follow the ground, it spans over it. So set the ground level to the height where the steps actually land, and the flight Gridsnap works out is the correct flight at that point. The rest of the slope shows up as posts of different lengths under the frame, which is a question for whoever builds it and not something a plan answers.
Where the fall is large enough that you would step down partway across the deck, that is two platforms at two levels rather than one sloping one — draw them as two terraces, with the drop between them stepped.
What the drawing has to show
A deck drawing a builder can price from needs five things, and it is usually the last two that are missing:
- The outline in real dimensions, measured from the house, not from a fence that might move.
- The height — deck surface relative to the floor inside and to the ground.
- The steps: how many risers, at what going, on which edge.
- The railing runs, with the openings marked — a railing drawn as an unbroken line around the deck is a railing somebody has to come back and cut.
- The board direction, because it decides the frame underneath.
What it does not need to show is the structure: joists, beams, footings and fixings are the builder's or engineer's work, and a layout drawing that guesses at them is worse than one that stays quiet. Export the dimensioned plan and let them size the frame.
Frequently asked questions
How big should a deck be?
Size it by what stands on it, not by a rule. A four-person dining set needs about 3.2 by 2.5 metres once you allow 800 mm behind each chair; a sofa and a low table need about 3.0 by 2.6; both together want around 6 by 3 metres. Below about 1.5 metres deep a deck is a landing rather than a place to sit.
How high should a deck railing be?
Gridsnap draws railings at 1100 mm above the deck, which is a common requirement for platforms more than roughly half a metre up. Whether your deck legally needs a guard, and at exactly what height, is set by your local building code — check it before you build.
How many steps will my deck need?
Take the drop from the deck surface to the ground and divide by about 150 mm. A deck 450 mm above the ground gets three risers of exactly 150. Gridsnap does this from the real heights — the deck sits 150 mm below your floor and the house stands on a plinth you set, so the drop is known and the flight redivides itself if either changes.
Do the steps make the deck bigger or smaller?
Smaller — and this catches people out. The step flight is cut out of the deck, not added beyond it, so every riser after the first eats one tread depth from the platform. A three-riser flight takes 640 mm (two treads of 320) off the edge it comes down. Draw the deck at the size you want to stand on and add the steps outside that.
What size are decking boards?
The drawing uses 145 mm centre to centre, which is the common board. You can set anything from 60 to 400 mm, and the board direction follows the deck outline — an L-shaped deck gets boards that stop at the notch instead of bridging it.
Can I plan a deck on sloping ground?
Partly, and it is worth being exact about the limit. Gridsnap models the site as one ground level, not a slope, so the step count it works out is the count at that level. On a sloping site, set the ground to the height where the steps actually land and the flight is right there; the posts under the rest of the deck get longer, which is a builder's question rather than a drawing one. Sloped terrain is not modelled yet.