Deck sizes, railing heights and baluster spacing

A deck is the one part of a house where the code numbers, the furniture numbers and the timber numbers all have to agree, and they are written in three different modules. The code works in inches and spheres, the furniture works in clearance bands, and the timber works in a 5.5 in board with a gap beside it. Get the order wrong and the deck comes out 4 in short of a full board, 2 in over the guard trigger, or 6 in too shallow for the chairs. The rest of outdoor room dimensions can absorb an inch. A deck cannot.

The 30 in trigger, and what "30 in above" actually means#

The single most consequential number on a deck is not the guard height. It is the height at which a guard becomes necessary at all.

Under the IRC, a guard is required where a walking surface is more than 30 in (762 mm) above the floor or grade below. The measurement is not taken at the deck edge. It is taken vertically, at any point within 36 in (914 mm) horizontally of the edge, to whatever surface is there. That phrasing is the whole game, because it means the trigger depends on the ground, and the ground is something you can build.

Two decks with identical framing can land on opposite sides of the line. A deck 30 in above a level lawn is exempt. The same deck 30 in above a lawn that falls away at 1 in 12 is not, because 36 in out from the edge the grade has dropped another 3 in and the vertical distance is 33 in.

Measure it properly: run a level line out from the deck surface, mark 36 in (914 mm) horizontally, and drop a tape to the ground at that point. That is the number the inspector uses. It is the same discipline that how to measure a room properly applies indoors, transferred to a surface that is not flat.

Guard heights, and the four standards that disagree#

Three things are worth pulling out. The UK splits the height by location rather than by occupancy, so the same house has 900 mm at the stair and 1,100 mm at the balcony, which is a 200 mm step in a rail that often has to be continuous. Australia allows a wider gap, 125 mm against the 100 and 102 mm used elsewhere, and it applies the barrier requirement from a 1 m fall rather than 762 mm.

Height is measured from the walking surface, and on a deck the walking surface is the top of the boards, not the top of the joists. Fit 1 in (25 mm) boards after setting the posts and every guard is 1 in short. Add a rubber mat, a tile overlay or a second deck surface later and the guard shortens again.

Many US jurisdictions amend the IRC upward to 42 in (1,067 mm) for decks. That is a local amendment rather than the model code, and it is worth confirming before buying posts, because the difference is not just 6 in of timber, as the load section below shows.

Balusters: why 96 in of rail takes 17 of them#

The rule is not a spacing. It is a sphere: no opening in the guard may allow a 4 in (102 mm) sphere to pass. That means the clear gap must be less than 4 in, and 4 in exactly fails.

Work it out for a 96 in (2,440 mm) bay between posts, using square balusters 1.5 in (38 mm) wide. With n balusters there are n + 1 gaps:

1.5n + g(n + 1) = 96

Set g at the 4 in limit and solve: 1.5n + 4n + 4 = 96, so 5.5n = 92 and n = 16.7. You cannot have 16.7 balusters, so 17 is the minimum. Put 17 back in and the gap becomes (96 - 25.5) / 18 = 3.92 in (100 mm), and the centre-to-centre spacing is 5.42 in (138 mm).

Sixteen balusters gives (96 - 24) / 17 = 4.24 in, which fails. There is no rounding to be had: the count jumps in whole balusters and the gap jumps with it.

Notice that the gap does not settle on a value and stay there. It jumps about between 3.45 and 3.92 in, because the baluster count is an integer and the bay width is not a multiple of anything. The 48 in bay wastes 14 per cent of its balusters on a gap far tighter than it needs, while the 96 in bay sits 0.08 in (2 mm) from failing. Build tolerance of 1/16 in in the wrong direction on a 96 in bay puts it over the line.

Our own recommendation is to design every bay to a 3.75 in (95 mm) gap and accept whatever count that produces, rather than solving each bay to the legal minimum. It costs at most one extra baluster per bay and it removes the argument on site. Better still, choose bay widths that divide cleanly: 72 in (1,830 mm) gives exactly 3.75 in with 13 balusters and no rounding at all.

Stairs get two relaxations. Guards on the open side of a stair may have openings that will not pass a 4 3/8 in (111 mm) sphere, and the triangular opening formed by the riser, the tread and the bottom rail may pass up to 6 in (152 mm). Both exist because a raked guard cannot hold a constant gap over a stepped nosing line.

7,200 lb-in at the base of every post#

The guard has to resist a 200 lb (0.89 kN) concentrated load applied in any direction at any point along the top, which the IRC sets out in its design load table. Infill is separately checked at 50 lb (0.22 kN) over any 1 sq ft.

Two hundred pounds sounds mild until it is multiplied by the lever arm. At the top of a 36 in (914 mm) guard, that load produces:

200 lb x 36 in = 7,200 lb-in (814 N-m)

of bending moment at the deck surface, where the post is fixed. If the post is bolted to a rim joist with two 1/2 in (13 mm) bolts spaced 5.5 in (140 mm) apart, the couple that resists that moment is 7,200 / 5.5 = 1,309 lb (5.8 kN) of tension in the upper bolt, before any prying is allowed for.

Now the notch. A 4x4 post is 3.5 x 3.5 in (89 x 89 mm) actual. Notching it to 1.5 in (38 mm) so it sits neatly against the rim leaves a section 1.5 in deep in the direction of bending, with a section modulus of:

3.5 x 1.5 x 1.5 / 6 = 1.31 in3 (21,500 mm3)

The bending stress at 7,200 lb-in is therefore 7,200 / 1.31 = 5,500 psi (38 MPa), against published bending design values for No. 2 softwood posts of roughly 850 to 1,000 psi (5.9 to 6.9 MPa), or about 1,400 psi with a short-term load duration factor. The notched post is short by a factor of four. This is exactly why the prescriptive deck guide from the American Wood Council details guard posts with blocking, hold-down hardware and through-bolts rather than a notch, and why an unnotched post with a bracket outperforms a notch every time.

Raise the guard to 42 in and the moment becomes 200 x 42 = 8,400 lb-in (949 N-m), a 17 per cent increase at the same connection.

Deck footprints, and the depth the guard steals#

The furniture arithmetic is the same as on paving, with one addition: a guard post and rail occupy about 5 in (127 mm) of depth off each railed edge, and a chair should not back into a baluster, so allow 42 in (107 cm) of pull-back rather than 36 in on any side that meets a rail.

The hot tub row carries a structural warning rather than a dimensional one: a filled six-person tub is a concentrated dead load far above the 40 psf (1.92 kPa) live load a deck is framed for, and the framing under it is a separate design, as hot tub dimensions, pad size and clearances sets out. The dining and lounge footprints come straight from the clearance arithmetic in patio sizes and layouts, and the pull-back changes on gravel and grass are covered in outdoor dining table sizes and clearances.

Board module matters at the setting-out stage. A nominal 5/4 x 6 board is 5.5 in (140 mm) wide, and with a 3/16 in (5 mm) gap the module is 5.6875 in (144 mm). A 12 ft (3,660 mm) deck width is 144 in, which divides by 5.6875 to give 25.3 boards. That leaves a 1.7 in (43 mm) rip along one edge, too narrow to fix cleanly. Twenty-five boards measure 142.19 in (3,612 mm) and 26 measure 147.88 in (3,756 mm), so set the deck to one of those and let the framing follow.

Deck stairs and the rail that changes datum#

Deck stairs are held to the same geometry as internal ones under the IRC: a maximum riser of 7 3/4 in (196 mm), a minimum tread of 10 in (254 mm), a minimum stair width of 36 in (914 mm), and a maximum variation of 3/8 in (9.5 mm) between the tallest and shortest riser in a flight. Those figures and the headroom rule are derived in stair dimensions: rise, run, headroom and handrails, and the stair rise and run calculator divides a total rise into equal risers without leaving an odd one at the bottom.

A handrail is required on stairs with four or more risers, set 34 to 38 in (864 to 965 mm) above the tread nosings, with a graspable profile: circular 1.25 to 2 in (32 to 51 mm) in diameter, and 1.5 in (38 mm) of clear space to any wall behind it.

The datum change catches people out. The level guard is 36 in above the deck surface. The stair handrail is 34 to 38 in above the nosing line, which is a different reference sloping at the pitch of the flight. A single top rail carried from the deck onto the stair therefore has to change both its height and its angle at the top newel, and the two datums only reconcile if the rail turns at the nosing of the top riser. Newel and rail sizes for that junction are set out in stair trim, newel and balustrade dimensions.

Outdoors there is one further constraint the code does not set: an outdoor step reads as steeper than an indoor one because there is no wall to give scale, and garden practice uses gentler proportions, which is covered in garden path widths and outdoor step dimensions.