Deck railing in Idaho

Idaho adopted the 2018 International Residential Code statewide, effective for projects submitted on or after 1 January 2021. Sections R312 and R311.7 of that code are what set guard and stair requirements, and they are the same in Meridian, Boise, Nampa, and Star.

The four numbers

30 inches: the trigger

A guard is required on any deck where the walking surface is more than 30 inches above the grade below. Under that height, none is required. This is the single most useful number at the design stage, because on flat ground a deck can often be set two steps down from the door and stay beneath it. That removes the rail cost entirely and leaves an open edge, which is a different and usually better relationship between the deck and the lawn.

36 inches: the guard

Standard prescriptive guard height. Guard posts are a minimum 4x4 nominal with an adjusted bending design value of at least 1,100 psi, and the joists or rim joists they attach to are a minimum 2x8 nominal. Hold-down anchors carry a minimum allowable tension load of 1,800 pounds for a 36 inch maximum guard height, installed to the manufacturer instruction.

Those hardware figures are the reason a guard is not trim. A rail that feels solid when you lean into it hard is doing what it was designed for; one that flexes is a real problem regardless of what it looks like.

4 inches: the sphere

Guard infill is sized so that a 4 inch diameter sphere cannot pass through the assembly. This governs baluster spacing, cable spacing, and any gaps at the ends of runs. Cable rail in particular needs enough tension and enough intermediate support that the cables do not spread under a push, which is where cheap cable installations fail their inspection.

Four risers: the handrail

Any stair with four or more risers needs a handrail on at least one side. The handrail sits 34 to 38 inches above the sloped plane touching the tread nosings, is graspable with no sharp corners, runs continuously from directly over the lowest riser to directly over the highest, and returns to the guard at each end. It may be interrupted by a guard post at a turn.

Three risers does not trigger it, which is exactly why so many low Meridian decks are built with three steps to the lawn.

Stairs, the rest of it

• Stairs are a minimum 36 inches wide. Cut stringers are minimum 2x12, and a cut stringer flight needs at least three of them; above 36 inches of width, cut and solid stringers combine at a maximum 18 inch spacing.
• Every landing is at least as wide as the stair it serves, and rectangular landings are at least 36 inches in the direction of travel.
• An intermediate landing is required once the total vertical height of a stairway exceeds 12 feet.
• Stringers bear on footings below the frost line, not on a patio slab poured above it. The footings guide covers why.

Choosing a rail system

Once the code minimums are satisfied, the rest is preference and money. Installed rates in the Treasure Valley, per linear foot:

• Wood, $45 to $80. Cheapest, warmest looking, and the only one that needs re-coating on the same cycle as a wood deck. Cedar holds up better than treated pine on a rail because it moves less.
• Composite, $75 to $130. Matches composite decking, needs no finishing, visually the heaviest of the systems because the components are chunky.
• Aluminum picket, $80 to $140. Slim profile, powder coated, essentially maintenance free. The most common upgrade on a composite deck.
• Stainless cable, $110 to $190. The clearest view. Needs terminal posts anchored for cable tension, which is a real structural difference rather than a hardware swap.
• Glass panel, $180 to $300. Fully transparent, blocks wind, and shows every water spot. In a valley with hard water that is a maintenance consideration people underestimate.

Retrofitting rail onto an existing deck

The limiting factor is almost always the posts, not the infill. Many decks built in the 2000s Meridian subdivision wave have rail posts lag screwed to the outside face of the rim joist, which was common practice and does not meet current hold-down detailing. Swapping the infill on those posts puts a modern rail on a connection that was never rated for it.

A proper retrofit replaces the posts and their anchoring, then fits the new system. The infill is the visible cost and the post work is the one that disappears out of a cheap quote. The repair page covers what else is worth checking while the rail is off.

Where these figures come from

The dimensional requirements above follow the 2018 International Residential Code as adopted by Idaho, and are set out in prescriptive form in the American Wood Council's free DCA6 residential wood deck construction guide, which cites the relevant code sections throughout. The ICC adoption record for Idaho confirms the code edition and effective date. Your local building official is the authority having jurisdiction and can approve alternative methods and materials.