Deck code checklist
Most deck inspection failures are not structural. They are dimensional — a guard that is 34 inches instead of 36, a baluster gap of 4½ inches, a riser that varies by half an inch. This page lists the dimensions that get measured, with the clause they come from.
Guards and railing
| Requirement | Value | Reference |
|---|---|---|
| Guard required above | 30″ above grade | IRC R312.1 |
| Guard height, residential | 36″ minimum | IRC R312.1.2 |
| Guard height, commercial | 42″ minimum | IRC R312.1.2 |
| Sphere that must not pass | 4″ | IRC R312.1.3 |
| Guard load at top rail | 200 lb concentrated | IRC R301.5 |
| Handrail height on stairs | 34″–38″ above nosing | IRC R311.7.8 |
The 4-inch sphere rule is the most common failure. It applies to the clear opening between balusters, between a baluster and a post, and under the bottom rail. Compute the opening, not the centre distance — see the railing calculator.
Stairs
| Requirement | Value | Reference |
|---|---|---|
| Maximum riser height | 7.75″ | IRC R311.7.5.1 |
| Minimum riser height | 4″ | IRC R311.7.5.1 |
| Minimum tread depth | 10″ | IRC R311.7.5.2 |
| Allowed variance across a flight | 3/8″ | IRC R311.7.5.1 |
| Minimum headroom | 6′8″ | IRC R311.7.2 |
| Handrail required at | 4 or more risers | IRC R311.7.8 |
That 3/8″ variance is across the entire flight, not between adjacent steps. In practice you should aim for zero — cut all stringers from one template, and check the first and last riser before you pour anything.
Footings and attachment
| Requirement | Value | Reference |
|---|---|---|
| Minimum footing diameter | 12″ | IRC R403.1.4 |
| Footing depth | Below local frost line | Local amendment |
| Footing bearing | Undisturbed soil | IRC R403.1 |
| Ledger fasteners | 1/2″ lag or through-bolt, per table | IRC R507.9.1 |
| Ledger flashing | Required above ledger | IRC R507.9.1.3 |
| Joist cantilever limit | 1/4 of adjacent span | IRC R507.5 |
Ledger attachment deserves more attention than it usually gets. The ledger is where most deck collapses originate — not from undersized joists. The fastener schedule depends on the ledger material, the rim material and the withdrawal capacity, and it is not something to estimate.
What this checklist is not
- Not a permit review. An inspector measures the building, not a web page.
- Not complete. Lateral load connections, stair stringer sizing, beam spans and soil bearing are all code-governed and all omitted here, because they cannot be reduced to a table.
- Not current for your area. Check which IRC edition your jurisdiction has adopted, and whether it amended these clauses.
If you want one reliable next step: call your building department and ask which edition they enforce and what frost depth they publish. That single conversation answers more than any checklist.
Questions people actually ask
How tall does a deck railing have to be?
36 inches minimum for a residential deck under IRC R312.1.2, measured from the deck surface to the top of the guard. Some jurisdictions require 42 inches, and commercial occupancies always do.
What is the 4-inch rule for deck railing?
A 4-inch sphere must not be able to pass through any opening in the guard — between balusters, between a baluster and a post, or under the bottom rail. It is IRC R312.1.3 and it is the most frequently failed deck inspection item.
When is a deck railing required?
When the deck surface is more than 30 inches above grade under IRC R312.1. Below that, the model code does not require a guard, though local amendments and insurers sometimes do.
What is the maximum stair riser height?
7.75 inches, with a minimum of 4 inches, and no more than 3/8″ variance across the whole flight (IRC R311.7.5.1). Treads must be at least 10 inches deep.
Do I need a permit to build a deck?
In most US jurisdictions, yes, and the threshold is usually tied to height (often 30 inches) or area. Unpermitted decks create disclosure problems when you sell. Ask your building department before you dig.