IRC R301.2 · R302.1 · 2021 / 2024 IRC

How to read IRC design criteria & fire separation tables

What every column of Table R301.2 means and who fills it in, plus how fire separation distance sets the wall rating, openings, and eaves under Table R302.1 — with a free link to the official code.

Need the actual tables? Read Tables R301.2 and R302.1 free at codes.iccsafe.org →

Table R301.2, column by column — what each one tells you

Column	What it drives on the job	Who fills it in
Ground snow load	Rafter, header, and girder sizing; footing widths in the foundation tables	Jurisdiction (from ASCE 7 maps or a local snow study)
Wind design — speed, topographic effects, special wind region, windborne debris zone	Whether prescriptive wall bracing, roof tie-downs, and cladding tables apply, and whether glazing needs impact protection	Jurisdiction
Seismic design category	Braced-wall requirements, anchor bolts, footing reinforcing, and whether masonry/stone veneer limits kick in	Jurisdiction
Subject to damage from — weathering	Minimum concrete strength and air entrainment for exposed concrete, brick grade	Jurisdiction
Subject to damage from — frost line depth	How deep the bottom of footings has to go	Jurisdiction (local number, often by county)
Subject to damage from — termite	Treated framing near grade, soil treatment, or physical barriers	Jurisdiction
Decay (where the edition includes it)	Where naturally durable or preservative-treated wood is required	Jurisdiction
Winter design temperature	Heating-equipment sizing (Manual J inputs)	Jurisdiction
Ice barrier underlayment required	Whether roofing needs an ice-barrier membrane at the eaves	Jurisdiction (yes/no)
Flood hazards	Date of the local floodplain ordinance and FIRM maps; triggers the flood-resistant construction section	Jurisdiction
Air freezing index and mean annual temperature	Frost-protected shallow foundation design	Jurisdiction (from climate data)

Column names paraphrased and grouped. Some editions add, split, or rename columns — read the table in your adopted edition. Our own summary, not the code text. Look up the tables free at codes.iccsafe.org and verify against the IRC edition (and local amendments) adopted by your AHJ.

How to read Table R302.1 (exterior walls), step by step

Step	What you do	Watch out for
1	Measure fire separation distance from the face of the wall to the lot line (or the centerline of a street/public way, or an imaginary line between two buildings on one lot)	Measured perpendicular to the wall face — not to the eave edge, and not to the neighbor’s house
2	Pick the right table: the one for dwellings without an automatic sprinkler system, or the one for sprinklered dwellings	Sprinklered homes get smaller distance thresholds — only if the whole dwelling is sprinklered to the IRC or NFPA 13D
3	Read the "walls" row: rated or not rated at your distance	A rated exterior wall has to be rated for fire exposure from both sides
4	Read the "projections" row for eaves, overhangs, and bump-outs	Projections have their own, tighter distance — and a minimum distance they can never cross
5	Read the "openings in walls" row: not allowed, limited to a percentage of wall area, or unlimited	Windows, doors, and vents all count as openings
6	Read the "penetrations" row for pipes, ducts, and vents through a rated wall	Penetrations close to the line must be protected like a rated penetration

Our own summary, not the code text. Look up the tables free at codes.iccsafe.org and verify against the IRC edition (and local amendments) adopted by your AHJ.

Worked example — new house on a narrow infill lot, no sprinklers

Input / step	Value	Source
Distance from side wall to lot line	4 ft	Site plan
Exterior wall rating on that side	1-hour, rated for exposure from both sides	Table R302.1, walls row (under 5 ft)
Openings allowed in that wall	Up to 25% of the wall area	Table R302.1, openings row (3 ft to under 5 ft)
Eave on that side	May not reach within 2 ft of the lot line; any projection closer than 5 ft needs a rated underside	Table R302.1, projections row
Frost depth for the footing	Whatever the jurisdiction wrote in R301.2 — say 36 in on this permit	Table R301.2 (local value)
Footing depth used	Bottom of footing at 36 in below grade	R403.1.4: deeper of the frost depth or 12 in below undisturbed ground

Worked example values only (the 36 in frost depth is a hypothetical local value). Look up both tables free at codes.iccsafe.org.

Common field mistakes

Mistake	What to do instead
Using a frost depth "everybody uses" instead of the jurisdiction’s number	Get the filled-in R301.2 from the building department — it’s often on the permit handout or website
Measuring fire separation distance to the neighbor’s house	Measure to the lot line (or to an imaginary line between buildings on the same lot)
Rating the wall but forgetting the eave	Check the projections row separately — eaves often need a rated soffit even when the wall layout works
Adding a window late in the job on a rated wall	Recheck the openings row before you frame it; a change can push the wall over its allowed opening percentage
Assuming the whole house is a "wind zone" because of the state	Wind speed, special wind regions, and debris zones are set locally in R301.2 — read the jurisdiction’s entry

Our own summary, not the code text. Look up the tables free at codes.iccsafe.org and verify against the IRC edition (and local amendments) adopted by your AHJ.

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The two tables you check before you draw a footing or a window

Read both tables free at codes.iccsafe.org. ICC offers free read-only access to the International Residential Code. This page explains how to use the tables; it does not reproduce them. It covers the 2021 and 2024 IRC — verify against the edition and local amendments your AHJ adopted.

Table R301.2 is a blank form the jurisdiction fills in. The IRC can’t know your snow load or your frost depth, so it hands each city or county a table to complete when it adopts the code. Those local values flow into almost everything downstream: footing depth, rafter and header sizes, bracing, roofing underlayment, termite protection, and whether flood-resistant construction applies. Before bidding a job in a new jurisdiction, get its filled-in table.

Table R302.1 is driven by one measurement: fire separation distance. That’s the distance from the building face to the lot line (or the centerline of a street, or an imaginary line between two buildings on the same lot). As the distance shrinks, the table steps up the requirements for the wall, the eaves and other projections, the openings, and the penetrations — and it has a separate, more lenient version for fully sprinklered homes.

Worked example. An unsprinklered infill house sits 4 ft from the side lot line. Under 5 ft, the wall needs a 1-hour rating from both sides, and openings in that wall are capped at 25% of the wall area. The eave can’t come within 2 ft of the line and needs a rated underside. The jurisdiction’s R301.2 lists (for this example) a 36 in frost depth, so the footing bottom goes at 36 in.

Next steps on the same job: size the footing with the IRC footing size guide, and frame with the joist and rafter span table guide and the wall framing, header, and fastening guide. Track the footing and framing inspections in an inspection request log — Field PM keeps inspections, photos, and daily reports with the job.

FAQ

Where do I find the numbers for IRC Table R301.2?+

The IRC prints Table R301.2 with blank cells — each adopting jurisdiction fills in its own ground snow load, wind speed, seismic design category, frost line depth, termite, decay, winter design temperature, ice barrier, and flood information. Ask the local building department for its filled-in table (often part of the adoption ordinance or permit handout). You can read the IRC itself free at codes.iccsafe.org.

How close can a house be to the property line without a fire-rated wall?+

In a dwelling without sprinklers, Table R302.1 requires a 1-hour exterior wall when the fire separation distance is under 5 ft. Sprinklered dwellings use a separate table with smaller thresholds. Local zoning setbacks are a separate rule and are often larger. Check the edition your AHJ adopted.

Can I put windows in a wall near the property line?+

It depends on the distance. Table R302.1 has an openings row that goes from not allowed very close to the line, to a percentage limit of the wall area, to unlimited farther away. In an unsprinklered house between 3 ft and under 5 ft, openings are limited to 25% of the wall area. Read the full row at codes.iccsafe.org.

What does frost line depth mean on a permit?+

It is the depth the jurisdiction writes into the frost line column of Table R301.2. Footings must bear below that depth (unless you use a frost-protected shallow foundation) and, in any case, at least 12 in below undisturbed ground under R403.1.4.

Did these tables change in recent IRC editions?+

The climatic table has been renumbered over time (older editions called it Table R301.2(1)), and its wind and snow columns have been revised as the underlying maps changed. R302.1 has been split into separate tables for sprinklered and unsprinklered dwellings for several editions. Details shift, so read the edition your AHJ enforces.

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