The Local Design Criteria Table

Why this matters

Most of the numbers that make a job in one county different from the same job in the next one live in a single table, and almost nobody in the field has read it. Whether a roof needs ice barrier at the eaves, how deep a deck footing goes, what temperature the heating load is sized to, whether a water heater must be strapped, which concrete mix a garage slab needs: the jurisdiction wrote each answer into one row when it adopted the code. Read that row once per jurisdiction, keep it, and half the "it depends" questions on a quote stop depending.

What the table is and who fills it in

The IRC, in Section R301.2, contains a table of climatic and geographic design criteria (numbered Table R301.2(1) in several editions). In the model code it is blank. The model code is not law; a state, county or city adopts it by ordinance, and when it does it fills in the table's cells with its own values, usually in the adopting ordinance or a local amendments document. The IRC's footnotes tell the jurisdiction where each value comes from: the code's own maps, a local study, the community's flood insurance records, and so on.

So the same edition of the IRC produces a different table in every jurisdiction that adopts it, and an unadopted IRC section number tells you nothing about a specific address. How to find a particular jurisdiction's filled-in table is the subject of How to Find the Local Design Criteria for a Jurisdiction.

Reading the row, entry by entry

Recent editions add a second block of Manual J design criteria beside the main table, with elevation, latitude, summer and winter design temperatures, coincident wet bulb, daily range and humidity entries. Where your jurisdiction has filled that block in, it is the set of outdoor conditions a load calculation is expected to use.

Each entry switches something on

The table does not do anything by itself. Other sections of the code point to it, so each cell is a switch for some requirement elsewhere:

• Ice barrier "yes" triggers the roofing chapter's ice barrier rule (R905.1.2 in the 2021 edition): at least two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet, from the lowest edge of the roof to at least 24 inches inside the exterior wall line, for the roof coverings that section lists, with an exception for detached accessory structures without conditioned floor area.
• Seismic design category D0, D1 or D2 triggers the mechanical chapter's anchorage rule (M1307.2 in the 2021 edition): water heaters strapped in the upper third and lower third of their height, with the lower strap at least 4 inches above the controls. That same section also reaches townhouses in category C.
• Weathering "severe" raises the minimum concrete strength for porches, steps and garage floor slabs exposed to weather in the concrete table (R402.2 in the 2021 edition), to 3,500 psi with air entrainment, against lower values for moderate and negligible exposure.
• Frost line depth sets the minimum depth for footings that must be protected from frost (R403.1.4.1 in the 2021 edition), with exceptions the section lists. The full story is in Frost Depth and What It Sets.
• Windborne debris zone "yes" triggers glazing protection for windows and doors in that zone.
• Termite switches on the termite protection methods, covered in Termite and Decay Zones.
• Flood hazards points to the flood-resistant construction section for sites in a mapped flood area; see How to Check a Flood Zone Before Quoting.

The basis traps

A number in this table is only comparable to another number on the same basis, and three entries change basis in ways that mislead people who compare across editions or across jurisdictions.

Wind speed. Editions from about 2012 onward state an ultimate design wind speed, tied to the wind provisions of ASCE 7 as referenced by that edition. Older editions stated a lower basic wind speed on a different basis. A 115 mph entry in a current table and a 90 mph entry in an old one can describe the same location. Never compare wind numbers across editions without knowing which basis each uses.

Ground snow is not roof snow. The table states the snow on the ground. The load on a roof is derived from it with factors for exposure, roof slope, thermal condition and drifting, and drifting against a taller wall or a parapet can far exceed the flat-roof figure. In mountain areas the code's map often defers to a local case study, and the jurisdiction's value may vary with elevation inside one county.

Winter design temperature is not the coldest day. It is a statistical design condition that a small share of winter hours fall below, chosen so that equipment is not sized for a once-in-decades night. The distinction is owned by Design Temperature Versus the Coldest Day Ever; the point here is that this cell is the number the load calculation uses, not the local record low.

Worked example: one row, three jobs

A shop works in a county whose adopted table reads (illustrative values): ground snow load 30 psf; wind speed 110 mph; topographic effects no; special wind region no; windborne debris zone no; seismic design category C; weathering severe; frost line depth 36 inches; termite slight to moderate; winter design temperature 6 F; ice barrier required yes; flood hazards with the community's NFIP entry in 1981 and its current flood maps dated 2016; air freezing index 1,200; mean annual temperature 51 F.

The roofing crew's reroof. Ice barrier "yes" means the roof needs ice barrier from the eave to at least 24 inches inside the exterior wall line. On a house with a 16 inch overhang measured horizontally, the target point sits 16 plus 24, or 40 inches, in from the eave edge in plan. On a pitched roof the material laid along the slope to reach that point is longer than 40 inches, so the crew locates the point from the wall line, measured in the attic or off the plans, rather than unrolling 40 inches along the deck. Where the building department reads the measurement differently, its reading governs. The detached shed out back, unconditioned, falls under the exception.

The plumber's water heater swap. Seismic category C does not trigger the IRC's strapping requirement for a single-family house, which reaches D0 through D2 and townhouses in C. So for a detached house in this county, the IRC alone does not require straps. That changes with the layer above it: California, for example, requires water heater bracing statewide by its own law regardless of category, and some local amendments add it. The plumber checks the local amendments page before deciding, and if this house were a townhouse, the strapping applies here under the IRC itself.

The concrete crew's garage apron and steps. Weathering "severe" puts exposed steps and the garage floor slab at 3,500 psi with air entrainment under the concrete table. A crew that orders a non-air-entrained mix because "it's just steps" gets surface scaling after a couple of freeze-thaw winters with deicer on them, which is exactly the damage the severe entry exists to prevent.

What the row does not tell any of them. The table is silent on how much water the lot holds, what the soil is, whether radon is present, which utility easements cross the yard, or what an HOA allows. It also does not give the energy code climate zone, which is set in a separate table in the energy provisions and owned by Climate Zones and Design Temperatures in Plain Terms. A crew that treats the design criteria row as the whole site story has read one of several records.

How to verify you read it right

For each job, note which cells you relied on, the value, and the edition and date of the adopted table: "Ice barrier required: yes, per county adopted 2021 IRC table, amendments dated 2023." When the county adopts a new edition, those notes show which past jobs were built to the old row and which to the new one, and a shop that keeps the row in a jurisdiction profile updates it once rather than rediscovering it on every quote.

References

• International Residential Code, Section R301.2 climatic and geographic design criteria, and Sections R402.2, R403.1.4.1, R905.1.2 and M1307.2, 2021 edition, as adopted by your jurisdiction
• ASCE 7, wind and snow load provisions, in the edition your adopted IRC references
• See related: How to Find the Local Design Criteria for a Jurisdiction
• See related: Frost Depth and What It Sets
• See related: Design Temperature Versus the Coldest Day Ever