Arpit Jain 18 min February 25, 2026

Idaho Structural Design Guide: Navigating Seismic, Snow Load, and Foundation Requirements

Code Requirements for Structural Design in Idaho: A Summary

Idaho's diverse geography presents unique structural design challenges, from high seismic zones in the central and eastern regions to extreme snow loads in the northern panhandle. Compliance requires a detailed understanding of the Idaho-adopted International Building Code (IBC) and International Residential Code (IRC), along with state and local amendments.

As of the latest adoption cycle, Idaho primarily enforces the 2018 I-Codes. For structural design, this means referencing the 2018 IBC/IRC, which in turn references ASCE 7-16 for load calculations.

Here are the key takeaways for architects, engineers, and builders:

• Seismic Design: Much of Idaho, including Boise and Pocatello, falls into Seismic Design Category (SDC) C or D. This mandates ductile detailing for structural systems, requires a site-specific geotechnical report to determine soil classification, and heavily influences the selection of lateral force-resisting systems per ASCE 7-16, Chapter 12.
• Snow Loads: Idaho provides a statewide ground snow load map, but many counties, especially in mountainous regions like Bonner County (Sandpoint), have their own detailed maps with significantly higher loads. The design roof snow load must account for elevation, roof geometry, and local amendments. Ground snow loads can range from 20 psf in the south to over 200 psf in the north.
• Frost Depth: Foundation footings must be placed below the mandated frost line to prevent heave. Idaho amends the IRC and IBC to provide a table of frost depths by county. These typically range from 24 to 36 inches. Frost-protected shallow foundations are a code-compliant alternative under IRC §R403.3 but require specific insulation and design detailing.

Location	Typical Seismic Design Category	Typical Ground Snow Load (Pg)	Mandated Frost Depth
Boise	SDC C or D	25-30 psf	24 inches
Pocatello	SDC D	35-40 psf	30 inches
Sandpoint	SDC C	70-150+ psf (Varies greatly)	30 inches
Idaho Falls	SDC D	35 psf	30 inches
Coeur d'Alene	SDC C	50-70 psf	24 inches

Note: The values above are typical and for preliminary planning only. Always verify with the local building department and a site-specific engineering analysis.

Why Idaho's Structural Codes Matter

Understanding Idaho's specific structural requirements is critical for project success, safety, and permit approval. The state's unique geological and climatological conditions are directly addressed through amendments and local enforcement practices. Failing to account for these can lead to costly redesigns, permit delays, and unsafe structures.

Key considerations in the project workflow include:

• Early Geotechnical Investigation: For commercial projects and homes in high-hazard areas, a geotechnical report is essential. It provides the soil site class for seismic calculations (determining SDC), foundation bearing capacity, and recommendations for frost protection.
• Integrated Design: The choice of lateral force-resisting system (e.g., shear walls, moment frames) directly impacts architectural layouts, MEP routing, and structural detailing. This must be coordinated early in the design phase.
• Jurisdictional Diligence: While the Idaho Division of Building Safety (DBS) sets the base codes, local jurisdictions (cities and counties) have the authority to adopt more restrictive amendments. Always start a project by confirming the specific snow load maps, frost depths, and administrative rules of the local building department.

Common misunderstandings include using the statewide snow map when a more restrictive county map exists, assuming a default soil class for seismic calculations on a major project, or not understanding the IEBC triggers for seismic upgrades during a renovation.

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For a new Type IIA, Group A-2 assembly occupancy in downtown Boise, what are the specific seismic force-resisting system requirements and detailing mandates per the Idaho-amended ASCE 7, and how do the local soil classifications impact the Seismic Design Category determination?

For a new Type IIA, Group A-2 building in downtown Boise, the design will be governed by high seismic requirements, likely resulting in a Seismic Design Category (SDC) D. This mandates the use of highly ductile lateral force-resisting systems with specific detailing as defined in ASCE 7-16 and material-specific standards like AISC 341 and ACI 318.

Here is a deeper explanation of the process and requirements:

1. Determine Seismic Design Parameters: Per the 2018 IBC, seismic design is governed by ASCE 7-16. For downtown Boise (Ada County), mapped spectral response acceleration parameters are approximately Ss = 0.85g and S1 = 0.32g. These values must be obtained from the ASCE 7 Hazard Tool or a similar source for the specific project address.

2. Impact of Soil Classification: The local soil classification is the most critical factor in determining the final design forces.

• Requirement: IBC §1613.2.2 and ASCE 7-16 §11.4.1 require a site-specific geotechnical investigation to determine the Site Class (A-F) for structures on sites with unknown soil properties or for structures assigned to SDC D, E, or F. For a Group A-2 occupancy of this scale in Boise, a geotech report is non-negotiable.
• Calculation: The Site Class determines the site coefficients, Fa and Fv (ASCE 7-16 Tables 11.4-1 and 11.4-2). For example, if the site is determined to be Site Class D (stiff soil), Fa would be 1.16 and Fv would be 1.96.
• Design Values: These coefficients are used to calculate the design spectral response acceleration parameters:
  • SDS = (2/3) * Fa * Ss
  • SD1 = (2/3) * Fv * S1
• SDC Determination: The Seismic Design Category is then determined using SDS and SD1 with ASCE 7-16 Tables 11.6-1 and 11.6-2. Given Boise's parameters and the common presence of stiff soils (Site Class D), the project will almost certainly fall into SDC C or, more likely, SDC D.

3. Seismic Force-Resisting System (SFRS) Requirements for SDC D: ASCE 7-16 Table 12.2-1 lists the permitted SFRS based on SDC. For SDC D, the code prohibits non-ductile systems and mandates those with high energy-dissipation capacity.

• Permitted Systems: Common choices for a Type IIA (non-combustible, protected) building include:
  • Steel: Special Steel Moment Frames (R=8), Special Concentrically Braced Frames (R=6), or Eccentrically Braced Frames (R=8). Ordinary Moment Frames are prohibited.
  • Concrete: Special Reinforced Concrete Shear Walls (R=6) or Special Reinforced Concrete Moment Frames (R=8). Ordinary concrete systems are prohibited.
  • Masonry: Special Reinforced Masonry Shear Walls (R=5.5).

4. Detailing Mandates: Assignment to SDC D triggers extensive ductile detailing requirements to ensure the system can withstand inelastic deformations during an earthquake.

• Steel: Design must comply with AISC 360 and the Seismic Provisions for Structural Steel Buildings (AISC 341). This includes requirements for protected zones, specific welding procedures and inspections, and capacity-based design principles where connections are designed to be stronger than the yielding members.
• Concrete: Design must comply with ACI 318, specifically the seismic provisions in Chapter 18, "Earthquake-Resistant Structures." This involves stringent requirements for confinement reinforcing in columns, boundary elements in shear walls, and ductile detailing of beam-column joints.
• Inspections: IBC Chapter 17 mandates a program of special inspections for seismic force-resisting systems, covering material verification, welding, high-strength bolting, and concrete reinforcement placement.

When retrofitting an unreinforced masonry (URM) building in Pocatello, what are the prescriptive vs. performance-based compliance paths allowed by the Idaho Building Code for seismic upgrades, and what level of alteration triggers a mandatory full building evaluation?

For retrofitting a URM building in a high seismic area like Pocatello, the Idaho-adopted 2018 International Existing Building Code (IEBC) provides three main compliance paths: Prescriptive, Work Area, and Performance. A substantial alteration, particularly one classified as a Level 3 Alteration under the Work Area Method, will typically trigger a mandatory seismic evaluation of the entire building.

1. Prescriptive Compliance Method (IEBC Chapter 5): This path is a simplified checklist approach. It is generally suitable only for very minor alterations that do not affect the structural system. For a URM building in Pocatello (likely SDC D), IEBC §506.4 specifically requires that any alteration increasing the seismic force in or capacity of a structural element by more than 5% must comply with the seismic requirements of the IBC for new construction, unless the entire building conforms to IEBC §1302 (Performance Method). This effectively pushes any significant URM work out of the purely prescriptive path.

2. Work Area Compliance Method (IEBC Chapters 6-12): This is the most commonly used path for renovations. The seismic upgrade requirements are directly tied to the extent of the work, classified into three levels.

• Level 1 Alteration (IEBC Chapter 7): Involves removal and replacement of existing materials. There are no specific structural triggers unless the work creates a new hazard.
• Level 2 Alteration (IEBC Chapter 8): Involves reconfiguration of space. IEBC §806.2 requires that where the alteration increases the story shear by more than 10%, the lateral-load-resisting system of that story must be brought into compliance with reduced IBC-level seismic forces (typically 75% of new). For URM buildings, it also triggers requirements for bracing of parapets and anchorage of walls to floors and roofs.
• Level 3 Alteration (IEBC Chapter 9): This is the trigger for a major evaluation. A project is a Level 3 alteration if the work area exceeds 50% of the building area. IEBC §906.1 requires a structural analysis of the entire building's lateral force-resisting system. The analysis must demonstrate that the building can resist reduced IBC-level seismic forces. This effectively mandates a full seismic retrofit for a URM building in Pocatello undergoing a major renovation.

3. Performance Compliance Method (IEBC Chapter 14): This path offers the most flexibility for complex or historic URM buildings where meeting prescriptive rules is impractical.

• Standard: This method relies on demonstrating that the retrofitted building achieves a specified life-safety performance objective using the methodologies of ASCE 41, Seismic Evaluation and Retrofit of Existing Buildings.
• Process: An engineer performs a detailed analysis of the existing structure (Tier 1, 2, or 3 evaluations), identifies deficiencies, and designs a retrofit scheme. The design might involve adding new steel frames, concrete shear walls, or applying fiber-reinforced polymers (FRP) to strengthen URM walls. The goal is to prove the retrofitted structure meets the performance goals of ASCE 41, providing an equivalent level of safety to the prescriptive code.

In summary, any alteration beyond minor cosmetic work on a URM building in Pocatello will trigger seismic upgrade requirements. A Level 3 Alteration (affecting >50% of the building area) or a significant change in structural loads mandates a full building evaluation and retrofit.

What is the specific minimum roof snow load (psf) required for a residential structure in Sandpoint, Idaho, accounting for the Idaho state snow load map and any local county amendments?

The minimum roof snow load for a residential structure in Sandpoint is determined by the Bonner County Building Department's specific snow load map, not the general Idaho state map. For the Sandpoint area, design roof snow loads typically range from 70 psf to over 150 psf, depending on the exact location and elevation.

• Governing Authority: Sandpoint is in Bonner County, which has adopted highly specific snow load requirements due to its mountainous terrain and proximity to major ski resorts. Per the Bonner County Building Department, designers must use the official Bonner County Snow Load Map.
• Code Reference: The Idaho Residential Code (based on 2018 IRC) references ASCE 7-16 for snow load calculations. However, IRC Section R301.2(5) allows local jurisdictions to establish ground snow loads by ordinance. Bonner County has done so.
• Determining the Load:
  1. Ground Snow Load (Pg): You must locate the project site on the Bonner County Snow Load Map to determine the ground snow load. For the city of Sandpoint and the immediate surrounding area, Pg is often in the 70-100 psf range. As you move to higher elevations (e.g., near Schweitzer Mountain), Pg can quickly exceed 200 psf.
  2. Roof Snow Load (Pf): The design roof snow load (Pf) is calculated from the ground snow load (Pg) using the formula in ASCE 7-16, Chapter 7: Pf = 0.7 * Ce * Ct * Is * Pg.
  3. Simplified Approach: For simple residential structures, the building department often provides prescriptive tables or a simplified minimum design roof snow load based on the ground snow load zone. For a typical residence in the Sandpoint area with Pg = 100 psf, the calculated flat roof snow load (Pf) would be 70 psf (assuming standard exposure, thermal, and importance factors).

It is critical to contact the Bonner County Building Department directly or consult their published maps for the correct ground snow load before beginning any design. Using the lower values from the general state map would result in a significant structural deficiency and permit rejection.

What is the mandated frost depth for a heated attached garage foundation in Idaho Falls, and is there an exception for frost-protected shallow foundations under the Idaho-adopted IRC?

The mandated frost depth for any foundation in Idaho Falls (Bonneville County) is 30 inches below the undisturbed ground surface. Yes, an exception for frost-protected shallow foundations (FPSF) is available under the Idaho-adopted IRC, which can be used for a heated attached garage.

• Mandated Frost Depth: The Idaho Residential Code amends Table R301.2(1) to specify frost line depths by county. For Bonneville County, this value is 30 inches. Per IRC §R403.1.4.1, the bottom of all exterior footings must be placed at or below this depth. The code does not make a direct exception for heated garages using conventional foundations; the footing must still reach the 30-inch depth.
• Exception for Frost-Protected Shallow Foundations (FPSF): The exception is found in IRC §R403.3. This section allows foundation depths to be reduced if the foundation is insulated according to its prescriptive requirements. This method is explicitly permitted for heated structures.
  • How it Works: FPSF design uses vertical insulation on the exterior of the foundation wall and horizontal wing insulation extending outward from the base of the footing. This traps the building's heat in the ground, preventing the soil under the footings from freezing.
  • Design Requirements: To use this exception for a heated garage in Idaho Falls, the design must strictly follow the insulation requirements detailed in IRC Figures R403.3(1) and R403.3(2) and Tables R403.3(1) and R403.3(2). The required R-value and dimensions of the insulation depend on the Air Freezing Index for the location.
  • Practical Application: For a heated attached garage, an FPSF design could potentially reduce the required footing depth from 30 inches to as little as 12 or 16 inches, saving significant excavation and concrete costs, provided the specific insulation detailing is correctly implemented and documented on the plans.

What is the frost line depth in Coeur d'Alene for footings?

The mandated frost line depth for footings in Coeur d'Alene (Kootenai County) is 24 inches.

This requirement comes directly from the Idaho Residential Code's amendment to Table R301.2(1), "Climatic and Geographic Design Criteria." This table provides a county-by-county list of design values, including the minimum depth to the bottom of footings to protect against frost heave. All exterior footings for new construction, additions, and in some cases, accessory structures, must extend to this 24-inch minimum depth below the final grade.

What is the roof snow load I should design for in Pocatello?

The design roof snow load for Pocatello (Bannock County) should be based on a ground snow load (Pg) of 35 to 40 psf, with the final design value depending on elevation and specific site conditions. For a typical flat or low-slope roof on a standard occupancy building, this results in a design roof snow load (Pf) of approximately 25 to 28 psf.

• Ground Snow Load (Pg): The Idaho State Snow Load Map indicates a ground snow load of 35-40 psf for the Pocatello area. The Bannock County building department should be consulted to confirm the exact Pg for the project site, as it can increase with elevation.
• Calculating Design Roof Snow Load (Pf): The design load is calculated per ASCE 7-16. For a simple calculation on a standard residential or commercial building (Importance Factor, Is = 1.0; Exposure Factor, Ce = 1.0; Thermal Factor, Ct = 1.0):
  • Pf = 0.7 * Ce * Ct * Is * Pg
  • Pf = 0.7 * 1.0 * 1.0 * 1.0 * (35 to 40 psf)
  • Pf ≈ 25 to 28 psf
• Minimum Load: Per IBC §1608.2, the minimum design roof snow load for low-slope roofs is 20 psf where Pg is 20 psf or less. Since Pocatello's Pg is higher, the calculated value governs.
• Other Considerations: The engineer of record must also evaluate potential snow drifts, unbalanced loads on gable roofs, and sliding snow, as required by ASCE 7-16, Chapter 7.

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Additional Considerations for Idaho Structural Design

Jurisdictional Variations: Beyond the State Code

While Idaho adopts a statewide building code, the most critical aspect of structural design is verifying local amendments. Authority Having Jurisdiction (AHJ) lies with the city or county, and they often adopt more stringent requirements.

• Snow Loads: Counties like Bonner, Valley, and Blaine publish their own detailed snow load maps that supersede the state map. These maps are based on localized studies and account for microclimates and elevation changes.
• Wind Loads: While many parts of Idaho have a base wind speed of 115 mph (Risk Category II), some local jurisdictions in foothill or canyon areas may have specific requirements or historical data indicating higher speeds.
• Administrative Amendments: Cities like Boise or Meridian may have their own administrative ordinances that affect plan submittal requirements, special inspection protocols, or local soil condition presumptions. Always check the AHJ's website or contact them directly at the start of a project.

Coordination Between Geotechnical and Structural Engineering

A seamless workflow between the geotechnical and structural engineer is non-negotiable for commercial projects and many residential projects in Idaho.

1. Geotechnical Report: The geotech provides the foundational data:
   • Site Class (A-F) for seismic calculations.
   • Soil bearing capacity for foundation sizing.
   • Recommendations on foundation type (e.g., spread footings, piles).
   • Frost depth confirmation and recommendations for subgrade preparation.
   • Information on expansive soils or high water tables.
2. Structural Design: The structural engineer uses this data to:
   • Accurately calculate seismic base shear (SDS and SD1 depend on Site Class).
   • Design footings that are safe against bearing failure and settlement.
   • Specify the correct lateral force-resisting system and its required ductile detailing based on the SDC.
   • Detail foundations to resist frost heave and other soil-related pressures.

Miscommunication or designing without a geotech report in a high-hazard area is a primary cause of plan review rejection and potential structural failure.

Common Mistakes and Misinterpretations

• Using Default Soil Class: Assuming Site Class D for a large project in SDC D without a geotech report is a code violation (ASCE 7-16 §11.4.1) and can lead to an inaccurate or unsafe seismic design.
• Ignoring Snow Drifts: Designing a roof only for the balanced snow load is a frequent error. ASCE 7-16 has extensive provisions for calculating surcharge loads from drifts at roof steps, parapets, and other obstructions. These often govern the design of localized framing.
• Incorrect Frost Depth for Unheated Buildings: For detached garages, sheds, or other unheated structures, the exception for frost-protected shallow foundations does not apply. Footings must extend to the full mandated frost depth.
• Applying IRC Prescriptive Bracing in High Hazard Zones: The prescriptive wall bracing methods in the IRC have limits. In areas with high seismic loads (like Pocatello) or high wind speeds, an engineered design is often required per IRC §R301.2.

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Idaho Structural Design FAQ

What building codes are currently in effect in Idaho? Idaho has adopted and enforces the 2018 International Codes suite, including the International Building Code (IBC), International Residential Code (IRC), and International Existing Building Code (IEBC).

Do I need an engineer for my residential project in Idaho? An engineered design is typically required if your project deviates from the prescriptive requirements of the IRC or is located in an area with high seismic risk, high wind speeds (>115 mph), or high ground snow loads (generally above 50-70 psf, depending on the jurisdiction).

Where can I find the official Idaho snow load map? The Idaho Division of Building Safety (DBS) provides a statewide map, but it is for general guidance. The legally binding snow load values are provided by the local city or county building department, many of whom have their own detailed maps.

Is Idaho a high seismic risk area? Yes, central and eastern Idaho, including the Boise, Pocatello, and Idaho Falls areas, are in regions of high seismicity, often falling into Seismic Design Category C or D. This requires specialized engineering and construction practices.

What is the difference between ground snow load (Pg) and roof snow load (Pf)? Ground snow load (Pg) is a statistically derived value of the weight of snow on the ground for a specific location. Roof snow load (Pf) is the actual load used for designing the building's roof structure, calculated from Pg and adjusted for factors like roof slope, exposure, and thermal conditions.

Can I use a monolithic "slab-on-grade" foundation in Idaho? Yes, but only if it is designed as a frost-protected shallow foundation with proper insulation per IRC §R403.3 or if the grade beam/thickened edge of the slab extends down to the required frost depth for your county (e.g., 24 inches in Kootenai County, 30 inches in Bonneville County).

Are pole barns exempt from the Idaho building code? A pole barn may be exempt if it is classified strictly for agricultural use as defined in IBC §202 and meets the exemption criteria of the local jurisdiction. If it is used for storage, as a workshop ("shop"), or for any commercial purpose, it is generally not exempt and must have a building permit.

What triggers the need for a special inspection for seismic systems? Per IBC Chapter 17, most designated seismic force-resisting systems in SDC C and higher require special inspections. This includes welding, high-strength bolting, placement of concrete reinforcing steel, and concrete testing to ensure the system is built exactly as designed.

How do I find out about my city's specific building code amendments? The best way is to visit the website for your local city or county building department. They typically publish their local ordinances, design criteria documents (like snow load maps), and plan review checklists online. A direct phone call to the plan review staff is also highly recommended.