Skylights Code Requirements in Iowa

Northeast Iowa's Driftless Area was unglaciated during the last ice age and features extensive karst topography with limestone and dolomite bedrock prone to dissolution. Counties with significant karst features include: Allamakee, Winneshiek, Clayton, Fayette, Delaware, Dubuque, Jackson, Jones, and portions of Howard, Chickasaw, and Buchanan counties. Features include sinkholes, caves, springs, disappearing streams, and underground drainage systems. Iowa Geological Survey has mapped thousands of karst features in the region.

Iowa is located in the heart of Tornado Alley and averages 50+ tornadoes per year. The entire state is at significant risk for tornadoes, straight-line winds (derechos), severe thunderstorms, and large hail. The 2020 Iowa Derecho (August 10, 2020) produced wind speeds exceeding 140 mph across a 770-mile path through central Iowa, causing over $11 billion in damage. EF3-EF5 tornadoes are not uncommon. Peak severe weather season is April through August.

Areas designated as Zone A, AE, V, or VE on FEMA Flood Insurance Rate Maps. Iowa has significant riverine flood risk along the Mississippi River (eastern border), Missouri River (western border), Des Moines River (central), Cedar River (eastern), Iowa River (eastern/central), and numerous tributaries. Major flood events include 1993 Great Flood, 2008 Cedar Rapids flood (500-year event), and 2019 Missouri River flooding. Local jurisdictions participate in National Flood Insurance Program (NFIP).

Iowa has the highest average indoor radon levels in the United States. The statewide average is approximately 8.5 pCi/L - more than double the EPA action level of 4.0 pCi/L. Over 70% of Iowa homes tested exceed the EPA action level. All 99 Iowa counties are classified as EPA Zone 1 (highest radon potential). The high radon levels are attributed to Iowa's glacial till soils, which are rich in uranium-bearing minerals, combined with well-sealed homes in cold climate.

Iowa averages 40+ severe hail events per year, concentrated from May through July. Western and central Iowa see the highest frequency of large hail (1.75 inch+). Roofing materials in Iowa should meet UL 2218 Class 3 or 4 impact resistance. Iowa insurance market increasingly requires or discounts impact-resistant roofing on residential properties.

Western and southern Iowa contain glacial till soils with high clay content that expand and contract with moisture changes. Des Moines lobe deposits and loess soils along the Missouri River bluffs are particularly problematic. Foundation design per IBC 1808.6 should account for soil expansion. Geotechnical investigation is recommended in areas with Pennsylvanian-age shale bedrock.

Iowa frost depths range from 36 inches in southern counties to 54+ inches in northern counties. Foundation footings must extend below the local frost line per IRC R403.1.4. Northern Iowa counties in Climate Zone 6A (Mason City, Decorah) have the deepest frost penetration and longest heating seasons. Frost-protected shallow foundations per IRC R403.3 are acceptable alternatives.

Iowa ground snow loads range from 20 psf in southern counties to 40 psf in northern counties per ASCE 7-22 Figure 7.2-1. Drift loads along parapets and adjacent structures must be designed per ASCE 7-22 Section 7.7. Northern Iowa lake-effect events from the Great Lakes can produce locally higher accumulations.

South-central Iowa counties including Marion, Monroe, Mahaska, Appanoose, and Wapello have historical underground coal mines that create subsidence risk. The Iowa Geological Survey maintains mine maps. Foundation and structural design in areas overlying abandoned mines must account for potential surface subsidence per IBC 1808.

Iowa has extensive Zone AE designations along the Mississippi, Missouri, Des Moines, Cedar, and Iowa rivers. The 2008 Cedar Rapids flood was one of the costliest in US history. Many Iowa communities require 1-2 feet of freeboard above BFE. The Iowa Flood Center operates a real-time flood monitoring network for 1,000+ communities.

Ice barrier underlayment is REQUIRED statewide per Iowa State Building Code (IRC R905.1.2). Iowa spans Climate Zones 5A (southern) and 6A (northern). Mean January temperatures range from 23°F (southern Iowa) to 14°F (northern Iowa). Iowa receives 30-50 inches of snow annually.

Eastern Iowa bluff lands along the Mississippi River and the Loess Hills in western Iowa contain WUI-designated communities. Iowa DNR Forestry Bureau identifies at-risk communities near forested areas. While lower risk than western states, drought conditions can create significant wildfire hazard. IWUIC guidelines apply where locally adopted.