Boulder County Building Publications
Residential Deck Requirements - B09

Residential construction in Boulder County follows the 2021 International Residential Code (IRC) for one‑ and two‑family dwellings, along with current county‑adopted amendments. The information below offers a general overview of residential deck requirements and standards to help guide project planning, but it is not a complete list of requirements that must be shown in construction documents.

Building Permit Requirements

Please apply online at Accela Citizen Access.

All construction drawings must be submitted in PDF format at the time of your online building permit application Boulder County Permit Records and Online Application Submittals.

Building permits are required for the construction of all decks to one- and two-family dwellings and townhouses that exceed 200 square feet, that are attached to a dwelling, and for freestanding decks that are elevated 30 inches or more above grade with supporting beams, joists, or posts.

Be Advised

Decks that are constructed on a parcel where the wind speed is 140 MPH (Vult) or more per the Boulder County wind map or where ground snow loads are greater than 70 pounds per square foot shall be designed by a licensed Colorado design professional, architect or engineer. Check for site specific design load requirements using the PropertySearch – look at the zoning and septic map layer.

The majority of unincorporated Boulder County exceeds the prescriptive construction requirements and will require a signed and sealed drawing prepared by a Colorado licensed design professional.

Construction Documents Requirements

Site Plan

Provide a site plan that shows the following:

• Location of the deck
• Distance from the deck to the property lines
• Distance from the deck to other structures on the lot
• North arrow

Deck Drawings

Construction drawings should show the proposed design and materials.

Floor plan to include following:

1. Proposed deck size with dimensions to scale with minimum 1/4 inch scale per square foot
2. Size and spacing of floor joists
3. Size and type of decking material
4. Size, location and spacing of posts including post connection to footing
5. Location of door and window openings adjacent to location of deck
6. Size of beams – also indicating post/beam connection
7. Species and grade of lumber to be used

Elevations to include the following:

1. Height of structure from established grade
2. Size and depth of footings
3. Guard height and spacing of intermediates
4. Stairway rise/run and handrail requirements
5. Clearance to overhead wires, as applicable

Deck Component Requirements

Footings (IRC section R507.3)

Assumed soil bearing capacity (IRC section R401.4.1) is 1500 pounds per square foot unless a geotechnical/soils report for the site is submitted.

1. Show size of the footing, including depth (minimum 30 inches), width, and length (IRC Section R403/R507.3.1).
2. Reference method of frost protection (IRC Section R507.3.3), as applicable.
3. Show the spacing of the footing placement.

Deck Posts (IRC section R507.4)

View IRC Section R507.4.

1. Show the size of the deck, and the locations and spacing of the columns/piers.
2. Show where wood posts are proposed to support the deck. Please also reference the wood species and the wood member dimensions.
3. Add references for proposed connections for posts to footings or foundation, as applicable.

Deck Beams (IRC section R507.5)

View IRC section R507.5.

1. Label the beam size and type of material including lumber species and grade.
2. Reference bearing points for the beam. Show all beam spans.
3. Note on the plans if the beam is intended to be a flush beam where the joists are required to bear on joist hangers or a ledger, or if the beam is to be a dropped beam with joists resting on top of the beam.

Deck Joists (IRC section R507.6)

View IRC section R507.6.

1. Show size, span, and spacing of joist members (IRC Table R507.6).
2. Show references to the drawings for deck joist connections to bearing locations.
3. Show any cantilevers.

Deck Walking Surface / Decking (IRC section R507.7 and Boulder County Building Code Amendment R390 Ignition Resistant Construction)

View IRC section R507.7 and R390.

The provisions of R507.7 are superseded by the specific requirements of BCBC amendment, IRC section R390.4.8.1.

1. Provide submittal for decking materials.
2. Decking shall comply with BCBC amendment, IRC section R390.4.8.1.
3. Wood-plastic composite (WPC) products are to be labeled with ASTM D7032.

Deck Vertical and Lateral Deck Supports (IRC section R507.8) and Deck Vertical and Lateral Supports at Band Joists (IRC section R507.9)

View IRC section R507.8 and R507.9

1. Decks shall be self-supported where positive connections to the primary structure are not able to be verified, as applicable (R507.8).
2. For Vertical Supports (IRC section R507.9.1), show deck ledger details including ledger lumber species, lumber grade, bearing, and connection details. Reference fastener spacing on ledger including fastener length and diameter. Refer to IRC Table and figure R507.9.2(2) for table and illustrative references.
3. Show Lateral Connection details and reference proposed hardware (IRC section R507.9.2), as applicable. Refer to IRC figure R507.9.2(1) and figure R507.9.2(2) for illustrative references.

Ledger Board Requirements (IRC section R507.9.1.3)

View IRC section R507.9.1.3

The following shall be shown or otherwise referenced on the construction documents. For prescriptive construction requirements, see all of IRC section R507.9.

1. Indicate the size of the ledger board.
2. List the type and dimension or size of fastener that attaches the ledger board to the structure.
3. Indicate the spacing between the fasteners or frequency and spacing.
4. Note the required penetration depth for ledger board fasteners.
5. Reference the flashing in the detail. See R390 for additional wall attachment requirements.

Deck Exterior Guards and Handrails

Refer to Residential Stairways, Handrails, Ramps and Guards – B32 – Boulder County

Show applicable construction details for deck guard rails, stairs to decks, and handrails. Connection details are required to resist design loads of IRC section R301.5.

Safety Glazing

Refer to Safety Glazing – B30 – Boulder County

Show how glazing adjacent to deck walking surfaces are compliant, including but not limited to windows and guards.

Ignition Resistant Construction and Materials

Refer to Ignition Resistant Construction – B37 – Boulder County

• Materials shall comply with requirements outlined in all sections of IRC section 507.2 and Boulder County Building Code amendments to IRC section R390.
• Third-party tested and listed materials may be required, including but not limited, to Ignition Resistant Construction compliance or structural compliance. Please provide the listing from an approved third-party agency (i.e. Underwriters Laboratory’s UL Product IQ, ICC Evaluation Services, Intertek, etc.).

Additional Resources for Deck Construction

• Free Access to Online ICC Codes
• Colorado Chapter of the International Code Council’s Building Guide-Decks (2021)
• American Wood Council’s Design for Code Acceptance. DCA6- 2015 Prescriptive Residential Wood Deck Construction Guide

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