Title of Rule and Other Identifying Information: Chapter 51-55 WAC, Amendments to the 2021 International Wildland-Urban Interface Code (IWUIC).
Purpose of the Proposal and Its Anticipated Effects, Including Any Changes in Existing Rules: Reconciling state amendments with section renumbering and model code modifications in the 2021 IWUIC; correcting errors and omissions.
Reasons Supporting Proposal: With the exception of the issues noted below, this represents section and reference numbering housekeeping.
Rule is not necessitated by federal law, federal or state court decision.
Name of Proponent: State building code council, governmental.
Name of Agency Personnel Responsible for Drafting and Implementation: Dustin Curb, 1500 Jefferson Street S.E., Olympia, WA 98504, 360-972-4158; Enforcement: Local jurisdictions having authority.
This notice meets the following criteria to use the expedited adoption process for these rules:
Adopts or incorporates by reference without material change federal statutes or regulations, Washington state statutes, rules of other Washington state agencies, shoreline master programs other than those programs governing shorelines of statewide significance, or, as referenced by Washington state law, national consensus codes that generally establish industry standards, if the material adopted or incorporated regulates the same subject matter and conduct as the adopting or incorporating rule.
Corrects typographical errors, makes address or name changes, or clarifies language of a rule without changing its effect.
Explanation of the Reason the Agency Believes the Expedited Rule-Making Process is Appropriate: This addresses clerical oversight.
THIS RULE IS BEING PROPOSED UNDER AN EXPEDITED RULE-MAKING PROCESS THAT WILL ELIMINATE THE NEED FOR THE AGENCY TO HOLD PUBLIC HEARINGS, PREPARE A SMALL BUSINESS ECONOMIC IMPACT STATEMENT, OR PROVIDE RESPONSES TO THE CRITERIA FOR A SIGNIFICANT LEGISLATIVE RULE. IF YOU OBJECT TO THIS USE OF THE EXPEDITED RULE-MAKING PROCESS, YOU MUST EXPRESS YOUR OBJECTIONS IN WRITING AND THEY MUST BE SENT TO Dustin Curb, State Building Code Council, 1500 Jefferson Street S.E., Olympia, WA 98504, phone 360-972-4158, email Dustin.Curb@des.wa.gov, AND RECEIVED BY September 5, 2023.
301 General.
301.1 Scope. Wildland urban interface areas shall be determined using the Washington wildland urban interface map (WA-WUI). WA-WUI designations are permitted to be modified, upon approval of a finding of fact in accordance with Section 302.
User note: The WA-WUI map is available at ((https://data-wadnr.opendata.arcgis.com/apps/wildland-urban-interface-viewing-app/explore))https://geo.wa.gov/maps/786aaa1dbbd748e6ae04bc43c8f127fe/explore.
301.2 Construction in wildland-urban interface or intermix areas. Where a structure is proposed to be constructed in an area designated by the WA-WUI map as wildland-urban interface or intermix, the construction shall comply with the provisions of this code.
301.3 Construction in wildlands areas. Where a structure is proposed to be constructed in an area designated by the WA-WUI map as wildlands, the applicable wildland urban interface area designation shall be based on a finding of fact in accordance with Section 302.
302 Wildland-urban interface area designations.
302.1 General. Wildland urban interface area designations are permitted to be established in accordance with this section.
302.2 Finding of fact. The applicable wildland urban interface designation shall be based on a finding of fact. The finding of fact shall comply with the provisions of Appendix E or is permitted to be based on the worksheet and procedures in Section 302.3.
302.3 Simplified wildland urban interface designation worksheet. The wildland urban interface designation is permitted to be established using the procedure outlined in Figure 302(1), using the worksheet in Figure 302(2).
302.3.1 Area to be evaluated. For the purposes of establishing structure and vegetation densities, the area covered by a square of 1320 feet on a side (40 acres) shall be evaluated. The square area shall be located such that the site under consideration is in its center, except where the square would overlap a water body shown on the WA-WUI map with a surface area greater than 200,000 square feet, the location shall be adjusted such that no part of the square overlaps the water body.
302.3.2 Structure density category. The structure density category shall be determined by counting the number of structures within the area to be evaluated per Section 302.3.1. The structure density category shall be determined as follows:
UNINHABITED: | 0 structures |
VERY LOW: | 1 structure |
LOW: | 2 to 8 structures |
MEDIUM: | 9 to 120 structures |
HIGH: | more than 120 structures |
302.3.3 Vegetation density category. Vegetation coverage within the area to be evaluated per Section 302.3.1 shall be determined in accordance with Chapter 9. Vegetation density shall be determined by dividing the vegetation coverage by 1,742,400 square feet (40 acres). Where the vegetation density is less than 50 percent, the vegetation density category for the site shall be nonvegetated. Where the vegetation density is 50 percent or more, the vegetation density category for the site shall be vegetated.
302.3.4 Proximity category. The distance from the site being evaluated to a high-density vegetated area shall be measured from the closest edge of the site boundary to the closest edge of the nearest high-density vegetated area. Where the distance is less than 1.5 miles, the proximity category shall be near. Where the distance is 1.5 miles or more, the proximity category shall be distant.
302.3.5 WUIC applicability. The WUIC shall apply, and the site shall be designated as intermix or interface in accordance with Section 302.3.6 under either of the following conditions:
1. The structure density category is very low to high, and the vegetation density category is vegetated.
2. The structure density category is very low to high, and the proximity category is near.
The WUIC shall not apply under either of the following conditions:
1. The structure density category is uninhabited, and the site is not located within an area designated as intermix or interface on the WA-WUI map.
2. The structure density category is uninhabited to high, the vegetation density category is nonvegetated, and the proximity category is distant.
302.3.6 Wildland urban interface area designation. Where required by Section 302.3.5, the site shall be designated as intermix or interface in accordance with Section 302.3.6.1 or 302.3.6.2.
302.3.6.1 Intermix designation. The site shall be designated as intermix where the structure density category is very low to high, and the vegetation density category is vegetated.
302.3.6.2 Interface designation. The site shall be designated as interface where the structure density category is very low to high, and the proximity category is near.
For the area to be evaluated in Section 302.3.1:
1. Determine structure density category (uninhabited, very low, low, medium, or high).
2. Determine vegetation density category (nonvegetated or vegetated).
3. Determine proximity category (near or distant).
4. Based on structure density, vegetation density, and proximity categories, determine if compliance with this code is required (WUIC applies, WUIC does not apply).
5. Where compliance with this code is required, determine wildland urban interface area designation (intermix or interface).
Figure 302(1). Outline of simplified procedure for determining wildland interface designation
1. Determine structure density category in accordance with Section 302.3.2. Numbers in table are the number of structures within the area determined by Section 302.3.1.
UNINHABITED | VERY LOW | LOW | MEDIUM | HIGH |
0 | 1 | 2 TO 8 | 9 TO 120 | MORE THAN 120 |
2. Determine vegetation density category within the area determined by Section 302.3.1.
NONVEGETATED | VEGETATED |
Less than 50% vegetated | 50% or more vegetated |
3. Determine proximity category to the nearest high-density vegetated area.
NEAR | DISTANT |
Less than 1.5 mi (2.414 km) | 1.5 mi (2.414 km) or more |
4. Use structure density, vegetation density, and proximity categories from above to determine if WUIC applies.
WUIC Applies | WUIC Does Not Apply |
• Structure density category is very low to high; and | • Structure density category is uninhabited; and |
• Vegetation density category is vegetated. | • The site is not located within an area designated as intermix or interface on the WA-WUI map. |
• Structure density category is very low to high; and | • Structure density category is uninhabited to high; and |
• Proximity category is near. | • Vegetation density category is nonvegetated; and |
| • Proximity category is distant. |
5. Where WUIC applies, the site shall be designated as intermix or interface as follows:
INTERMIX | INTERFACE |
• Structure density category is very low to high; and | • Structure density category is very low to high; and |
• Vegetation density category is vegetated. | • Proximity category is near. |
Figure 302(2). Worksheet for simplified procedure for determining wildland interface designation
302.4 Review of wildland-urban interface areas. The code official shall review for approval evaluated areas for new or modified findings of fact. Where a new or modified findings of fact are approved, the code official shall recommend to WADNR a modification to the wildland-urban interface areas mapping.
Section 501 General.
501.1 General. Buildings and structures hereafter constructed, modified, or relocated into or within the wildland-urban interface area shall meet the construction requirements of Sections 501.4 through 501.8.
EXCEPTIONS: | 1. Buildings and structures with fire hazard severity determined in Section 502 and with ignition-resistant construction classification determined in Section 503. |
| 2. Accessory structures not exceeding 200 square feet (((11))18.5 m2) in floor area and where located not less than 50 feet (15,240 mm) from buildings or structures containing habitable spaces. |
| 3. Agricultural buildings located not less than 50 feet (15,240 mm) from buildings or structures containing habitable spaces. |
501.2 Objective. This section is not adopted.
501.4 Roof covering. Roofs shall have a roof assembly that complies with a Class A rating when tested in accordance with ASTM E108 or UL 790. For roof assemblies where the profile allows a space between the roof covering and roof deck, the space at the eave ends shall be firestopped to preclude entry of flames or embers or have one layer of 72-pound (32.4 kg) mineral-surfaced, nonperforated cap sheet complying with ASTM D3909 installed over the combustible roof deck.
EXCEPTIONS: | 1. Class A roof assemblies including those with coverings of brick, masonry, or an exposed concrete roof deck. |
| 2. Class A roof assemblies also include ferrous or copper shingles or sheets, metal sheets and shingles, clay or concrete roof tile or slate installed on noncombustible decks or ferrous, copper or metal sheets installed without a roof deck on noncombustible framing. |
| 3. Class A roof assemblies include minimum 16 oz/sq. ft. (0.0416 kg/m2) copper sheets installed over combustible roof decks. |
501.4.1 Roof valleys. Where provided, valley flashings shall be not less than 0.019-inch (0.48 mm) (No. 26 galvanized sheet gage) corrosion-resistant metal installed over a minimum 36-inch-wide (914 mm) underlayment consisting of one layer of 72-pound (32.4 kg) mineral-surfaced, nonperforated cap sheet complying with ASTM D3909 running the full length of the valley.
501.5 Exterior walls and projections other than decks. Exterior walls and projections other than decks, of buildings, or structures, or accessory structures attached to buildings or structures with habitable spaces, shall be constructed with one of the following methods, with materials extending from the top of the foundation to the underside of the roof sheathing:
1. Materials approved for not less than one hour fire-resistance rated construction on the exterior side;
2. Approved noncombustible materials;
3. Heavy timber or log wall construction;
4. Fire-retardant-treated wood on the exterior side. The fire-retardant-treated wood shall be labeled for exterior use and meet the requirements of Section 2303.2 of the International Building Code; or
5. Ignition-resistant materials, complying with Section 503.2 on the exterior side.
EXCEPTION: | Unenclosed accessory structures attached to buildings with habitable spaces and projections, such as decks, attached to the first floor of a building, if when the structure is built with building materials at least two inches nominal depth and the area below the unenclosed accessory structure is screened with material with openings no greater than 1/4-inch maximum to prevent accumulation of combustibles and to prevent embers from coming in underneath. |
501.6 Decks and appendages. The material of decks, porches, balconies, and stairs shall be constructed with any of the following materials:
1. Ignition-resistant material that complies with the minimum performance requirement of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Any material that complies with the minimum performance requirements of Section 503.2 when attached exterior wall covering is also either noncombustible or ignition-resistant material.
5. Heavy timber construction consisting of the following:
5.1. Posts shall be a minimum of 6 inches x 6 inches nominal dimension.
5.2. Beams shall be a minimum of 6 inches x 8 inches nominal dimension.
5.3. Joists shall be a minimum of 4 inches x 8 inches nominal dimension spaced at no greater than 24 inches on center.
501.6.1 Clearance. Decks with less than 48 inches of clearance from finished grade to deck joists shall be enclosed with screen material with openings no greater than 1/4-inch maximum to prevent accumulation of combustibles and to prevent embers from coming in underneath.
501.6.2 Walking surfaces. The walking surface material of decks, porches, balconies, and stairs shall be constructed with one of the following materials:
1. Ignition-resistant material that complies with the performance requirements of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Where the deck, porch, balcony, or stairs are constructed of heavy timber in accordance with Section 501.6, natural wood decking products shall be:
4.1. 2-inch nominal dimension lumber; or
4.2. 5/4-inch nominal hardwood (i.e., teak, mahogany, or other approved hardwood).
5. Material that complies with the performance requirements of Section 501.6.2.1 when tested in accordance with ASTM E2632 and when attached exterior wall covering is also composed of only noncombustible or ignition-resistant materials.
EXCEPTION: | Wall material shall be permitted to be of any material that otherwise complies with Section 501.5 when the decking surface material complies with the performance requirements of ASTM E84 with a Class B flame spread index. |
501.6.2.1 Material in Section 501.6.2, Item 5. The walking surface material shall be tested in accordance with ASTM E2632 and shall comply with the following condition of acceptance. The ASTM E2632 test shall be conducted on a minimum of three test specimens and the peak heat release rate shall be less than or equal to 25 kW/ft2 (269 kW/m2). If any one of the three tests does not meet the conditions of acceptance, three additional tests shall be run. All the additional tests shall meet the condition of acceptance.
501.7 Exterior glazing. Exterior windows, window walls and glazed doors, windows within exterior doors, and skylights shall be tempered glass, multilayered glazed panels, glass block, or have a fire protection rating of not less than 20 minutes.
501.8 Vents. Attic ventilation openings, foundation or underfloor vents, or other ventilation openings in vertical exterior walls and vents through roofs shall not exceed 144 square inches (0.0929 m2) each. Such vents shall be covered with noncombustible corrosion-resistant mesh with openings not to exceed 1/4 inch (6.4 mm), or shall be designed and approved to prevent flame or ember penetration into the structure.
1. Attic ventilation openings shall not be located in soffits, in eave overhands, between rafters at eaves, or in other overhang areas. Gable end and dormer vents shall be located not less than 10 feet (3048 mm) from lot lines. Underfloor ventilation openings shall be located as close to grade as possible.
502 Fire hazard severity.
502.1 General. The fire hazard severity of building sites for buildings hereafter constructed, modified, or relocated into wildland-urban interface areas shall be established in accordance with Table 502.1. See also Chapter 8.
502.2 Fire hazard severity reduction. The fire hazard severity identified in Table 502.1 is allowed to be reduced by implementing a vegetation management plan in accordance with Chapter 7.
TABLE 502.1
FIRE HAZARD SEVERITY
(No change to the table)
504 Class 1 ignition-resistant construction.
504.7 Appendages and projections.Accessory structures attached to buildings with habitable spaces and projections other than decks, porches, balconies, or stairs, shall be not less than 1-hour fire-resistance-rated construction, heavy timber construction, or constructed of one of the following:
1. Approved noncombustible materials.
2. Fire-retardant-treated wood identified for exterior use and meeting the requirements of Section 2303.2 of the International Building Code.
3. Ignition-resistant building materials in accordance with Section 503.2.
504.8 Decks and appendages. The material of decks, porches, balconies, and stairs shall be constructed with any of the following materials:
1. Ignition-resistant material that complies with the minimum performance requirement of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Any material that complies with the minimum performance requirements of Section 503.2 when attached exterior wall covering is also either noncombustible or ignition-resistant material.
5. Heavy timber construction consisting of the following:
5.1. Posts shall be a minimum of 6 inches x 6 inches nominal dimension.
5.2. Beams shall be a minimum of 6 inches x 8 inches nominal dimension.
5.3. Joists shall be a minimum of 4 inches x 8 inches nominal dimension spaced at no greater than 24 inches on center.
504.8.1 Clearance. Decks with less than 48 inches of clearance from finished grade to deck joists shall be enclosed with screen material with openings no greater than 1/4-inch maximum to prevent accumulation of combustibles and to prevent embers from coming in underneath.
504.8.2 Walking surfaces. The walking surface material of decks, porches, balconies, and stairs shall be constructed with one of the following materials:
1. Ignition-resistant material that complies with the performance requirements of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Where the deck, porch, balcony, or stairs are constructed of heavy timber in accordance with Section 501.6, natural wood decking products shall be:
4.1. 2-inch nominal dimension lumber; or
4.2. 5/4-inch nominal hardwood (i.e., teak, mahogany, or other approved hardwood).
5. Material that complies with the performance requirements of Section 504.8.2.1 when tested in accordance with ASTM E2632 and when attached exterior wall covering is also composed of only noncombustible or ignition-resistant materials.
EXCEPTION: | Wall material shall be permitted to be of any material that otherwise complies with Section 501.5 when the decking surface material complies with the performance requirements of ASTM E84 with a Class B flame spread index. |
504.8.2.1 Material in Section ((504.8.1))504.8.2, Item 5. The walking surface material shall be tested in accordance with ASTM E2632 and shall comply with the following condition of acceptance. The ASTM E2632 test shall be conducted on a minimum of three test specimens and the peak heat release rate shall be less than or equal to 25 kW/ft2 (269 kW/m2). If any one of the three tests does not meet the conditions of acceptance, three additional tests shall be run. All the additional tests shall meet the condition of acceptance.
504.9 Exterior glazing. Exterior windows, window walls and glazed doors, windows within exterior doors, and skylights shall be tempered glass, multilayered glazed panels, glass block or have a fire protection rating of not less than 20 minutes.
504.10 Exterior doors. Exterior doors shall be approved noncombustible construction, solid core wood not less than 1 3/4 inches thick (44 mm), or have a fire protection rating of not less than 20 minutes. Windows within doors and glazed doors shall be in accordance with Section 504.8.
EXCEPTION: | Vehicle access doors. |
504.11 Vents. Attic ventilation openings, foundation or underfloor vents, or other ventilation openings in vertical exterior walls and vents through roofs shall not exceed 144 square inches (0.0929 m2) each. Such vents shall be covered with noncombustible corrosion-resistant mesh with openings not to exceed 1/4 inch (6.4 mm), or shall be designed and approved to prevent flame or ember penetration into the structure.
504.11.1 Vent locations. Attic ventilation openings shall not be located in soffits, in eave overhangs, between rafters at eaves, or in other overhang areas. Gable end and dormer vents shall be located not less than 10 feet (3048 mm) from lot lines. Underfloor ventilation openings shall be located as close to grade as practical.
504.12 Detached accessory structures. Detached accessory structures located less than 50 feet (15,240 mm) from a building containing habitable space shall have exterior walls constructed with materials approved for not less than 1-hour fire-resistance-rated construction, heavy timber, log wall construction, or constructed with approved noncombustible materials or fire-retardant-treated wood on the exterior side. The fire-retardant-treated wood shall be labeled for exterior use and meet the requirements of Section 2303.2 of the International Building Code.
504.12.1 Underfloor areas. Where the detached structure is located and constructed so that the structure or any portion thereof projects over a descending slope surface greater than 10 percent, the area below the structure shall have underfloor areas enclosed to within 6 inches (152 mm) of the ground, with exterior wall construction in accordance with Section 504.5 or underfloor protection in accordance with Section 504.6.
EXCEPTION: | The enclosure shall not be required where the underside of exposed floors and exposed structural columns, beams and supporting walls are protected as required for exterior 1-hour fire-resistance-rated construction or heavy timber construction or fire-retardant-treated wood on the exterior side. The fire-retardant-treated wood shall be labeled for exterior use and meet the requirements of Section 2303.2 of the International Building Code. |
505 Class 2 ignition-resistant construction.
505.7 Appendages and projections.Accessory structures attached to buildings with habitable spaces and projections, other than decks, porches, balconies, or stairs, shall be not less than 1-hour fire-resistance-rated construction, heavy timber construction or constructed of one of the following:
1. Approved noncombustible materials.
2. Fire-retardant-treated wood identified for exterior use and meeting the requirements of Section 2303.2 of the International Building Code.
3. Ignition-resistant building materials in accordance with Section 503.2.
505.8 Decks and appendages. The material of decks, porches, balconies, and stairs shall be constructed with any of the following materials:
1. Ignition-resistant material that complies with the minimum performance requirement of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Any material that complies with the minimum performance requirements of Section 503.2 when attached exterior wall covering is also either noncombustible or ignition-resistant material.
5. Heavy timber construction consisting of the following:
5.1. Posts shall be a minimum of 6 inches x 6 inches nominal dimension.
5.2. Beams shall be a minimum of 6 inches x 8 inches nominal dimension.
5.3. Joists shall be a minimum of 4 inches x 8 inches nominal dimension spaced at no greater than 24 inches on center.
505.8.1 Clearance. Decks with less than 48 inches of clearance from finished grade to deck joists shall be enclosed with screen material with openings no greater than 1/4-inch maximum to prevent accumulation of combustibles and to prevent embers from coming in underneath.
505.8.2 Walking surfaces. The walking surface material of decks, porches, balconies, and stairs shall be constructed with one of the following materials:
1. Ignition-resistant material that complies with the performance requirements of Section 503.2.
2. Exterior fire-retardant-treated wood.
3. Noncombustible material.
4. Where the deck, porch, balcony, or stairs are constructed of heavy timber in accordance with Section 501.6, natural wood decking products shall be:
4.1. 2-inch nominal dimension lumber; or
4.2. 5/4-inch nominal hardwood (i.e., teak, mahogany, or other approved hardwood).
5. Material that complies with the performance requirements of Section ((505.8.1.1))505.8.2.1 when tested in accordance with ASTM E2632 and when attached exterior wall covering is also composed of only noncombustible or ignition-resistant materials.
EXCEPTION: | Wall material shall be permitted to be of any material that otherwise complies with Section 501.5 when the decking surface material complies with the performance requirements of ASTM E84 with a Class B flame spread index. |
505.8.2.1 Material in Section ((505.8.1))505.8.2, Item 5. The walking surface material shall be tested in accordance with ASTM E2632 and shall comply with the following condition of acceptance. The ASTM E2632 test shall be conducted on a minimum of three test specimens and the peak heat release rate shall be less than or equal to 25 kW/ft2 (269 kW/m2). If any one of the three tests does not meet the conditions of acceptance, three additional tests shall be run. All the additional tests shall meet the condition of acceptance.
505.9 Exterior glazing. Exterior windows, window walls and glazed doors, windows within exterior doors, and skylights shall be tempered glass, multilayered glazed panels, glass block or have a fire protection rating of not less than 20 minutes.
505.10 Exterior doors. Exterior doors shall be approved noncombustible construction, solid core wood not less than 1 3/4 inches thick (45 mm), or have a fire protection rating of not less than 20 minutes. Windows within doors and glazed doors shall be in accordance with Section 505.8.
EXCEPTION: | Vehicle access doors. |
505.11 Vents. Attic ventilation openings, foundation or underfloor vents or other ventilation openings in vertical exterior walls and vents through roofs shall not exceed 144 square inches (0.0929 m2) each. Such vents shall be covered with noncombustible corrosion-resistant mesh with openings not to exceed 1/4 inch (6.4 mm) or shall be designed and approved to prevent flame or ember penetration into the structure.
505.11.1 Vent locations. Attic ventilation openings shall not be located in soffits, in eave overhangs, between rafters at eaves, or in other overhang areas. Gable end and dormer vents shall be located not less than 10 feet (3048 mm) from lot lines. Underfloor ventilation openings shall be located as close to grade as practical.
505.12 Detached accessory structures. Detached accessory structures located less than 50 feet (15,240 mm) from a building containing habitable space shall have exterior walls constructed with materials approved for not less than 1-hour fire-resistance-rated construction, heavy timber, log wall construction, or constructed with approved noncombustible materials or fire-retardant-treated wood on the exterior side. The fire-retardant-treated wood shall be labeled for exterior use and meet the requirements of Section 2303.2 of the International Building Code.
505.12.1 Underfloor areas. Where the detached accessory structure is located and constructed so that the structure or any portion thereof projects over a descending slope surface greater than 10 percent, the area below the structure shall have underfloor areas enclosed to within 6 inches (152 mm) of the ground, with exterior wall construction in accordance with Section 505.5 or underfloor protection in accordance with Section 505.6.
EXCEPTION: | The enclosure shall not be required where the underside of exposed floors and exposed structural columns, beams and supporting walls are protected as required for exterior 1-hour fire-resistance-rated construction or heavy-timber construction or fire-retardant-treated wood on the exterior side. The fire-retardant-treated wood shall be labeled for exterior use and meet the requirements of Section 2303.2 of the International Building Code. |
507 Replacement or repair of roof coverings.
507.1 General. The roof covering on buildings or structures in existence prior to the adoption of this code that are replaced or have 50 percent or more replaced in a 12-month period shall be replaced with a roof covering required by Section 501.4 or based on the type of ignition-resistant construction as determined by Section 501.1 Exception 1.