2021 IRC Chapters 9 & 10
This lesson covers what keeps water out of a finished roof, what governs putting a new roof over an old one, and the masonry fireplace and chimney provisions a GCHS inspection often checks on the same visit as the roof above them. The numbers below come from 2021 IRC Sections R905, R908, R702.7, R1001, and R1003, checked against the code text. Always confirm the edition and any local amendment a given city has adopted.
Section R905.2.2 sets a 2:12 minimum roof slope for asphalt shingles. Between 2:12 and 4:12, the code requires doubled underlayment, a 19-inch starting strip followed by 36-inch-wide sheets overlapped 19 inches so coverage under the shingles is effectively two layers deep (Table R905.1.1(2)), because water drains slowly enough on a low slope to be driven back under a single layer's laps. At 4:12 and steeper, underlayment is applied shingle fashion with a minimum 2-inch lap, except that where wind design is required by Figure R301.2.1.1 the table calls for the two-layer method on any slope. Fastening matters as much as slope: Section R905.2.6 sets four nails as the minimum for a standard strip shingle, with manufacturer instructions often calling for six in high-wind areas, which govern where stricter. Two- and three-nail installations are a routine and preventable cause of wind-blown shingle claims.
In areas with a history of ice forming along the eaves and backing water up under the covering, as designated in the local climatic design criteria table, Section R905.1.2 requires an ice barrier, either two layers of cemented underlayment or a self-adhering polymer-modified bitumen sheet, extending from the lowest roof edge to a point at least 24 inches inside the exterior wall line, and on roofs 8:12 or steeper also at least 36 inches up the slope from the eave edge. That reach matters because an ice dam can back water up well past the eave; a barrier stopping at the fascia does not protect the point where the dam forms. Whether it applies is a jurisdictional call in the adopted design criteria table, not contractor preference, and once required, the section lists no substitute for the membrane (detached accessory structures without conditioned floor area are excepted).
Drip edge at the eave must extend below the sheathing and back onto the deck, under Section R905.2.8.5 not less than 1/4 inch below the sheathing (so water clears the fascia) and not less than 2 inches back onto the deck, fastened at not more than 12 inches on center, with segments overlapped at least 2 inches. At a roof-to-wall intersection, step flashing, individual pieces woven in with each shingle course so each laps the one below and is covered by the next, sheds water onto the shingle beneath rather than behind it; sealant along that joint is maintenance, never a substitute. An open valley works the same way at larger scale: a lining goes down before the shingles (metal at least 24 inches wide, or two plies of mineral-surfaced roll roofing, per R905.2.8.2), with the shingles cut back from the centerline so the lining does the actual waterproofing. A skylight needs the same layered approach, a properly flashed curb or the manufacturer's integral kit, woven into the courses in sequence.
Section R908.3 makes removal of the existing coverings down to the deck the default, and R908.3.1.1 bars a recover where the existing roof has two or more applications of any type of covering. A roof already carrying two layers cannot receive a third; the accumulated weight, the lost nailing base, and the impossibility of inspecting or fastening into a deck buried under two old layers all require complete removal first. Even short of that limit, a recover is never permitted where the existing covering is water-soaked or deteriorated past serving as an adequate base, or consists of slate, or of clay, cement, or asbestos-cement tile, materials a new asphalt layer cannot fasten into or bed against. A tear-off is also the one point in a reroof when the deck is actually visible, so verifying sound, properly fastened sheathing, and replacing anything deteriorated, belongs at that stage; it cannot be checked once the new covering is on.
R-value measures a material's resistance to conductive heat flow at a given thickness; higher is better, and R-values of layers in series add together for an assembly total. Ceiling insulation levels come from the energy provisions by climate zone. Vapor retarders in frame walls are governed separately, by Section R702.7, which calls for the retarder on the interior side of the wall; for the ceiling, Section R806.2 asks for a Class I or II retarder on the warm-in-winter side only in Climate Zones 6, 7, and 8 when the reduced attic vent ratio is used. Moisture in conditioned indoor air moves toward a colder surface, so in a cold climate a vapor retarder belongs on the warm-in-winter side, the room side, below the insulation, not above it; a retarder on the attic side instead puts a condensing surface right where the moisture arrives. Vapor retarders are classified by permeance into Class I (0.1 perm or less), Class II (over 0.1 up to 1.0 perm), and Class III (over 1.0 up to 10 perms). Table R702.7(2) sets which classes are permitted on the interior side of a frame wall by climate zone, and no wall vapor retarder is required in Climate Zones 1, 2, and 3. R-value markers or a posted insulation certificate let installed thickness and R-value be verified after the fact, since blown insulation settles and is easy to under-install in hard-to-reach corners.
In the field
A homeowner who builds a plywood storage platform on ceiling joists, over existing blown insulation, creates two problems worth flagging on the same visit: the insulation under the platform is compressed and no longer performs at its rated R-value, and the joists were very likely sized for an uninhabitable attic with no storage load, not whatever gets stacked on the platform later. Both are common in older GCHS rehab houses where a previous owner added attic storage without consulting a span table.
Section R1001.10 sets hearth extension dimensions by opening size: under 6 square feet, the extension reaches at least 16 inches in front and 8 inches beyond each side; at 6 square feet or larger, those figures grow to 20 and 12 inches, since a larger opening throws sparks and embers farther. The hearth itself, under the fire, must be at least 4 inches thick (R1001.9.1); the extension in front, protecting the floor rather than supporting the fire, can be thinner, at least 2 inches (R1001.9.2). Reversing those two figures is a frequent mistake. The firebox needs a minimum depth of 20 inches (R1001.6), keeping the fire back enough that heat and sparks stay contained and giving the smoke chamber above the geometry to draft correctly. Combustible forms used to build the hearth extension must be removed once the masonry cures; leaving them in place puts combustible material directly beneath the surface meant to protect the floor from embers.
Section R1001.11 sets clearance from the fireplace to combustible framing at not less than 2 inches from the front and sides and 4 inches from the back face. Trim and mantels are stricter: nothing combustible within 6 inches of the opening, and within 12 inches, combustible material may project no more than 1/8 inch for each inch of distance from the opening. A deep mantel mounted close above an opening is a common violation and a real fire risk, since sustained heat exposure can char wood over years before it ignites.
A masonry chimney's walls must be solid masonry units, or hollow units grouted solid, at not less than 4 inches nominal thickness, plus an approved flue lining, clay tile, a listed liner, or other approved material, since the lining contains the hot gases and acidic condensate that would otherwise attack the mortar joints over time. Combustible framing must clear the exterior surface of the chimney by at least 2 inches where the chimney is inside the building, reduced to 1 inch where it sits entirely outside the exterior walls and sheds its heat outdoors rather than into surrounding framing. The footing must be at least 12 inches thick and extend at least 6 inches beyond the face of the foundation or support wall on every side (R1003.2), since a tall, narrow chimney has a genuine overturning tendency that a wider, thicker footing resists. Where a chimney's width, measured perpendicular to the roof slope, exceeds 30 inches, a cricket or saddle is required on the ridge side (R903.2.2), since the flat catchment area behind a wide chimney otherwise collects enough water, snow, and debris to defeat ordinary flashing.
Termination height follows the 3-2-10 rule: the chimney must extend at least 3 feet above the highest point where it passes through the roof, and at least 2 feet higher than any part of the building, including the ridge, within 10 feet horizontally, whichever produces the taller result. Where a spark arrestor is installed, its mesh openings must be sized between 3/8 inch and 1/2 inch: smaller openings clog with creosote and restrict the flue, while larger openings let burning embers through. A factory-built, listed metal fireplace and chimney is a different animal from masonry entirely; it must be installed exactly to the terms of its listing and the manufacturer's instructions, and mixing masonry clearance rules with a listed assembly invalidates both.
A masonry chimney passes through a roof plane 6 feet from the ridge, the tallest part of the building within 10 feet. Under the 3-2-10 rule the chimney needs at least 3 feet above its own roof penetration point, and separately, because the ridge sits within that 10-foot radius, at least 2 feet of clearance above the ridge itself. If the ridge is 4 feet higher than the penetration point, a chimney only 3 feet above that point would sit 1 foot short of the 2-foot ridge clearance, so the ridge clearance governs here, not the flat 3-foot figure: the chimney needs at least 6 feet above its penetration point (4 feet to reach the ridge height, plus 2 more) to satisfy both parts of the rule at once.
1. Under Section R905.2.2, asphalt shingles may be installed on a roof slope no lower than:
2. In an area with a history of ice damming, Section R905.1.2 requires the ice barrier to extend from the eave edge to a point at least:
3. An existing roof already carries two layers of asphalt shingles, and a third is proposed. Under Sections R908.3 and R908.3.1.1, the inspector should require:
4. A recover over an existing roof covering is not permitted, regardless of how many layers already exist, when the existing covering:
5. The minimum number of fasteners for a standard strip asphalt shingle under Section R905.2.6 is:
6. Which statement correctly describes R-value?
7. Under Section R702.7, where a vapor retarder is used on a cold-climate exterior frame wall, it belongs:
8. The hearth extension for a masonry fireplace with an opening of 6 square feet or larger must extend, at minimum:
9. Minimum thickness for a masonry fireplace hearth and its hearth extension are, respectively:
10. The walls of a masonry chimney must be constructed of solid masonry units (or hollow units grouted solid) not less than:
11. A masonry chimney footing must be at least 12 inches thick and extend beyond the face of the foundation or support wall on all sides by at least:
12. A chimney passes through the roof 4 feet from the ridge, and the ridge is the tallest part of the building within 10 feet. If the ridge sits 5 feet higher than the roof penetration point, the chimney's termination must reach at least:
13. Where a spark arrestor is installed on a chimney, its mesh openings must be sized:
14. A chimney passing through a roof measures 36 inches wide, perpendicular to the roof slope. What does the code require on the ridge side of that chimney?