2021 IBC Chapters 14 and 15
Water is the most common enemy of a building, and Chapters 14 and 15 of the 2021 IBC are the code's answer to it. Chapter 14 covers exterior walls: the weather-resistive barrier, flashing, and the veneers that face the building. Chapter 15 covers the roof: drainage, flashing, fire classification, underlayment, coverings and reroofing. Stained ceilings, rotted sills, leaking window heads and roofs that blow off usually trace back to an item in these two chapters. The Gulf Coast adds wind and driving rain to the mix, so the details matter more here than in a drier climate.
Section 1402.2 sets the goal. Exterior walls provide a weather-resistant envelope. That envelope includes flashing under Section 1404.4, a water-resistive barrier behind the exterior veneer under Section 1403.2, and a means of draining water that gets into the assembly back out to the exterior. The section also requires protection against condensation. The idea is layered: the veneer sheds most water, the drainage path and barrier handle what gets past it, and flashing directs water out at every place where the wall is interrupted.
Section 1403.2 requires not fewer than one layer of water-resistive barrier material attached to the studs or sheathing, with flashing under Section 1404.4, forming a continuous barrier behind the veneer. Acceptable materials are No. 15 felt under ASTM D226 Type 1, a product meeting ASTM E2556 Type I or II, a barrier tested to ASTM E331 as described in Section 1402.2, or another approved material installed to the manufacturer's instructions. The inspector's eye goes to the laps and the terminations, which is where water finds its way in.
Section 1404.4 requires flashing that prevents moisture from entering the wall or redirects it to the surface or to the water-resistive barrier as part of a drainage means. It is required at the perimeters of exterior door and window assemblies, at penetrations and terminations of wall assemblies, at wall intersections with roofs, chimneys, porches, decks, balconies and similar projections, at built-in gutters, and at similar places. Copings get flanged flashing on both sides and ends, sills get flashing under them, and projecting trim gets continuous flashing above it. Self-adhered flashing at windows and doors follows AAMA 711 and fluid-applied flashing follows AAMA 714. Section 1404.4.1 warns against wall pockets or requires caps or drips to protect them.
For masonry, Section 1404.4.2 puts the flashing and weep holes of anchored veneer (under 1404.6) not more than 10 in. above the finished ground at the foundation. At other supports such as floors, shelf angles and lintels, the flashing goes in the first course of masonry above the support. Water that runs down the back of the veneer has to be caught and sent out through the weep holes.
Section 1404.6 says anchored masonry veneer complies with Sections 1404.6 through 1404.9 and with Sections 12.1 and 12.2 of TMS 402 (2016 edition). That means the details that inspectors most want to check, the spacing of the ties that hold the veneer to the backing and the size of the air space behind the veneer, live in TMS 402 Chapter 12 and not in the IBC chapter text. This lesson therefore describes them only in general terms: ties are required at a set spacing, the air space has to stay clear so water can drain, and both are checked against the approved design and the adopted code. In seismic design categories C through F, Section 1404.6.2 adds the requirements of TMS 402 Section 12.2.2.11.
Stone veneer under Section 1404.7 over wood stud backing calls for wire mesh, two layers of water-resistive barrier under Section 1403.2, and wire ties at the spacing in that section. Adhered masonry veneer, where the units are stuck to the backing rather than tied, is covered by Section 1404.10.
Section 1502.1 says roof drainage complies with Section 1502, with Section 1611 (rain loads), and with Chapter 11 of the International Plumbing Code. Where roof perimeter construction would trap water, such as a low-slope roof behind parapets, Section 1502.2 requires secondary (emergency overflow) drains or scuppers. Scuppers may not be smaller than 4 in. in any opening dimension and must be sized to keep ponding within the design depth of Section 1611.1 (Section 1502.3). Section 1503.2 and its locations list in 1503.2.1 require flashing at wall and roof intersections, gutters, changes in roof slope or direction, roof openings, penetrations and parapet intersections.
Section 1505 classifies roof coverings by fire test, ASTM E108 or UL 790, as Class A, B or C. Table 1505.1 sets the minimum class by construction type:
| Minimum roof covering class | Construction types |
|---|---|
| Class B | IA, IB, IIA, IIIA, IV, VA |
| Class C | IIB, IIIB, VB |
Footnotes matter. Non-classified coverings are permitted on Group R-3 and Group U buildings with at least 6 ft of fire separation from the leading edge of the roof (footnote b). Footnote c allows No. 1 cedar or redwood shakes on buildings not over two stories, not over 6,000 sq ft of projected roof area, with 10 ft to lot lines on all sides. Footnote a subjects a roof to the wildland-urban interface code or a fire district where they apply. Section 1505.2 describes Class A roofs as effective against severe fire exposure, and brick, masonry and exposed concrete deck qualify.
Section 1507.1.1 requires underlayment for shingles, tile, metal panels and similar coverings to conform to the chapter standards, labeled to ASTM D226, D1970, D4869 or D6757, applied under Table 1507.1.1(2) and attached under Table 1507.1.1(3). The wind column in Table 1507.1.1(1) splits at a design wind speed of 140 mph. For asphalt shingles, below 140 mph the underlayment may be D226 Type I or II, D4869 Type I through IV, or D6757. At 140 mph and above it must be D226 Type II, D4869 Type IV or D6757. Coastal sites can sit at or above that threshold, so check the design wind speed on the plans before accepting a material. Ice barriers are covered in Section 1507.1.2.
Slope decides which roof covering can be used at all. Section 1507.2.2 allows asphalt shingles only on slopes of 2:12 (17 percent) or greater, and from 2:12 up to 4:12 double underlayment is required. Low-slope systems have their own minimums: built-up roofs need at least 1/4:12 (1/8:12 for coal-tar built-up), and modified bitumen, single-ply and liquid-applied systems need at least 1/4:12.
A worked example on slope. A carport roof rises 2 ft over a 10 ft horizontal run. Slope in twelfths is rise divided by run times 12, or 2 divided by 10 times 12, which is 2.4:12. That is above the 2:12 minimum for asphalt shingles but below 4:12, so double underlayment is required. If the same roof rose only 1 ft over 10 ft, the slope would be 1.2:12, too flat for asphalt shingles, and a low-slope membrane would be needed instead.
The number changed. In the 2021 IBC reroofing is Section 1512, and Section 1511 is rooftop structures, so older references to 1511 for reroofing are out of date. Section 1512.1 says recovering or replacing a roof follows Chapter 15, with exceptions that exempt an existing low-slope roof from the 1/4:12 minimum where positive drainage exists, and exempt it from secondary drains where positive drainage exists (existing required secondary drains may not be removed unless they are replaced). Section 1512.2 defines roof replacement as removal of all existing layers of the roof assembly down to the deck, although an adhered ice barrier may stay and be covered by another. A roof recover, new covering over the existing one, is permitted under 1512.2.1 where installed per the manufacturer, for example a complete separate system such as standing seam metal, or a coating over an existing coating or compatible roof without tear-off. Section 1512.2.1.1 says a recover is not permitted where the existing roof is water soaked or deteriorated, is slate, clay, cement or asbestos-cement tile, or already has two or more applications of any type of roof covering. That means a roof with one existing layer may be recovered, and a roof with two may not. Flashings are reconstructed per the manufacturer (1512.5).
In the field
A community hall GCHS was asked to re-roof already had two layers of asphalt shingles, and the contractor's bid offered to "just put a third layer over it" to save tear-off cost. Under Section 1512.2.1.1 a recover is not allowed when there are already two or more applications, so the roof must be replaced down to the deck. Once the old layers came off, the deck showed soft spots at a valley, and the wall flashing had to be rebuilt under Section 1512.5. Check the count of existing layers at the eave or a test cut before the bid is accepted, and confirm that the plans show the wind speed used for the underlayment and the flashing at every wall-roof joint.
Chapter 15 also carries wind performance requirements for roof coverings (Section 1504) and Lesson 12 covered the wind speed and windborne debris region. The 140 mph split in the underlayment table follows the same idea: the higher the design wind speed, the tougher the material. On a Gulf Coast roof, find the design wind speed first, then match the underlayment, fastening and flashing to it.
1. Under Section 1403.2, the minimum number of layers of water-resistive barrier behind exterior veneer is:
2. Which of these is an acceptable water-resistive barrier under Section 1403.2?
3. An inspector measures the flashing and weep holes of a brick veneer on a Group B office at 16 in. above the finished ground at the foundation. Under Section 1404.4.2, this is:
4. Section 1404.4 requires flashing at:
5. Where do the tie spacing and air space requirements for anchored masonry veneer come from?
6. Under Section 1404.7, stone veneer over wood stud backing calls for how many layers of water-resistive barrier?
7. An inspector reviews a Group B office of Type IIB construction. The minimum roof covering classification in Table 1505.1 is:
8. Under footnote b of Table 1505.1, a Group R-3 or Group U building with at least 6 ft of fire separation from the leading edge of the roof may use:
9. Under Section 1507.2.2, asphalt shingles may be used only on roof slopes of at least:
10. A shed roof rises 3 ft over a 15 ft horizontal run and is to receive asphalt shingles. The slope and the underlayment requirement are:
11. Table 1507.1.1(1) divides its underlayment requirements at a design wind speed of:
12. A Group M store has two layers of asphalt shingles, and the contractor proposes a third layer over them. Under Section 1512, this is:
13. In the 2021 IBC, reroofing is located in Section:
14. Under Section 1502.3, a roof scupper may not be smaller than what dimension in any opening?