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ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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Roofing — steep-slope and low-slope

Overview: Roofing assemblies consist of two parts:

There are two main categories of roofing:

Steep-slope roofing

roof shingles
Typical geometry of roofing shingles, adapted from Edward Allen, How Buildings Work.

roof shingles
Typical geometry of roofing shingles, showing "toplap" and exposure.

roof shingles
Flashing at steep-sloped "valley."

The most common roofing element for steep-slope roofing is the shingle which can be fabricated from many materials, as long as they shed water: wood, asphalt, fiberglass, clay, concrete, and metal are all used. The basic principle is a staggering of the horizontal "courses," and an overlapping of the shingles along the slope. As seen in the image below, the actual shingle length must be somewhat longer than twice the exposure in order to maintain the watershedding capability:

section through shingle roof showing substrate, underlayment, and shingles at a slope of x : 12 (rise : run)

The slope is indicated by the ratio in inches of "rise" to "run;" i.e., x : 12 where x varies from about 4 up to 12 or more (where 12 : 12 is a 45-degree angle). Below about 4 : 12, the roof looses its ability to shed water reliably, and a more rigorous and water-tight underlayment system must be used.

The roof pitch, not commonly used, is the ratio of "rise" to twice the "run," i.e., the pitch is one-half of the slope. A slope of 6:12 is equivalent to a pitch of 6:24 or 3:12.


The underlayment, or roofing felt, provides temporary waterproofing of the exposed substrate during construction as well as a second line of defense after completion. Additionally, it can act as an air barrier, and as a cushion, especially when used with brittle shingles (clay tile, concrete).

Gutters, downspouts, and scuppers

roof shingles
Parts of a gutter (from Architectural Graphic Standards).

scupper :scupper
Scuppers at (left) the Basilica of the Sacred Heart of Jesus of Paris, commonly known as Sacré Coeur Basilica; and (right) in a Red Hook, Brooklyn industrial building (photos by J. Ochshorn, 2010 and 2007 respectively).

Ronchamp, Le Corbusier, scupper
Scupper at Ronchamp (Notre Dame du Haut, Le Corbusier; photo by Tim Brown Architecture)

scupper detail
Boxed scupper detail (A href="">image source.

Flashing and drip edges

drip edge at roof eave
Metal drip edge at roof eave

Where two materials intersect, or where the roofing terminates, flashing is used as a transitional device to maintain the continuity of the water protection. Two applications of flashing are:

Metal roofing

Metal roofing is detailed so that it can readily adjust to thermal stresses that might otherwise cause separation at joints. Two types of seams are shown below:

standing seam and flat seam in metal roofing

photo of metal roofing

The metal roof shown above is from Indiana Steel Roofing.

Galvanic corrosion

showing galvanic corrosion

When two dissimilar metals are brought into contact with an electrolyte, electric current flows. This is the principle of a battery, and also is the often unintended consequence of using different metals that are exposed to water (which acts as an electrolyte). Where galvanic action is present, one of the metals will corrode (the "anode") as the other gains ions (the "cathode"). The so-called galvanic potential between metals is tabulated in many sources, including Architectural Graphic Standards. Some selected metal combinationss and their galvanic potential (high = avoid direct contact; possible = direct contact is not advisable; and low = in most cases, no problems arise from the metals in contact) are shown below:

Avoid mixing metals within flashing systems; do not use nail or screw materials that differ from the metals they are fastened through: in other words, use aluminum nails with aluminum flashing; copper nails with copper flashing; etc.

Where two metals must be brought into contact in an exterior application (where water is likely to be present), a coating should be applied between the points of contact.

Common steep-slope roof forms

The terms and geometries shown below are commonly encountered in steep-slope roof construction:

common roof forms and vocabulary

common roof forms and vocabulary

The "soffit" and "fascia" (the underside of a projecting roof is the soffit; the vertical surface where the roof projects beyond the wall surface is the fascia) are shown in the details above. Note that these two terms are also used in other contexts. Also note that a parapet (not shown) is a vertical extension of an exterior wall up beyond the plane of the roof, originally mandated for fire protection, but also useful as a guard rail.

Ice dams

Ice dams occur whenever melting snow on a roof (over a heated or poorly insulated space) freezes upon reaching a roof overhang (over an unheated space). The result can be merely dangerous (icicles posing a hazard to people below) or also damaging to the building itself, in cases the melted water is prevented from draining from the roof by the dam of frozen water (ice) and works its way under shingles and into the building itself.

Ice dam formation
Melting snow freezes over eave, forming ice dam (left); eave and ridge vents, along with insulation, keep attic cold and prevent ice dams (right)

Ice dam formation in Ithaca, NY
Ice dams in Ithaca, NY: typical Collegetown house (left) and Cornell's Foundry Building (right) — photos by J. Ochshorn.

Ice dam formation in Ithaca, NY — Schwartz Center for the Performing Arts, Cornell
Ice dams in Ithaca, NY: Schwartz Center for the Performing Arts, Cornell (left) with interior water damage (right) — photos by J. Ochshorn.

Ice dam formation in Ithaca, NY — Squash Courts
Ice dams in Ithaca, NY: Grumman Squash Courts at Cornell — photos by J. Ochshorn.

unvented assemblies with closed-cell insulation
The 2006 IRC allows unvented roof assemblies if two conditions are satisfied: (1) no interior vaopr barriers are installed on the ceiling side — attic floor — of the assembly; and (2) air-impermeable insulation is applied in direct contact with the structural roof deck. Image shows closed-cell polyurethane foam sprayed into attic rafters; in this case, the attic becomes part of the heated/cooled interior space of the house.

Low-slope roofing

common roof forms and vocabulary
common roof forms and vocabulary
Schematic representation of low-slope roofing (top) and my own analysis (bottom), from Edward Allen, How Buildings Work, 2nd edition

Low-slope roofing may look flat, but except in rare circumstances, it has a minimal slope to eliminate rain water, either over the side (perhaps through a scupper), or internally to a roof drain. The minimum slope, often specified in building codes, is typically 1/4" rise per foot of run, or more. Thus, the roof shown below (assuming a 1/4" slope) has a change of elevation from its high point along the edge to its low point at the two roof drains (marked "R.D.") of 30 x 1/4 = 7.5 inches.

geometry of typical roof drains

typical internal roof drain
Typical internal roof drain on steel deck (left) or concrete slab (right), from Architectural Graphic Standards.


Ponding is a phenomenon associated with low-slope roofs with inadequate slope and/or excessive deflection. Where water can pool (or "pond") in a depressed area of the roof, the added dead load of this water causes additional deflection; in a rain storm, this additional deflection soon fills with more water, causing more deflection, and so on, possibly leading to structural collapse. To prevent ponding, design low-slope roofs with adequate slope, and prevent excessive deflection of structural supports.

Back-up drains

Most building codes require back-up drains on low-slope roofs, so that if the primary drains become clogged or unusable, water will not build up.

ponding and back-up drains
Mechanism causing "ponding" (top); typical backup or overflow drain (bottom).

Substrates for membranes

The basic condition for a low-slope roof membrane is a surface to which it can be applied. This surface (substrate) is typically either wood (plywood, OSB), concrete, or rigid insulation:

different substrates for low-slope roofs

In the inverted membrane roof (protected membrane roof) shown above, the insulation must be able to be in contact with water; the membrane and the insulation are typically loose-laid (unattached) and held in place with gravel or concrete pavers acting as ballast (weight). This is the only roof type that corresponds to the "rules" in Ronald Brand's Architectural Details for Insulated Buildings (see notes from week 11a).

Membrane roof types

For low-slope membrane-type roofs, there are two main categories:

Edited (abridged) version of the original "Performance at the Top" video produced by the Polyurethane Division, The Society of the Plastics Industry, Inc. (view on YouTube).

In steel-framed buildings especially, the single-ply membrane is often placed over rigid insulation, which is placed directly over the corrugated steel decking. The video shown in class promoted the use of polyisocyanurate insulation boards, but other insulation materials can also be used as a substrate.

Unlike traditional built-up roofing, many single-ply systems do not require cant strips at 90-degree transitions. A composite EPDM detail based on typical manufacturer's recommended base flashing and parapet details is shown below. See Firestone RubberGard for more information.

EPDM details at base flashing and parapet

EPDM fully-adhered system
Fully adhered system (EPDM)

Addition to J. Ochshorn house showing roofing Addition to J. Ochshorn house
Case study: J. Ochshorn house addition, Ithaca, NY showing roofing (left) and finished project (right).

Some images of my own EPDM roofing adventure are shown below. More details can be found here.

EPDS roof installation
Design-build addition: photos by J. Ochshorn

J. Ochshorn cutaway view of ventilation under EPDM roof J. Ochshorn deck over EPDM roof
Low-slope roof with ventilation prevents ice dams from forming at gutters or scuppers: (1) cool air enters at vent under gutter; (2) attic joist batt insulation is continuous; (3) vent space is created with ripped 2x lumber above insulation and below roof substrate; (4) vent space continues into parapet wall; (5) cool air exits from conventional gable vents on the outside of the parapet wall; and (6) a flat roof deck is "floated" on the sloping EPDM roof membrane using pressure treated lumber and wood-composite decking. (J. Ochshorn house addition, Ithaca, NY, 2004)

Liquid- or sprayed-applied roofing

There are several classes of roofing systems that are applied as liquids or sprayed directly onto a substrate. Typically, these systems can conform to irregular geometries, and often avoid the need for sealants, joints, and flashing that are points of weakness in single-ply or built-up roofing systems. Some examples are:

Green roofs

A variant on the membrane roof, most often accomplished with single-ply membranes, is the so-called "green" roof, in which various landscape products are grown over a series of layered elements, the bottom of which is a single-ply roof membrane, while others include grids to promote drainage, XPS rigid insulation, and a board to block root growth (and thereby protect the roofing membrane). See, for example, this information page from Hydrotech, whose system is shown below:

Hydrotech green roof system

In this system, the roofing membrane is a hot, fluid-applied rubberized asphalt with a minimum 25% recycled content.

In the following, more generic, image of a green roof, the author makes the point that drainage layers must be present both above and below the insulation layer; the insulation layer "can get saturated if it is not drained above and below."
Guidelines for green roofs
PMR = "protected membrane roof"; IRMA = "inverted roof membrane assembly." Image and caption from Lstiburek, "Seeing Red Over Green Roofs," ASHRAE Journal, June 2011, p. 71 (reproduced here).

Construction of Milstein Hall video series by J. Ochshorn (Part 10—Green Roof).

Update showing seemingly continuous repair of the Milstein Hall green roof membrane since its installation in 2010; this short video taken Nov. 1, 2017 by J. Ochshorn