The difference between the inspection of steep-slope roofs and low-slope roofs is mainly one of complexity.
Steep-Slope: Typical Construction
Steep-slope construction is generally as follows, starting with the roof support structure:
- conventional rafters or trusses;
- plywood or oriented strand board (OSB) roof deck;
- underlayment of asphalt-saturated felt or a synthetic material;
a. a mechanically-fastened (but sometimes adhered, as with tile) roof-covering material that acts as the primary weather barrier;
b. asphalt shingle or roll roofing;
c. clay or concrete tile;
d. metal panel or shingle;
e. wood shake or shingle; and
f. slate (stone or composite).
- Flashing is typically sheet metal bent to a purpose/roofing-based profile.
- Steep-slope roofs typically drain over the edge, often into gutters with downspouts.
Low-Slope Roofing Systems
The greater relative complexity of low-slope roof systems compared to steep-slope systems results in a substantial increase in the number of opportunities for mistakes in roof design, material transport, storage, jobsite handling, installation, maintenance, repair, and roof replacement. The components included in low-slope roofing are generally more sensitive to variations in moisture and temperature levels than those in steep-slope roofing, both during and after installation. This is mainly because water lingers on low-slope roofs longer, giving it more time to find a way into the roof system or to negatively affect the roof structure because of water weight. Contrarily, steep-slope roofs that shed water quickly are less likely to leak or become damaged from water weight.
Additionally, roof system component compatibility is sometimes an issue. Compatibility issues may only appear under certain circumstances, such as in the presence of moisture or material defects. In some cases, incompatible materials may be used anyway, but separated by a layer of material that prevents direct contact between the two.
Low-Slope Assembly: Typical Construction
Low-slope assemblies are generally constructed as follows, starting with the roof support structure:
- In addition to those materials listed for residential roofs, commercial roof support structures may include metal trusses or concrete posts/columns;
- the roof deck;
- a base, separator or slip sheet that separates the deck from the insulation;
- thermal insulation (typically rigid board insulation);
- a cover board, when the membrane is chemically incompatible with the insulation, or when insulation alone will not adequately support the loads to which it will be exposed, including roof-mounted equipment, workspace, or extensive foot traffic.
- In addition to those mentioned for residential roofs, commercial roof membranes (which may or may not be the weather surface) include:
b. modified bitumen;
- thermoplastic (TPO/PVC); and
- thermoset (EPDM)
Each membrane type has application requirements that are typically far more stringent than commonly used residential roof-covering materials, including temperature windows and restrictions related to moisture.
Note: In some installations, the position of the membrane and insulation are reversed.
- coatings or surfaces:
a. flood coats;
b. aggregates; or
c. liquid-applied (brush or roller).
2. Flashing may be a combination of metal (typically for terminations, but more commonly consisting of a material closely related to the field membrane.
3. Drainage may be over the edge, through scuppers in a parapet wall, or through internal drains (which may include sumps).
4. Other accessories:
a. cant strips;
b. equipment curbs;
c. penetrations components.
Note: Some commercial buildings, such as condominiums and apartments buildings, may have a combination of steep-slope and low-slope roofing systems.