Last week we touched on a residential roof inspection, which is generally a sloped roof 2:12 pitch or higher. Roofs on commercial buildings are generally low-slope, or "flat" roofs. These present some big differences in design and layout, and though some defects will be similar, there are some different kinds of failures to look for as well. Lets take a look!
Low-slope roofs are sometimes referred to as "flat roofs", but technically they arent usually flat. They are sloped anywhere from 1/4:12 up to 3:12. A "dead level roof" is actually flat but they are uncommon. There has to be some sloping designed into the roof to move water to the drainage systems, otherwise ponding(retention of water for more than 48 hours in conditions conducive to drying) may occur. Only a half-inch of water on a 2,000 square foot flat roof can weigh as much as a small car. This additional dead load can cause substantial structural issues within the roof assembly leading to leaks and degradation, or if left unchecked can lead to roof collapse.
Ponding on a "flat" roof assembly
There are 3 main components of a low-slope roof: the structural roof deck, thermal insulation, and the roof membrane. On most roofs the roof membrane is the top layer, though on a Protected Membrane Roof(PMR) the insulation is installed on top of the membrane. PMR roofs are considerably less common. Some other features found on a flat roof are flashings, coatings/surfacings, air retarders(barriers), vapor retarders(barriers).
The 5 main types of roof deck material are steel, cementitious wood fiber(sometimes referred to by its manufacturer name, Tectum), structural concrete(both normal-weight and lightweight), wood panel, and wood planks/boards. Some common defects involving a commercial roof deck are:
1.) Rust/Corrosion
2.) Insufficient space between roof deck and electrical components/fuel piping
3.) Mechanical fasteners piercing electrical components
4.) Lack of supplemental support for penetrations
5.) Water stains, rotting, paint deterioration
6.) Evidence of movement
Rust/Corrosion
A badly deteriorated roof deck
A natural gas distribution pipe attached directly to deck.
Low-slope roof thermal installation provides: strength, dimensional stability to limit thermal expansion/contraction, sound attachment points, thermal resistance, moisture resistance, fire resistance, and impact resistance. The 5 main types of thermal insulation are:
1.) Rigid board insulation(foam board, fibrous, cellular glass, mineral wool, perlite)
2.) Dual-Purpose structural deck and insulating planks
3.) Lightweight insulating concrete(LWIC)
4.) Sprayed polyurethane foam(SPF)
Some insulation may be tapered to provide proper sloping for drainage purposes!
Common defects of the thermal insulation include: moisture absorption, face delamination, blistering, shrinkage, cupping, and edge cavitation. Due to the insulation generally being located underneath the roof membrane a core sample or cut test(both invasive procedures performed by a specialist) these defects can be hard to observe.
A low-slope roof membrane can be mechanically attached, fully-adhered, loose-laid/ballasted, or liquid-applied(sprayed on). A built-up roof(BUR) is comprised of 3 to 5 layers of asphalt felt laminated with coal tar, pitch, or asphalt, and will be finished with a smooth surface or ballasted with crushed stone. BUR roofs comprise roughly 20% of the low-slope roof market. Some common defects are: blistering, splitting, ridging, wrinkling, water damage, and UV damage.
Blistering
Tenting at a parapet wall
Billowing of roof membrane
UV damage of a sprayed-on roof membrane
With a commercial roof being one of the most vast and expensive components to replace on a commercial building, you dont want to leave your investment up to chance. It is important to hire a commercial property inspector familiar with low-slope roofs and their common defects. The useful service life of a commercial roof is only 10-15 years and the damage caused to the building structure caused by a failed or damaged roof can be substantial. When hiring your next commercial property inspector, remember to #havemooktakealook !
Photos provided by CCPIAs website www.ccpia.org
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