With all the science that has taken place in the roofing industry over the past half-century, one wonders why so many mysteries and crimes remain to be solved.
For the building owner or manager, most roofing decisions revolve around money. Oftentimes, lowest first cost prevails. A lightweight roof system usually means lower costs in terms of footings, framing, and other structural components. Since the 1950s, steel columns and decking have dominated for most of the country while wood joists and plywood/OSB continue to be a factor on the West Coast.
With the deck decision made, next comes thermal insulation. With a massive, monolithic concrete deck, we could skip the insulation altogether. For wood structures, flexible glass fiber batts could be placed between wood joists on the underside of the decking. This is still viable when condensation isn’t a factor. Structural wood fiber decking has insulating and acoustical value, so no additional insulation may be needed. Lightweight insulating concrete provides some R-value, but needs a sub-deck for support. The main advantage of insulating concrete is the ability to slope the fill to the drains. And, of course, steel decking requires a leveling board to bridge the ribs (flutes) of the deck. So, Watson, Question No. 1 is: What kind of deck do we have?
Vapor retarders have been and continue to be a design factor in colder climates, but more attention today is paid to air barriers. Question No. 2: What are our internal humidity conditions relative to our climate, and do we need a retarder?
And, of course, decisions have to be made on the roof membrane.
Membrane Choices of the Good Ol’ Days
- Coal tar pitch
- Organic felt
- Asbestos felt
- Glass fiber felt
- Mineral surfaced cap sheets
- Smooth surfaced
- Glaze coat
- Emulsion coat, fibrated or not
- Cut-back, fibrated or not
- Flood coat and aggregate
- Asphalt treated wood fiber
- Paper-faced glass fiber board
- Foamed glass
Some of the changes today would, of course, include the dominance of cellular foam thermal insulations, the use of cover boards, use of polymer-modified bitumens, single-ply roof membranes, reflective coatings, photovoltaics, and vegetated roofs.
Things Have Changed, But What’s the Mystery, and Has a Crime Been Committed?
First – more than two-thirds of low-slope roofing activity pertains to reroofing, re-cover, or replacement. In this case, we need to know:
- What is the designed load capacity of the structure? Not a simple question. It may take a qualified structural engineer to assess what was designed and whether significant alterations have affected the load-bearing capacity. Building codes generally required a minimum of 20 psf for buildings in regions that did not get much snow load, 30 pounds further north, and perhaps 40 in special cases.
- If you were thinking of putting on a new ballasted EPDM roof, the nominal amount of ballast is 10 psf, and recently developed criteria to qualify as a “cool” roof requires 17 psf. Furthermore, wind design following ANSI/SPRI RP-4 calls for as much as 20 psf in corners and perimeter areas.
- You’re probably going to add a lot more insulation. Foam boards don’t weigh much, but they do insulate. Snowmelt may be a lot slower, and the melt-water weight could be significant.
- If we do add several inches of new insulation, do we have adequate height at flashings, windows, and doorsills?
- Will the services of a design professional be required to prepare and seal contract documents?
- How are you going to attach that new insulation? Will the installation meet local and national code, and wind-uplift requirements? If you’re hoping to leave the old roof systems in place, you must verify that any wet insulation in the old roof is removed. (Roof vents have long since been proven to be ineffective in drying out wet insulation.)
- How will you find the wet insulation? When the old insulation was wood fiber, perlite, or glass fiber, the R-value diminished significantly, which could be spotted with an infrared, neutron backscatter, or capacitance scans of the roof. With board-foams, the roof could be actively leaking, but the boards don’t hold enough water to allow the heat transfer to be easily seen. (Do you know what insulation is on the roof now?)
- If you’re going to mechanically fasten the new insulation through the old membrane and insulation, is the fastener the correct one for the roof deck? If the new insulation is several inches thick to achieve today’s energy requirements, are fasteners long enough, or even too long so that they engage the deck flutes (an FM no-no)? If you use the new insulation as a way to taper the roof, are there fasteners long enough for the highest thicknesses of the taper system?
- If you’re planning to use a mechanically fastened single-ply system, we have the same questions about the length of the fasteners. Also, if the fasteners do pass through some wet insulation in the old roof, are they corrosion resistant? Wet organic insulation fibers become acidic, and will chew up screws if they’re not well protected. There is an ASTM test (ASTM G87), called the Kesternich acid test, that evaluates this corrosion resistance. This is now an FM Global requirement for mechanical fasteners to meet FM Class 1.
- If the existing roof has performed well, why would we consider something else now?
- When overlaying an old roof, what is the durability of the existing insulation likely to be during the installation of the new roof? For instance: perlite and glass fiber were tried-and-true roof insulations for BURs in particular. However, when re-roofing over either, problems can occur since the roof is “temporarily” stressed by the re-roofing operations. Old perimeter fasteners can pop through and boards can compress down to the decking substrate and create low spots with ponding as a result.
- What process should be used to select an appropriate new roof membrane? Is the warranty relevant? Is the thickness of the membrane related to performance or durability? Would a cover board (e.g. gypsum board) enhance puncture (hail), fire, or wind performance? Are we picking a competent manufacturer? One who has done his research and development of his product well, and supplies products that actually meet stated ASTM and FM requirements? Will the new roof impact aesthetics? A brilliant white roof coating might reduce peak energy demand, but if it produces glare complaints, is that acceptable?
- Are there pending issues of system durability or manufacturer misrepresentation?
- Are we thinking out of the box? Would switching to a sprayed-in-place polyurethane foam, protected membrane roof (PMR), or metal roof system solve our problems?
- Is the preferred roof system compatible with the building’s operations? For example, is oil, fat, or grease, or organic acids (soap) exhausted onto the roof surface? Hot exhausts? Silt, vegetation growth, or food byproducts? Will installation of new self-drilling insulation fasteners drop particulates into the interior of the building? Would dropped particles interfere with the operation of the building or the production of on-site manufactured product?
- Is the roofing contractor(s): pre-qualified to bond his work; an installer pre-approved by the roofing system manufacturer; able to install the chosen system; able to understand the contract documents; capable of performing proper quality control of his work-product; able to produce a warranty at the end of the job; willing to work productively with an outside quality assurance observer; and willing to provide, for a fee, the periodic preventive maintenance after the installation is complete that is necessary to assure maximum roofing longevity?
- Is the selected system one in which repairs can be made by in-house personnel?
- Are we thinking of adding photovoltaics to the new roof? Many PV systems generate heat that may raise the membrane temperature to the point where it softens or rapidly degrades.
- If we put a new roof on that meets “cool roof” criteria, are there rebates or other incentives for us? Does that mean we need to periodically clean the roof surface? Does that mean we need to periodically recoat the roof?
- Whatever kind of roof we select, are we clear about what steps we need to take to insure the trafficked roof areas are protected adequately from compression damage for many years to come? Increased insulation thicknesses have more likelihood of being squashed. Insulations and membranes that are attached with fasteners are more susceptible to membranes being punctured when stepped on, rolled over, or abraded by dragging. Foam or bonding adhesive installed membranes can suffer from membrane loosening and facer delaminating if inadequately protected. The following are examples of sources of installations that may create issues with most roof systems:
- Any constant traffic access via a roof hatch or roof ladder (fixed or unfixed) and any areas of the roof utilized (even temporarily) as a work platform or area.
- Window-washing activities, equipment, and tie-offs to flashed curbs.
- HVAC contractor access and traffic areas, fasteners, Freon or oil spills, loose rooftop unit panels gashing the roof, pipes installed after the roof need to be flashed properly.
- Cell-phone company mountings, cable trays, conduits, electrical connections.
- Satellite antennae installations; ballasted units need to have protection placed below and concrete pavers or whatever is used to add ballast needs to be freeze/thaw resistant to prevent fragmented concrete causing puncture damage and loss of ballast weight occurring.
- Solar PV installations may have many of the same issues as a number of the above, including increased maintenance traffic, hidden seam areas, ballast methods that need to provide durability as well as wind security.
Doctor Watson, where are our roofing files? Do we have answers to any of these questions? When is the last time we had a moisture survey done? Do we have potentially complicating factors, such as asbestos under the deck that might become airborne during a tear-off? Will reroofing operations affect the occupants of the building? Do we have a 5-year plan and budget?
And, lastly, where is the phone number of that roof consultant?