Roof System Selection - Design Parameters

by Steve Nelson, RRC

In the April 1995 issue, we focused on how future maintenance requirements can influence roof system selection. Our discussion continues with consideration of roof traffic as a project design parameter.

Anticipating Roof Traffic
In the performance of their primary duty to shed water, roofs must not only withstand the elements of nature, but also the use and abuse of the human element in the form of rooftop traffic. The degree to which a roof is affected by traffic varies with the levels of intensity and concentration of the traffic. Some buildings have minimal service traffic and perhaps an occasional maintenance inspection. Conversely, in heavy industrial settings, the roof may be considered another floor of operation and may be used for general pedestrian circulation and material storage.

Generally, the most common areas of concentration are at points of access such as doorways, hatches and ladders. Traffic can be expected to be relatively heavy where directed through restricted passageways defined by the layout of the roof. In addition, the areas surrounding frequent destinations such as mechanical equipment or the building perimeter in the case of window washers, are often staging sites for service activity.

Avoiding Potential Damage
By recognizing the nature of roof traffic and identifying areas of concentration, one can properly assess the general durability needs of a roof during the system selection process. Minimizing the impact of traffic will allow the roof to perform to its full potential.

Common considerations are:
Light but frequent traffic channeled to specific locations can result in erosion of surface coatings, granules and stone ballast. Eventually, abrasion of the membrane, punctures and other physical damage can occur. Walkpads, pavers or additional layers of membrane can be specified to protect these defined pathways.

If traffic is heavy but undirected across an open roof, general durability of the roofing membrane becomes important. In this case, built-up or modified bitumen roof systems may be preferred to single-ply systems for all around wearability and puncture resistance. In extreme cases, such as patio decks, heliports, heavy equipment runways, etc., an insulated roof membrane assembly (IRMA) may be the only choice durable enough for the long-term. In an IRMA system, the membrane can be protected from traffic impact by insulation board and pavers or a concrete topping slab.

Compression damage can result from heavy traffic in concentrated areas. Flexible roof membranes, primarily single-plies, are unable to disperse weight when concentrated in a small area. Consequently, the load is carried by the underlying substrate material, usually the roof insulation board. If the compressive strength of the board is inadequate, it may be crushed. As a result, the board becomes thinner, causing fasteners to protrude into the membrane.

The top facer skin may also separate from crushed insulation board. Should this occur on a fully adhered single-ply roof, the attachment of the membrane is seriously compromised, leaving the roof susceptible to wind damage. Sub-strate materials with high compressive strength, such as foam glass, or rigid walkway protection such as concrete pavers, should be considered when heavy traffic is anticipated. Flexible walkpad material may not be enough to disperse traffic loads.

Flashings can become kick plates below access door thresholds, at the base of ladders and hatches, and around equipment curbs. Use of protective metal counterflashing with a long skirt to cover the flashing membrane will help prevent physical damage. Use of wood cants in lieu of fiber cants will provide more rigid support for built-up flashings.

Flexible, reinforced single-ply membranes usually fare better than rigid or non-reinforced materials. On multi-story buildings requiring safety lines for such activities as window washing, sheet metal or other abrasion resistant material should be specified where line tie-offs occur to prevent "rope burns" through penetration flashings.

Collateral damage to roof components that act as obstacles to traffic can be avoided by accommodating traffic flow over or around obstacles. One way to do this is to provide elevated ramps to carry walkway traffic up and over raised expansion joints and major pipelines. Fixed ladder access at roof level changes can also be specified to avoid having pipes and mechanical equipment used as climbing apparatus.

When selecting roofing materials and system components, carefully consider how the roof will be used in the course of daily operations. When traffic is moderate and directed, use of protective walkway materials may be sufficient. However, when traffic is heavier or undirected, the overall durability and puncture resistance of Punctures in base flashing created by ropes the roof system becomes critical to from window washing equipment. successful performance.