What to Consider When Selecting a Roof System (Part 3)

You know you need a new roof, but where do you start? The most common method is to call three roofers to get independant proposals and then compare. But how do you know what to get proposals for? How do you pick a roof system that is best suited to your building? A step-by-step look at the various factors that affect the roof (and the needs of the building) is the best approach to answering those questions.

Yesterday we discussed the importance of the building's characteristics in regards to selecting the right roof system. Today we will review evaluation of the existing roof and how that should play in to your decision.

Evaluate the Existing Roof
Existing deck construction and whether the roof drains properly or not will influence the kinds of roofs that can be installed. Decks with a slope greater than 2:12 can use most membrane systems, as well as roof systems like shingle or tile designed to combine watershed and
aesthetics. Slopes lower than 2:12 must be a water barrier system because they do not drain as quickly.

The type of deck may be the deciding factor in how the roof is fastened. Roofs over steel, lightweight insulating concrete, wood, gypsum and cementitious wood fiber planks generally are best installed with fasteners, while those on structural concrete are best adhered.

The condition of the deck is also important to consider. If the deck is in poor condition, re-covering the existing membrane is not a good option as the installation may ignore life safety issues. Ballasted installations, vegetated roofs or heavy re-covers may overload the structural capacity of the roof.

Drainage requirements should also be identified. If water stays on the roof more than 48 hours, the roof has a ponding problem. Because most manufacturers do not warrant areas of ponded water, the new roof should incorporate a way to remove the water, such as new drains or tapered insulation. The number and size of overflow drains should be identified. If the elevation of the roof gets higher due to a re-cover or from adding more insulation, it may partially or completely block overflows or bring the roof assembly to a non-code-complying height. This will mean that the overflows may have to be moved, added or changed, which is an expensive process. Increasing the thickness of the roof also may cause through-wall flashings and door thresholds to be too low.

It's important to identify anticipated abuse. If the roof has equipment that requires regular maintenance, there will be foot traffic on the roof. However, abuse to the roof can happen from window washing equipment and personnel, smokers and vandalism. If abuse is possible, the resulting design should incorporate extra toughness in the form of extra thickness or extra plies.

Tomorrow we will review a checklist of questions for you to ask the roofing contractors you request bids from.

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