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Built-Up / Flat Roofs

A “built-up roof” is often referred to as a “Flat Roof”.

It is generally a series of layers successively laminated together with asphalt. The asphalt is heated to a liquid form and then mopped onto a base sheet. More layers of base sheet are laid into more layers of hot asphalt until the desired number of layers is achieved.

Built-up roofing is the most common type of roof on structures with low slope. It is normally coated with white, tan or aluminum coating, although up until the early 80’s built-up roofs were installed with a gravel covering. The gravel, like its modern counterparts (coatings) was intended to protect the asphalt from the sun.

Common Problems With Flat Roofs

Built-up roofing is a very durable material and in fact is considered commercial roofing in most areas of the country. Considering the fact that it sits in the Arizona sun, a well installed and well maintained built-up roof might last 25 or even 30 years. However, the average life expectancy falls at around 20 years because of common problems in installation and maintenance.

Ponding Conditions

A built-up roof conforms to the structure being roofed. Although I can’t conceive of someone intentionally building a structure that does not drain well, the fact is, Tucson is full of roofs with some level of ponding water.

Sometimes the ponding is the result of scupper drains being installed too high in the wall or the framework of the structure simply creates an uphill slope to the drains. Some older structures were simply built to be “Dead Level”. Settling of the structure then created low spots.

Regardless of the reason, ponding water on a roof can cut into the life expectancy of a roof and add significantly to the maintenance costs. Roof coating is applied over the roof to reflect harmful rays away from the roof. Ponding water can degrade the coating in the area prematurely leaving the area vulnerable to sunlight. Once sunlight is allowed to do its damage, the resulting leak can cause significant interior damage as the opening is being fed by the reservoir of water in the pond.

Solutions to ponding will vary to match conditions on the roof. If the problem is that the drains are too high on the wall, the solution is simply lowering the drain. Limited areas can be re-sloped and in some cases supplemental deck drains can be retrofitted to correct poor drainage. In very severe cases, it may be in the owner’s best interest to hold out until the roof needs to be replaced and re-slope the entire structure at that point.

Blistered Coating

Blisters in coatings had its beginnings in this valley in 1997. It is largely the result of a federal requirement for manufacturers to phase in VOC standards. There is a painfully large amount of information on the web if you Google “VOC standards”. For our purposes, the 1997 phase-in affected materials used in the manufacture of most, if not all, exterior coatings.

Unlike most other coatings on the market, roof coatings need a measure of permeability. Solvents in the asphalt of a roof must and will vent or “off-gas” over time. If the roof coating traps this vapor, the result will be an accumulation of these vapors between the roof and the first layer of coating applied to the roof where the bond is weakest.

The new formula of coatings reacted badly when applied over the older formulas in previous coatings. This resulted in lowering the permeability of the coating overall which had the effect of creating the blisters that many of you are dealing with today. Indeed, some manufacturers are still struggling today with a formula that both meets the standard and will avoid blistering, even when applied over newer versions of coatings.

There are no good solutions to blistered coatings. The one thing everyone must take away from this lesson is if you have a roof that has a tendency to blister, each subsequent re-coating will likely prompt more blisters regardless of the coating used. The truth is that the permeability of the overall matrix goes down as the mill thickness of the coating overall goes up. It is also true that re-coatings are necessary to achieve the maximum life expectancy of your roof. As a result we generally recommend re-coating a roof that blisters less often. The idea is to allow the older coating to wear as thin as possible, therefore increasing permeability, before coating it again.

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