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Moisture Control
The Tale of Low-Slope Venting

by Lawrence P. Evensen, President, All Style Industries, LLC
March-April 2009

Once upon a time in Visalia, California, a repairman walking across a commercial flat roof suddenly broke though, falling 30' to the concrete floor. After checking limbs and body parts, he got to his feet and staggered out an exit. Limping to his truck, he told others, "I just fell the roof. It's falling apart, and if you're smart, you won't go up there."

This ten-year-old had a roof installed to modern codes and standards. Even so, it had dangerous defects that created an environment for the proliferation of simple groups of plant life. These agents of destruction (fungi and mold) discolor surfaces, lead to odor problems, and deteriorate building materials to the point that they become unsafe, as in the case of the Visalia building.

A roof is designed to keep water out. The idea that "safe and dry" exists when exterior water can't pass through a roof covering is not necessarily true since moisture problems are not caused by exterior leaks alone, but are also the result of water vapor inside a structure. Today's structures are more insulated, air tight, and energy efficient than ever before. The advantage of using less fuel is a double-edge sword since air-tight structures have a higher concentration of moisture vapor-laden air, and that air is trapped inside the building.

Therefore, it's imperative to examine conditions that create water and water vapor within the building envelope.

The Story of Water
All matter exists in one of three states, depending on pressure and temperature subjected to its mass of atoms. As a mass of matter switches from a solid, liquid, or gas, the conversion of its physical state is described as "changing phase" or "phase transition". When a material shifts physical state during a phase transition, heat holding capacity, and temperature variations are a by-product.

Roofers are familiar with materials that change phase when roofers asphalt is melted. Roofing asphalt is hard and brittle when cold, so in order to use it, it is put into a roofer's kettle and heated. As the asphalt is heated, it changes from a solid into a liquid. If the roofer continued to add heat, the asphalt would again change phase into vapor. Roofers know that asphalt vapor is explosive gas and that kettle fires are extremely dangerous.

Water is another phase change material and the culprit behind the Visalia building's roof failure.

How Roof Venting Works
The largest class of low-sloped roofing systems protecting commercial and industrial buildings in the United States is built-up roofing (BUR). Representing one-third of the multi-billion dollar industry, BUR roofing systems out number the growing market share of single-ply or other roofing products installed each year. However, no matter what class of roofing system installed, allowances for the escape of trapped water inside the structure must be considered since each type of roofing membrane can and will malfunction due to the thermodynamic cycle of water.

The typical low-slope roof covering may consist of a structural roof deck, a near-impermeable vapor barrier, insulation, and an impermeable roof cover. Water can be introduced into a roof system in one, or all of three common pathways. First, roofing materials used to construct an assembly can have high moisture content due to relative humidity or from exposure to rain when the roof is assembled. Second, water can pass around the installed impermeable roof membrane due to damage, poor installation techniques, or faulty design. Finally, water can migrate up to the roof assembly from inside the structure. No matter how water enters a roof system, it will change phase between solid, liquid and gaseous states and cause problems.

When vapor barriers are installed, they can promote a cycle where vapor rising up to a cool impermeable roof cover reaches the dew point and changes back into a liquid. Now heavier liquid water will cascade down to the near-impermeable vapor barrier, where the heat-cool process can begin again. These cycles waste enormous quantities of energy that negate the value of the insulation and create conditions that promote the life cycle of fungi, mold, and mildew. Structural components can rot to the point of failure, and the integrity and useful life of the building may be jeopardized.

The bottom line is that a sound and robust roof design must include provisions for water vapor within the assembly and provide escape routes for that moisture.

The Bernoulli principle impacts vapor movement out of openings on a roof as wind passes across the openings. Lower pressures within the opening will draw air up and out of the building. This phenomenon is called the Venturi effect and is the same dynamic principle demonstrated when air blows across the open top of a straw and pulls liquid up the straw.

In reality, a roof is not blown off a building by wind. What actually happens is that when the air pressure on the top side of the roof is sufficiently lower than the air pressure inside, the higher pressure literally pushes the roof off the building.

A Dryer Roof
A completely closed roof system is a myth since even under perfect conditions flat roof seals are unpredictable. The opaque cloud of water condensation exhibited inside sealed double pane windows is proof the water cycle can negatively affect even the tightest "sealed systems". It's folly to expect that "sealed" roof assemblies can remain perfectly dry when factory made double pane windows fail with regularity.

Provisions for venting a roof will enhance the possibility of long life and a healthy building envelope by using one or all of the following venting methods.

• Breather Vents: Breather Vents can provide venting for a low-slope roof assembly. A roof breather vent can be fabricated in many cone diameters and lengths with cap designs that provide a one-way exit of wet vaporous air from within a roof assembly.
  
  The use of pre-made vents provide a venting option, however, under closer scrutiny the manufacturer's recommendation of one 4" breather for each 93 square meters (1000 ft squared or 10 squares) of roof area causes concern. With a cone diameter of 10.16 centimeters (4") each breather's overall venting area is 81.03 square centimeters (12.56 in squared). The NRCA 1/150 guideline provides that for every roofers square, the venting area should include .66% or 619.35 cm squared (.06m squared, 96 in squared, or .67 ft squared) of venting area.
  
  Since a 4" breather provides enough venting area for about 1.12 m squared (13 ft squared), a roof would need eight breather vents for every roofers square to satisfy the 1/150 venting rule. That's a lot of vents!
  
  
• Wall Vents: For many roof projects the utilization of breather venting can significantly increase venting of trapped water vapor. Another additional over-looked method of venting for a flat roof is by using wall vents.
  
  Rarely are buildings totally without roof to wall details. A roof wall can be found as part of parapets, equipment platforms, roof penetration platforms, and at the intersection between lower and higher plane areas of a structure. Providing a successful wall termination requires the same waterproofing flashing with a cover procedure used to enable the law of gravity to work.
  
  A roof-to-wall vent is easy to add to roof-to-wall termination details. In theory, if air passage is provided behind the roof base flashing that extends under the impermeable roof cover, then water vapor that accumulates within the insulation layers will have a means to escape. A vent behind the base flashing allows the dynamic forces to drive water vapor out from within the roof. The challenge is providing a practical and reliable air escape channel scheme.
  
  A solution is an off-the-shelf venting mat that is designed to provide vapor venting for steep slope hip and ridge roof designs. A ridge vent mat is a randomly-aligned natural fiber product that is made by heat-curing a latex bonded polyester mesh. Natural fiber vents provide airflow while providing a barrier to most ambient water, dust and pests.
  
  
• Penetration Vents: The total number of breather vents necessary to satisfy a NRCA venting solution makes using only breather vents expensive. In order to have sufficient venting for 93 square meters (1000 ft squared or 10 squares) up to 80 commercial vents would be required.
  
  There is an easy way to get additional free ventilation by converting roof penetrations, which already populate a roof, into breather vents.
  
  Over the centuries, roof construction evolved to include two waterproofing techniques: flashings and counter flashings. Flashings and counter flashings create a rise in the roof's membrane that is high enough to keep the elements from entering the waterproofing membrane then placing a cover or counter flashing over the rise. Pipes, conduits, vents, and support legs use a sleeve or 'jack' flashing to create a rise in the roof's level.
  
  As a general rule of thumb, roof product manufacturers recommend using a flashing or inserting roof jacks for projections through a roof's membrane not lower than 203 mm (8"), and not higher than 356 mm (14") above the finished roof level on low slope roof applications. Each respective manufacturer's standard roof specifications include penetration details for plumbing vents, electrical conduits, HVAC line sets, domestic water lines, natural gas, and all sorts of other roof penetrations. Each roof penetration can be matched to a pipe-flashing jack with a proper outside diameter.
  
  The fact is that most penetration details are not designed using the flashing/counter flashing method, but is mechanically attached to the roof penetration. A typical installation includes caulk at the union of the flashing opening and compressing the assembly using a stainless steel band clamp. No allowance is made for thermal, seismic movement, or trapped water vapor.
  
  By properly utilizing counter flashings, roof penetrations can provide unique opportunities for roof designers. If the top of the flashings are not mechanically clamped, but remain open, then the roof design would allow for thermal and seismic movement, and provide vapor escape avenues with free venting areas.
  
  We can even take this concept further if each flashing jack was sized larger than the roof penetration. An oversized flashing could provide greater amounts of free ventilation areas. Also, since a building can include hundreds, if not thousands of roof penetrations, this venting technique can be exploited at little cost to the building owner.

The Tail of the Tale
The person that fell through the Visalia roof, and the owner of the building ended up in court. Neither knew that this roof had a dead fall that would catch some unlucky soul who entered its trap.

The Visalia building is a tilt-up manufacturing warehouse. The diaphragm roof deck included large steel and wood beams that allowed the deck to span its great width and length. As the building aged, the 3.05m (10') long rafters began to sag. This created cupping areas across the deck.

You now know that this roof provided the perfect water cycle environment. Vapor rose to the bottom of the impermeable roof cover during cool nights. The condensed liquid was absorbed into the fiberboard insulation. As the insulation became saturated, the heavy water accumulated across the surface of the sagging asphalt-coated plywood sheathing. Mildew, mold and fungi enjoyed an ideal environment to feast on wet wood. The trap was now set, since this was taking place under what appeared to be a perfectly sound white built-up roof cover.

Historically, the "water that falls from the sky" design approach has been the waterproofing criterion for flat roofs. The thermodynamic cycles affecting them have been set aside or ignored in favor of this one-sided approach.

By including breather, wall, and storm collar penetration venting in their arsenal, creative designers have the opportunity to make building envelopes more energy efficient, safe and healthy. Armed with these new understandings, additional venting techniques and solutions can be developed. And that's no fairy tale.

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