Water Resistant vs. Water-Shedding Metal Roofs - What's the Difference?

Published May 4, 2018 by Whirlwind Team

water resistant vs water shedding metal roof

Water is a crucial component of life, but too much of it in the wrong place does great damage. The roof of your building is the primary defense against keeping outside water from coming in. Repelling the water from your roof is accomplished using either a water resistant or water shedding roof system.

Let’s look at the difference between a water shedding metal roof and a roof that is water resistant. We’ll also look at which type of roof is suitable for your project.

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Definitions

Waterproof literally means proof against water; water cannot enter. Making your roof waterproof takes a few extra steps to ensure everything is right and tight when the roof is installed.

Water-resistant or hydrostatic roofs are for low slope roofs (less than 3:12 slope) to handle slow moving water from a roof. These types of roofs do not rely on a secondary membrane to prevent leakage into the building.

Water-shedding or hydrokinetic roofs, (3:12 and greater slope) on the other hand, simply means fast moving water. A water shedding roof on the building will normally rely on a secondary membrane to resist water entering the building.

Water Shedding Roofs

The easiest way to ensure the roof sheds water effectively is by designing the slope to be at least 3:12, preferably higher. The higher the slope, the faster the water runs off into the gutters, downspouts or drainage system.

A roof with a 3:12 slope or greater is considered a steep slope and is the minimum slope to provide effective water-shedding. Conventional roofs, particularly residential roofs, tend to be 4:12 to 9:12.

A roof that sheds water still needs a little assistance to keep water from entering the structure. Since a water-shedding roof is typically an architectural roof, decking and underlayment are already required.

  • Use a solid deck covered with a moisture barrier or membrane.
  • The moisture barrier should be #30 felt at a minimum.
  • A peel-and-stick membrane is a better, although more expensive solution.
  • A peel-and-stick membrane is more tear resistant and will self-seal to fasteners such as nails and screws.

Avoid intricate trim details and roof profiles. Every non-continuous point on the roof is an invitation for water to enter. Reduce the number of valleys, hips and other effects as much as possible.

Water Resistant Roofs

Waterproofing requires you to block moisture out by using techniques to seal your roof against water. If you have a low slope roof or one with many architectural elements such as multiple gables and valleys, waterproofing may be a necessity.

A roof with a low slope may not shed water efficiently and experience pooled or standing water for a period of time. Water will always seek to run downward; if there is an opening, it will get through and create water damage to anything underneath the roof, from the decking and insulation to the building’s contents.

If water gets trapped in the end lap or the purlin, it can cause premature corrosion in the secondary framing members or panel.

  • A roof with a slope of less than 3:12 is generally considered to have a low slope.
  • If there are panels or areas of the roof what are slightly lower than the surrounding materials, it will hold standing water.
  • Other places water sneaks in is through exposed fasteners and through nesting panels, which may experience wicking, drawing water underneath the edge.

Waterproofing is best approached via multiple aspects of the roof.

  • The most water resistant roof is a standing seam roof with seams at least two inches high. Ensure the roof can move with thermal expansion and contraction without damage.
  • Mechanical seaming of the joints between panels eliminates areas where water may enter between panels.
  • Do not use exposed fasteners. Opt for concealed fasteners for the entire roof. Fasteners do not always last the life of the roof and become a primary entry point for moisture.
  • Use foam and metal closures along with manufacturer approved sealant to ensure a watertight fit between panels, at the end laps and around fasteners. Do not allow the sealant to be exposed to the outside environment as it will shorten the life of the material.
  • Include proper ventilation to protect against condensation on the underside of metal panels and trim.

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To completely waterproof your roof, add insulation to combat condensation. Metal panels transfer heat quickly and easily in the absence of insulation. As heat is transferred, condensation is left behind on the metal surface where it can speed the rate of corrosion and rust. 

Insulation with a vapor barrier will provide the best protection against condensation. However, in the event a roof leak occurs anyway, insulation that becomes damp must be replaced rather than left to hold water against any attic or roof materials.

A steel roof is an investment that returns itself over time. Steel is an environmentally friendly material that is resistant to insects and other pests while providing a strong barrier against hail. Steel roofs are manufactured to exacting standards so they can withstand high wind uplifts and heavy snow loads.

For green construction purposes, steel earns LEED points and Energy Star designations and certifications. Steel is 100% recyclable, and most steel contains at least 25% recycled material. In fact, steel can be recycled endlessly without losing any of its strength.

A lesser-known fact about steel is its high strength to weight ratio. A steel roof is many times lighter than an asphalt roof and significantly lighter than a wood frame or tile roof. A lighter roof requires less material for framing, decreasing the weight on the primary frame and the foundation.

Finally, a steel roof can be designed to reflect solar radiation and re-emit heat every bit as easily as it sheds water. A white “cool roof” can reduce interior temperatures and energy bills while shedding rain.

Insulation helps stop heat transfer to reduce heat in summer and retain heat in winter. In both conditions, condensation is avoided while maintaining a comfortable interior.

A water-shedding roof is one that allows water to run off quickly instead of pooling on the roof panels. Roofs with slopes of 3:12 and above are efficient water-shedding roofs. Water barriers such as membranes and felt added to the decking required for architectural roofs provide further protection from moisture.

If the roof slope is lower than 3:12, the roof should be waterproofed using various techniques and methods to keep water from entering between the roof panels, through fasteners holes and other openings. Sealants, standing seam roof designs and concealed fasteners all work together to make a roof waterproof.

When designing your metal roof, make an early decision about how to protect against water and incorporate the recommended techniques and materials into your design before beginning your project.

Metal Roofing Guide

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