Steep Slope, Low Slope

What is a hydrostatic roof? What is a hydrokinetic roof, and how do the two differ? For one thing, the types of buildings they’re typically used on are different. Hydrostatic and hydrokinetic metal roofs allow for thermal expansion and contraction in different ways. Flashing, trim and other details are different. Moreover, the materials used to clad hydrostatic and hydrokinetic roofs have changed over time. In the past 100 years, the range of applications in which metal roofing can be used has expanded due to advancements in metal roofing technology.

Hydrostatic and hydrokinetic roofs are used on different types of buildings and are installed differently

By Christopher Brinckerhoff
Hydrostatic roof. Photo courtesy of Building Research Systems

Roof Type Comparisons

Generally speaking, a hydrostatic roof, or low-slope roof, has a slope between 1/4:12 and 3:12, and can withstand hydrostatic water pressure, meaning standing water. They are often used on warehouses and other large, boxy, industrial and commercial buildings.

Hydrokinetic roofs, or steep roofs, on the other hand, have a 3:12 slope or greater, and water sheds off the surface in a short time period. They are sometimes called architectural roofs, and are often used on residential buildings, churches and other small buildings for which attractive aesthetics are a factor. Steep-slope roofs typically have a water barrier membrane underneath metal cladding.

One person who is deeply knowledgeable about hydrostatic metal roofing is Cody Rodden, president at Edmond, Okla.-based Building Research Systems Inc., which licenses hydrostatic standing seam roof systems. He has worked in the metal roofing industry for more than 40 years, and says there are no metal-to-metal connections in hydrostatic applications; there is always some sealant between plies and seam joints.

“On the hydrostatic the biggest difference is there are no exposed fasteners that are not in conjunction with some kind of sealant, that is a non-skinning butyl type product,” Rodden says. “The fasteners that do penetrate a hydrostatic panel are often covered from the outside by a flashing detail, so if you do have any situations where you have water damming, it will not be impacted, except in the valleys, end-laps and at eaves. In areas such as some valleys where you do have exposed fasteners with sealants typically with the hydrostatic panel, these are proven details. These details can be likened to a bolted connection where the fastener acts as a bolt, and the heavy gauge material it is fastened into acts like a nut. As a fastener is tightened, it draws the plies together and compresses sealant between them, forming a water-tight connection. Sealant location is critical in hydrostatic roof systems.”

Some hydrostatic panels have what is called a nut, washer, bolt connection, the bolt being the fastener, the washer being the panel, and the sealant. The bolt part is usually a 16-gauge or heavier piece of flat-gauge structural steel in order to secure the nut, bolt compression joint.”

Hydrokinetic metal roof cladding can be metal tiles or standing seam panels. In contrast, most often hydrostatic metal cladding is standing seam panels.

Some standing seam panels can be used on hydrostatic or hydrokinetic slopes, depending on how they’re installed. Bossier City, La.- based McElroy Metal Inc. manufacturers metal roof panels that can be used in hydrostatic or hydrokinetic applications.

It depends on how they’re installed and detailed, says Jamey Purdy, technical specialist, engineering, at McElroy Metal, who has been an instructor for the company’s installation certification class for contractors for the past 16 years. “There’s not really a hydrokinetic versus a hydrostatic roof panel; hydrokinetic and hydrostatic are basically just details.” Purdy says. “Any roof could be hydrokinetic or hydrostatic depending on how you seal it.”

Expansion and Contraction

All metals expand and contract in conjunction with temperature changes. A major challenge for metal roof designers and installers is to make sure water can’t get inside, but the metal cladding can still move as it thermally expands and contracts without compromising the integrity of the roof assembly.

To prevent water infiltration, oftentimes, hydrostatic and hydrokinetic metal roofs direct thermal expansion/contraction movement in different directions, Purdy says. “Usually a hydrostatic system is going to have a pinned eave, meaning that it’s going to have exposed fasteners along the eave, and tape sealants in that line of fasteners. Any thermal movement goes toward the ridge, hip or headwall. That way the eave is stationary, and any expansion or contraction goes up the panel.”

Hydrokinetic metal roofs are often pinned at the opposite end, Purdy says. “You pin the ridge, or the high side of it, and all the thermal movement comes down the roof. Typically you cleat the panel, and it is an open area. You rely on underlayments to get any kind of water infiltration back out of the roof.”

Another way hydrokinetic metal roof cladding, such as panels with exposed fasteners, can be allowed to expand and contract is by way of a purlin system, Purdy says. “The purlin actually will flex to allow for some thermal movement. Or, if it’s over a solid substrate like plywood, the panel will actually buckle in between fasteners. So that’s where you have to worry about any kind of leakage into the side-lap of an exposed fastener panel. One thing that we generally recommend is sometimes you’ll have to put a stitch fastener in there to keep the side-lap together.”

Another big part of preventing water infiltration and, at the same time, allowing thermal expansion and contraction to occur in hydrostatic standing seam roof systems is attachment clips, says Stephen Patterson, PE, a registered roof consultant at RCI Inc., Raleigh, N.C., and president at Fort Worth, Texas-based Roof Technical Services Inc. Like Rodden, Patterson has also worked in the roofing industry for more than 40 years. “You basically attach the panels with clips rather than attaching the panels with screws,” he says. “The clips are inside the rib of the panel and, in most cases, the clip interrupts the continuous sealant in the rib. As a result, the way the panels are clipped becomes very important. All the panel seams have to be waterproof, and the panels have to be able to expand and contract and accommodate the expansion and contraction of the roof panels without breaking the waterproofing.”

Patterson describes two of the different attachment clip designs in the market. “In one design of the panel, the clip does not interrupt the sealant; there is a continuous sealant,” he says. “And, in the other type, there is a potential for a void at either end of the clip that can leak.”

In conjunction with the attachment clips, Rodden says rake plates also have a role in thermal expansion and contraction. “A hydrostatic panel, as built and featured by a designer to allow for thermal expansion and contraction using a series of floating clips, uses a rake plate where the gable allows the roof to move in unison when it starts to move along with the clips.”

Private residence, Ponte Vedra Beach, Fla.

Penetrations and Flashing

Proper flashing is critical to preventing water infiltration on low- and steep-slope roofs. However, sealing, welding and soldering are more thorough and extensive processes on hydrostatic details.

“On a hydrokinetic roof, you’re just overlapping materials so that water runs off,” Patterson says. “On a hydrostatic roof, you’ve got to make sure that the joints are completely waterproof. The flashing is every little interruption, every little transition, and has got to be waterproof, so the flashings become very critical. It requires a lot more attention to detail, introducing sealants, soldering or welding. And the joints have to be able to resist expansion and contraction, so the level of difficulty between hydrostatic and hydrokinetic details is tremendous. It takes a lot more detailing, a lot more craftsmanship, to make sure that they are waterproof.”

The type of sealant used can make the difference between a flashing detail that keeps water out versus one that allows it inside, Purdy says. “We want to use something that’s a non-curing type sealant like butyl sealant because if you use something like urethane or silicone, those cure and will actually separate; they won’t stay tacky. So if anything makes the lap come apart, the sealant just breaks away. Whereas a butyl type sealant, it has flex like chewing gum. It’ll expand with an opening and then close back.”

In some circumstances, hydrostatic detailing is needed on steep-slope roofs, Patterson says. “Most leaks on hydrokinetic roofs occur at flashing details where hydrostatic water can build up, like around chimneys and valleys. If water runs in the back of a chimney and doesn’t run off fast enough to get around it, water builds up and causes a leak.”

Wherever a roof is interrupted, such as chimneys and valleys, those particular areas may end up being under water at certain times, Patterson says. “During heavy, intense or wind-driven rains, if leaves settle in a valley, water backs up, and suddenly a hydrokinetic roof becomes hydrostatic. That’s why leaks occur in valleys and at chimneys during certain types of rain, because the flashing detail is not hydrostatic.”

Simplified roof designs with minimal rooftop penetrations lend themselves well to hydrostatic details, Patterson says. “As soon as the flow of water is interrupted in the metal roof, you create a problem that you got to deal with. It’s got to be 100 percent waterproof, so simplicity in low-slope metal roofs, or hydrostatic metal roofs, is critical.”

Another key to minimizing rooftop penetrations is communication among project team members during construction, Rodden says. “You can’t have a roofing contractor go out and install a roof, do a wonderful job and make it watertight, and then have another trade, like a plumber, come through and not know how to seal it up. So you have to have inter-trade collaboration for a successful roof.”

When it comes to dealing with penetrations, Rodden says the quality and application of sealants is a pivotal step. “Your details have to be as good as the roof that you’re putting down,” he says. “The roof penetrations have to be addressed just as seriously as the sealants that are going to help keep it watertight. Regardless of whether it’s a hydrokinetic or hydrostatic, both are dependent on sealants.”

Sterilite Corp., Ennis, Texas, Photo: Tony Corso

The Sealant Bump

In fact, metal roofing technology improved greatly during the past 100 years, and sealant technology was an essential part of it.

Richard Starks Jr., PE, vice president of engineering and product development at Building Research Systems, sums up the difference between old sealants and new sealants in one word: longevity. “A roof used to be designed for a lifespan of 20 years,” Starks says. “And now with experience and improved coatings, we are expecting roofs to stay on for 40 or 50 years. So sealants have to meet that longevity.”

Rodden says sealants were very rudimentary in the 1960s and gradually improved over the decades. “In the last eight to 10 years, you’ve seen a marked improvement in metal roof sealant technology. The sealants now are probably 50 to 60 percent better than those when they first started out, and in some cases, probably 100 percent better.”

Cobalt Stables, Kansas City, Kan.

Metal Roofing Evolution

Metal roofing today is based on technology that is hundreds of years old. Rodden says, “In Europe, the old hand-turned, hand-seamed standing seams that are hundreds of years old were a forerunner of the hydrostatic panels today. They set the stage that this open framing, hydrostatic panel is a good method of construction.”

Metal roofing grew in the U.S. in the late 1930s and early 1940s, and then got boost in the 1960s with the development of mechanical seamer technology, Rodden says. “Back in the mid-‘80s, a large number of component companies were developing standing seam roof systems, and the evolution of standing seam had just exploded in the industry. Before that, there was only a panel that was through-fastened over open framing, and it didn’t accommodate for thermal expansion and contraction like a standing seam would. They were not hydrostatic; they were hydrokinetic.”

On old churches with copper or other metal roofs, where builders had to roof over small, flat areas, they soldered or welded small pieces of metal together, perhaps a foot square, Patterson says. At that time, metal was too labor-intensive to be efficiently installed in large, low-slope applications.

“With the advent of sealants, the first metal roof panels that were commonly used were the standard metal ribbed panels, where they simply overlapped the panels and screwed or nailed them together,” Patterson says. “Then the process became, on the lower roofs, that when installers lapped the sheets together, they introduced a layer of butyl between the laps so all the laps were joined together to make the panels waterproof.”

Today, installers can form panels that are tens of feet long on job sites, which has made metal cladding a viable material for many more roofing applications than it had been in the past, Patterson says. “Now, instead of dealing with a 12-inch by 12-inch piece that you’re hand soldering with a soldering iron, suddenly you’re putting on a sheet that’s 30 feet long or longer by 2 to 3 feet wide with sealant between the joints. And then screws with rubber washers were developed so the rubber washer on top sealed the screw holes. Now we’re building long panels out of rolls of steel which are rolled and formed on-site; it’s just amazing.”