Metal Construction News, February 2019

SPECIAL FEATURE Hydrostatic roof Steep Slope Low Slope By Christopher Brinckerhoff Associate Editor What is a hydrostatic roof What is a hydrokinetic roof and how do the two differ For one thing the types of buildings theyre 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 roofi ng can be used has expanded due to advancements in metal roofi ng technology Roof Type Comparisons Generally speaking a hydrostatic roof or lowslope 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 roofi ng 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 roofi ng 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 fl ashing 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 fl at 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 theyre 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 theyre installed and detailed says Jamey Purdy technical specialist Hydrostatic and hydrokinetic roofs are used on different types of buildings and are installed differently Photo courtesy of Building Research Systems Private residence Ponte Vedra Beach Fla 16 METAL CONSTRUCTION NEWS February 2019