Metal Construction News, February 2019

engineering at McElroy Metal who has been an instructor for the companys installation certifi cation class for contractors for the past 16 years Theres 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 cant 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 infi ltration 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 its 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 infi ltration 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 fl ex to allow for some thermal movement Or if its over a solid substrate like plywood the panel will actually buckle in between fasteners So thats 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 youll have to put a stitch fastener in there to keep the sidelap together Another big part of preventing water infi ltration and at the same time allowing Private farmhouse Maryland 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 roofi ng 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 waterproofi ng 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 Sterilite Corp Ennis Texas Photo Tony Corso February 2019 METAL CONSTRUCTION NEWS 17 www metalconstruct ionnews com