Direct Fastening Technical Guide , Edition 22
3.6.1.3 HEAD STYLE SELECTION
HWH ( HHWH ) ( High ) Hex Washer Head : Washer face provides a bearing surface for the driving sockets . |
PPH ( PPFH ) Phillips Pan ( Framing ) Head : Conventional head for general applications and provides low profile fastening . |
PFH Phillips Flat Head : Used primarily in wood to countersink and seat flush without splintering the wood . |
PWH Phillips Wafer Head : Large head provides the bearing surface necessary to seat flush in soft materials . |
PBH Phillips Bugle Head : Used primarily for fastening drywall , plywood or insulation board to steel studs . |
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PTH ( MPTH ) ( Modified ) Phillips Truss Head : Large head and low profile provides surface area needed to attach wire lath to metal stud . |
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PPCH Phillips Pancake Conventional Head : Head for general applications and provides low and flat profile . |
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PFTH Phillips Flat Truss Head : Lowest profile head available for attaching metal to metal . |
PFHUC Pancake Framing Head Undercut : Used for countersinking where a full head taper would cause stand-off of the screw . |
SHWH Slotted Hex Washer Head : Hex washer head with slot in center to provide additional drive connection . |
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TPCH Torx Pancake Head : Pancake head with a star drive to prevent strip during installation and provide a decorative drive finish . |
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3.6.1.4 SEALING CRITERIA
Sealing washer screws offer greater weather resistant fastenings where moisture or condensation is a factor . The washer helps seal the hole to better prevent moisture from dripping into the fastener threads from the fastened material side , reducing corrosive build-up . As added protection against corrosion , all sealing washer screws come standard with Kwik-Cote coating . The torque control or depth gauge of the electric screwdrivers help ensure that the optimal seal is applied ( Reference Section 3.6.1.7 ).
3.6.1.5 LENGTH SELECTION
Length of the screw ( L )
Depending on the screwhead , there are two different ways to measure the overall length of a screw .
For HWH / HHWH , PPH , PTH , PFTH , SHWH and PPCH screws , the overall length is measured from the bottom of the washer under the head to the point of the screw .
For PWH , PFH , PBH and PFHUC screws , the overall length is measured from the top of the head to the point of the screw .
Maximum total thickness ( MT )
The maximum total thickness ( MT ) for all screws is the length of the threads reduced by the first three threads ( protruding past the back-side of the base material ). See drawings above and below .
The maximum total thickness ( MT ) describes the maximum thickness of all attachments to be fastened plus the base material .
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