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Roofing tube, PP
Polypropylene tubes are suitable both for metal screws with a diameter of 4.8 mm and for concrete screws with a diameter of 6.3 mm.
A mechanical system with a plastic tube and metal screw allows cheaper installation by choosing a longer tube and a shorter screw, because plastic is usually cheaper than metal. The use of plastic at the same time breaks the thermal bridge through the thermal insulation created by the fastener and thus saves energy in the lifespan of the building. What is more, between the tube and the screw there are certain moving when the tube is pressed down. This is so called “telescopic effect”. Thus, the tube “sinks” into the thermal insulation when someone steps on and the screw head does not pierce the membrane. The softer the mineral wool is, the shorter the tube should be. 20% shorter tube puts us on the safe side.
By installation someone should make sure himself that the head of the tube is flat to the surface and not coned inwards. Beware: even a small dent in the center of the tube down could show a significant run-out of the bearing capacity of the connection!
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