You should always use fasteners and washers that are compatible (made from the same materials) with the roofing materials you are using. For example, if you are using corrugated stainless steel sheet metal, you will want to use steel screws as well (rather than say, copper, which will accelerate corrosion). They should have the same longevity, so you do not have one corrode before the other. Because fasteners are a low-cost component of your overall structure, there is no point in scrimping on their quality.
Screws for fasteners are measured in three parts: Gauge, thread pitch (tpi), and length.
Screws for fasteners are formatted in these three numbers:
Example: 12 — 14 X 50
(gauge) — (pitch) X (length)
Fasteners must have a coating system meeting AS 3566 Class 3 or Class 4.
- AS 3566 Class 3 screws are sufficient for most exterior conditions at least 400m from the ocean.
- For severe conditions or close proximity to the ocean, you should use fasteners meeting AS3566 Class 4 requirements.
Nails should be galvanised steel, while blind rivets should be stainless steel or aluminium. For sealing washers, you should use EPDM materials, which do not have high levels of conductive carbon black.
There should be between 1 and 3 threads visible past the support you are fixing. If you are fixing ribs, the “shank protection” segment should not quite reach the support. For this reason, choose screws that are longer than the material you are fastening.
To fix to steel thicker than 3MM, use self-drilling screws with pre-drill holes.
The screw should be embedded to the same degree as it would without insulation.
Sealing washers are used to help weather proof the fastener. They should be tightened only until they are secure, but be careful not to overtighten washers as they may split and become useless.