Roofing Screw Caps for Corrugated Bitumen Sheets

Introduction

A simple replacement for the plastic washers normally used with nails when fixing corrugated bitumen roofing sheets.

These lightweight roofing sheets are normally fastened with special roofing nails fitted with a large plastic washer. The washer increases the bearing area under the nail head and prevents the relatively soft bitumen material from being pulled through or damaged by the fastener.

I prefer using wood screws, so I designed this simple 3D-printed load-spreading cap for use with 4 mm wood screws with countersunk/conical heads.

The screw passes through the centre of the cap, while the underside provides a much larger bearing surface against the roofing sheet. This spreads the load from the screw head and helps prevent it from cutting into or pulling through the bitumen material. It also helps prevent water from leaking through the screw hole.

Designed for

• Corrugated bitumen roofing sheets

• 4 mm wood screws (3.5 mm aro also totally fine)

• Screw lengths of approximately 60–70 mm

• Countersunk/conical screw heads are assumed, but other head shapes will work, too

The cap is deliberately simple and easy to print. It is intended as a practical alternative when you want to use wood screws instead of the nails supplied for this type of roofing. Remember that you need approx. 9 screws for every square meter of roofing.

The model is dimensioned around 4 mm screws, but the concept can easily be adapted to other screw sizes. If you need a different screw diameter, washer diameter, or shape for another type of roofing sheet, scale the model, or let me know.

Printing & Material

The model has no special printing requirements. It can be printed with a standard nozzle and normal layer heights; no supports are required.

For convenience, the gcode file and also the 3mf slicer file have rows of the caps arranged for printing in one go. There are 90 of them, which is good for approx. 10 m² of roofing.

The stl file has a single cap in case you want to arrange them differently.

Since the part is intended for outdoor use, choose a filament suitable for long-term exposure to sunlight, temperature changes and moisture.

PETG is a good general-purpose choice and is easy to print. ASA is preferable if maximum resistance to UV and weathering is required. PLA is not recommended for permanent outdoor use, particularly where the part may be exposed to direct sunlight or high temperatures.

The part is a simple load-spreading cap rather than a structural component, so there is no particular need for high infill. A few perimeters and a moderate infill should be sufficient.