Butyl tapes in sheet-metal joints

What it is for

Where two sheets overlap, a thin gap forms. At low pitches, in wind or under snow, water creeps into it and gets behind the sheet. A strip of butyl in the lap fills that gap. It is used mainly:

• in the side and end laps of trapezoidal sheeting at low pitches,
• in sandwich-panel joints and their flashings,
• under flashing trims where the trim sits on a flat surface,
• for joints in the vapour control layer (a system tape specified by the membrane manufacturer),
• as a sealing layer under chimney flashing – ASB.sk describes a flashing made from a flexible strip with a butyl-rubber adhesive layer.

How to apply it properly

1. The surface must be dry, clean and free of dust and grease. The tape will not stick to a dusty or frozen surface.
2. Place the tape in the lap so that it sits above the fixing screw, not below it – otherwise water gets round the tape alongside the screw.
3. Once in place, press it down along its full length with a roller or by hand. Butyl needs pressure, not heat.
4. Peel the release liner off gradually so the tape does not get dirty.
5. Join lengths of tape with an overlap, not butt to butt.

When butyl is not enough

• As the only weather protection at an exposed point. Tape in a lap supplements a correctly laid sheet; it will not rescue a sheet laid against the flow of water.
• In a joint that moves. Long trims expand in the heat. If the joint is bonded rigidly, the tape cannot take the movement.
• On porous masonry without preparation. A primer specified by the manufacturer, or a different system, is used there.

Butyl, sealant or a seam?

• PE foam fillersfilling the ribs of trapezoidal sheeting
• Chimney flashingwhere butyl and sealant are used most often