Side Lap Waterproofing for Vertical Metal Roof Sheet Joints

Side laps are the vertical joints that run up the slope where the edge rib of one roof sheet overlaps its neighbour. They are long, narrow and easy to overlook, yet on windy sites they let in more water than most owners expect. ETCE seals side laps with a continuous gun-applied bead of Promax Bonding Adhesive that bonds to both sheets and moves with them.

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Zamil Plastics – Dammam

Why side laps leak even on steep roofs

Water should run straight down past a side lap, and in calm rain it usually does. The trouble comes with wind. Shamal gusts across the Eastern Province and central Najd drive rain sideways and even uphill, forcing it under the overlapping rib. Capillary action then pulls it further in, and it drips from the purlin line well away from where it entered. Fine dust blown into the same gap keeps the joint damp and wears down any old sealant.

Because a side lap runs the full length of every sheet, even a modest roof has hundreds of metres of them, and the sealant must stay bonded through daily heat cycling on metal far hotter than the air. Promax Bonding Adhesive is rated for surface temperatures up to 120 °C, so it keeps its grip on the sheets through the hottest part of the Saudi summer.

Signs of failing side laps

Leaks that appear only when rain comes with strong wind from one side
Drips that follow the line of a sheet edge down the slope
Visible gaps or lifted ribs where sheets overlap
Brittle, cracked or missing sealant along the joint
Rust staining along the edge of an overlapping sheet

How we seal side laps

Joint inspectionEvery side lap is checked for lifted ribs, loose fixings and corrosion, and problem sections go into the photo report.
Fix movement firstWhere an overlap has opened because a screw has backed out or rusted, the fastener is replaced so the joint is tight before sealing.
Clean and dryDust, loose rust and old sealant are cleared from the joint line and the metal is dried.
Continuous beadPromax Bonding Adhesive is applied by gun in one unbroken bead along the lap, bridging both sheets.
Tool and checkThe bead is tooled into full contact with both surfaces and inspected along its length for any breaks.

Materials & workmanship

Promax Bonding Adhesive applied by sealant gun along the full lap
Rated for roof surface temperatures up to 120 °C
Bonds to steel, aluminium, rubber and other roofing materials
Applied from the roof, zone by zone, with the building in full use

Photos from ETCE projects

Side Laps (Vertical Joints) - Zamil Plastics, Dammam

Side Laps (Vertical Joints) - Alfa Laval, Riyadh

Side Laps (Vertical Joints) - Guardian Industries Company Ltd., Dammam

Site photos from ETCE projects in Saudi Arabia and Bahrain. Tap to enlarge.

Frequently asked questions

What is the difference between a side lap and an end lap?

A side lap runs up the slope, where the edge rib of one sheet overlaps the next sheet sideways. An end lap runs across the slope, where an upper sheet overlaps a lower one. They leak for different reasons, so we treat them differently.

Why use a sealant bead here instead of an EPDM strip?

Water runs alongside a side lap rather than straight into it, so the aim is to close the narrow gap between the ribs. A continuous bead of flexible adhesive does that neatly along the joint, while EPDM strips are used on details that water flows against, such as end laps and upstands.

Will ordinary silicone from a hardware shop do the same job?

Many general-purpose sealants are not made for metal roof temperatures in a Saudi summer and soon harden or lose their grip. Promax Bonding Adhesive is rated for surface temperatures up to 120 °C, and we apply it only after the joint has been cleaned and dried.

Do you need to seal every side lap on the roof?

Not always. The survey shows which laps are open, rusted or already leaking, and on smaller jobs we can seal just those runs. On older roofs where the original sealant has failed everywhere, treating all side laps together is usually better value.