End Lap Waterproofing: Sealing Horizontal Metal Roof Joints

Where one roof sheet ends and the next begins further down the slope, the two overlap to form an end lap: a horizontal joint that crosses every rib of the roof. All the rainwater from the sheets above has to pass over it. ETCE's end lap waterproofing covers each of these joints with a 1.2 mm EPDM strip bonded across its full width, so water flows over the top instead of creeping into the overlap.

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Zamil Plastics – Dammam

Why end laps are the weak line on long roofs

An end lap faces the flow of water head-on. On long warehouse roofs, and especially low-pitch ones, water slows as it reaches the joint and can be drawn into the overlap by capillary action. Dust and sand blown in by Shamal winds settle along the step of the lap, hold moisture against the metal and hide the first signs of corrosion. In the Eastern Province, humid salty air speeds up rusting at the cut edge of the lower sheet, which sits damp inside the joint.

Sealant beads alone struggle here because the joint moves every day as the sheets expand in the sun and contract at night. EPDM is a flexible rubber membrane that stretches with that movement instead of cracking, and it shades the metal beneath from UV. Once bonded across the whole lap it forms a continuous waterproof skin over the joint that typically lasts beyond 20 years.

Warning signs at the end laps

A line of drips across the building at the same distance from the ridge
Rust bleeding out from under the step where sheets overlap
Sand and dirt built up along the lap line after dust storms
Old mastic or tape that has dried out, cracked or lifted
Stained insulation directly below a horizontal joint

Our end lap sealing method

Survey and mark-upEach end lap is inspected along its full length, and corroded or lifted sections are photographed for the report.
Cleaning the lapSand, loose rust and failed old sealant are removed and the surface is dried so the adhesive grips sound metal.
Cutting the strip1.2 mm EPDM is cut to a width that covers the overlap with a margin on both sheets, to the length of each lap.
Bonding across the ribsThe strip is laid over the joint on Promax Bonding Adhesive and pressed into every rib and valley of the profile.
Edge checkAll edges are inspected so no gap is left where water or wind could get under the strip.

Materials & workmanship

1.2 mm EPDM rubber, supplied in rolls and trimmed on site to suit each lap
An adhesive bond made for sun-baked sheets, rated to 120 °C surface temperature
Stays flexible and UV-stable through daily expansion and contraction
Warranty of up to 15 years on the treated laps

Photos from ETCE projects

End Laps (Horizontal Joints) - Zamil Plastics, Dammam

End Laps (Horizontal Joints) - ATCO Group – East & West Factory, Dammam

End Laps (Horizontal Joints) - Alfa Laval, Riyadh

End Laps (Horizontal Joints) - Guardian Industries Company Ltd., Dammam

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

Frequently asked questions

What exactly is an end lap on a metal roof?

It is the horizontal joint where an upper roof sheet overlaps the top of the sheet below, running across the slope. Long roofs use several sheets in each run, so a building can have several lines of end laps between ridge and gutter.

Why not just run sealant along the joint?

A bead of sealant sits on a narrow line where all the sheet movement is concentrated, so in Saudi heat it tends to split or peel within a few seasons. An EPDM strip spreads that movement over a wide, flexible band bonded to both sheets.

Will the EPDM strip follow a corrugated or trapezoidal profile?

Yes. The membrane is soft enough to be worked into each rib and valley. That matters, because any void left under the strip would only move the leak rather than stop it.

Can end laps be treated on their own?

They can, and on many roofs they are the main source of leaks. Our survey still checks the side laps and fasteners nearby, because a leak blamed on an end lap sometimes starts at a screw just above it.