How to Install Mechanically Fastened TPO: Plates, Rows, Perimeter Zones and Welded Laps

Step by Step

1. 1Step 1: Confirm the deck, insulation and wind design

   Walk the deck and confirm it is sound, dry and clean. Wet or soft plywood, corroded steel and rotten edges must be repaired first, because a fastener is only as strong as what it grips. Check the insulation: boards should be tight, staggered and attached per the design. See how to install tapered insulation and crickets.

   Get the fastening schedule in writing. Wind uplift design divides the roof into a field, a perimeter and corners, each with its own fastener spacing and row count. Do not guess, and do not apply the field spacing across the whole roof. The perimeter zone width depends on building height and width, and is often from 4 to 10 feet or more, so confirm it on the project specification.

2. 2Step 2: Lay out the sheets and mark the fastener rows

   Snap chalk lines for each sheet edge. Place the first sheet where it gives the roof the most wind protection, typically starting at the low edge so each following sheet laps with the flow of water. A lap that runs across the slope can act as a small dam in heavy rain, especially on low slopes.

   Sheets are commonly 10 feet wide with a 6 inch lap for a single fastener row along the seam. Perimeter and corner zones often use narrower sheets, for example 5 feet, so more fastener rows can be added in those zones. Plan which sheets are narrow before unrolling, not after.

   Tip: Unroll the membrane and let it relax in the sun for a few minutes. A sheet that has been tightly rolled for days and is put down cold will wrinkle and buckle.

3. 3Step 3: Position the first sheet and fasten the edge

   Set the sheet with its fastening edge on the chalk line and hold it with light sandbags or weights, not with random fasteners. Place the plates at the fastening edge, on the chalk line, with the plate centred so the lap will later cover it with room to spare. Fasteners are typically spaced 12 inches on centre in the field of the roof and 6 inches in perimeter and corner zones, but confirm the spacing on the manufacturer's specification and the wind design.

   Seat plates flat. A plate pulled down too tight dimples the membrane and can crush the insulation and cut the scrim. A plate left proud rocks and wears a hole through the sheet as it moves. Use a depth-setting driver and check the first few fasteners each morning and whenever the deck type changes.

   Watch out: Fasteners must go into the structural deck at the full embedment the manufacturer requires. A fastener that only grips the insulation or barely enters the deck holds nothing. Pull-out tests on the first fasteners of a job are the quickest way to find out.

4. 4Step 4: Lap the next sheet over the plates

   The sheet edge of the new sheet goes over the plates and past them. Measure: a 6 inch lap with the plates about 1-1/2 to 2 inches from the edge of the lower sheet leaves a clean weldable width of at least 1-1/2 inches beyond the plates. If the plates creep too far in from the edge, the weld lands on top of a plate and the seam is not probe-able.

   Wrinkles and fishmouths at the lap must be removed before welding. Pull the sheet flat, cut a small relief if necessary, and patch the relief later with a round piece of TPO that is welded and probed like any other seam.

5. 5Step 5: Clean the lap and make a test weld

   TPO that has been outside for even a few hours picks up dust and moisture, and welds badly over it. Wipe the lap with a clean dry rag, or with the manufacturer's cleaner if the membrane is dirty or weathered, and let it flash dry.

   Make a test weld on scrap membrane from the same roll, cool it and peel it by hand. A correct weld fails by tearing the sheet, not by splitting at the weld line. Repeat the test at the start of each session, after a long break and whenever the weather, power supply or sheet changes. The full procedure is in how to hot-air weld TPO seams.

6. 6Step 6: Weld the lap

   A robotic welder is used on the long straight laps and a hand welder with a 1-1/2 inch nozzle and a silicone roller for the rest. Start with the machine at the manufacturer's recommended settings, then adjust using test welds. Nozzle temperatures are commonly in the range of several hundred degrees Celsius and speed in the range of several feet per minute for robotic machines, but the test weld determines the working value on the day.

   Maintain the same travel speed and keep the nozzle fully under the top sheet. Too slow burns the membrane and leaves a brown scorch mark and a brittle weld. Too fast leaves a seam that looks closed but peels apart. Check the squeeze-out along the edge: it should be a small, continuous bead.

7. 7Step 7: Probe every seam and mark the defects

   Probing means running a blunt, rounded probe along the seam edge with light pressure, checking for any spot where the top sheet lifts or where the probe slides in. Do it after the weld has fully cooled, and do it to every seam, including hand welds at details and corners.

   Mark defects with a pen, reweld them and probe again. A seam that passes probing on the day it is made has a good chance of holding for the life of the roof. A seam that was never probed is an open question.

8. 8Step 8: Repeat across the roof, then treat the perimeter and details

   At walls, parapets and curbs, the membrane runs up as flashing and terminates on a termination bar. See how to flash TPO walls, curbs and corners. At the edge, a TPO-coated metal fits the perimeter: see how to install TPO-coated edge metal.

   Where the membrane meets a drain, taper and sump details decide whether water reaches it. See how to install a flat roof drain. Finish with a full walkover to check for wrinkles, stray fasteners and unwelded T-joints.

Why Corners and Perimeters Are Fastened Harder

Fastener Notes

Common Mistakes

• Using the field spacing everywhere. Corners and perimeters see the highest suction and need the tighter spacing or extra rows. A uniform pattern is the classic cause of edge blow-offs.
• Fasteners that miss the deck. They hold the insulation, not the roof. Pull-out tests on the first fasteners prevent it.
• Plates set too far in from the sheet edge. The weld lands on top of the plate and cannot be probed, leaving an unverified seam.
• Over-driving or under-driving plates. Crushed insulation and cut scrim on one side, loose plates that wear through the sheet on the other.
• Welding without a test weld. A cold weld looks perfect and opens in winter.
• Laps running across the slope. They hold back water and collect dirt, and a pinhole there will leak every time it rains.