How to Flash TPO Penetrations: Pre-Molded Boots, Split Boots, Field Wraps and Sealant Pockets

Step by Step

1. 1Step 1: Choose the flashing type for the pipe

   A pre-molded boot is a cone of TPO with a flat flange. It is slipped down over the pipe and welded to the membrane. It is the best choice for any round pipe that can be lifted out of the way or that has a free end. Boots are sold in size ranges with stepped rings, so measure the pipe outside diameter and cut the boot at the ring that gives a snug fit, not one that hangs loose.

   A split boot has a slit and a welded closure, and is used where the pipe is already in place and the top cannot be removed. A field-wrapped flashing is cut from unreinforced membrane, wrapped around an odd shape such as a square post or cluster of conduits, and welded at every seam. Pipe clusters can be avoided by relocating pipes or using a manufactured penetration housing.

2. 2Step 2: Prepare the pipe and the membrane

   Pipes should be clean, dry and smooth where the boot seats. Wire brush rust, file burrs and remove old sealant. A rough pipe wears through a boot as the pipe moves. Plastic pipes are fine but may need a light scuff. Galvanized and steel pipes that will stay outdoors should be painted or otherwise protected.

   Confirm the pipe is secure and will not move or settle. A pipe that shifts in the roof structure pulls the boot apart at the flange. Also confirm that the pipe is at least about 12 inches from walls, curbs and other penetrations so there is room to weld and probe the flange. Closer than that and the detail becomes difficult to inspect and repair.

3. 3Step 3: Cut the field membrane and fit the boot

   Where the membrane was laid over the pipe position, cut a clean hole just large enough for the pipe, no bigger. The boot flange then laps on top of the membrane in every direction. A boot sits on top of the field sheet, it does not go underneath it, so water runs over the joint and not into it.

   Flange size matters. A common minimum is about 2 to 3 inches of flange beyond the base of the boot cone, giving enough width for a weld of 1-1/2 inches or more. Confirm on the manufacturer's specification. Check that the pipe, with the boot slipped over it, stands vertical and that the flange is not tented.

4. 4Step 4: Weld the flange to the field sheet

   Wipe both surfaces and make a test weld and peel test as described in how to hot-air weld TPO seams. A circular weld is harder than a straight one. Work in overlapping segments of a few inches, using the silicone roller right behind the nozzle and keeping the nozzle angle steady around the curve.

   Weld the flange continuously with no skips, and finish by overlapping the start of the weld so there is no gap where you began. A little squeeze-out at the edge means the sheets fused. Do not hold the welder in one place: a scorched ring around a pipe is a failed weld.

5. 5Step 5: Clamp and seal the top of the boot

   The top of the boot is a simple friction fit against the pipe, so it is held with a stainless steel clamp, tightened so the boot is firm without crushing it. Stainless is used because galvanized clamps corrode at the exposed top edge of a pipe in a wet climate.

   Apply a bead of the manufacturer's approved sealant around the top edge of the boot, where it meets the pipe, to shed water. The sealant is a secondary barrier. The weld and the boot geometry stop the water. Sealant exposed to sun needs inspection and renewal every few years, and it is one reason penetrations are included in roof maintenance visits.

6. 6Step 6: Use a split boot or field wrap for pipes already in place

   For a pipe that cannot be slipped over, cut a split boot or a piece of unreinforced membrane to wrap around the pipe with a lap of at least 1-1/2 inches on the vertical seam. Weld the seam the full height, probe it, and then weld the base flange to the field sheet.

   The place where the vertical seam meets the base weld is a T-joint and needs a round patch with rounded corners, typically 3 inches or larger, welded over the junction. Skipping it leaves a pinhole channel that looks closed and leaks.

7. 7Step 7: Use a sealant pocket only where nothing else fits

   A sealant pocket, sometimes called a pitch pan, is a metal or plastic enclosure welded to the membrane and filled with pourable sealer around the penetration. It is a last resort: the sealer shrinks, cracks and needs to be topped up as it ages, so pockets need regular inspection and refilling.

   Wherever possible, avoid the need for one: relocate the pipes, use a manufactured penetration housing or ask the manufacturer for a wrapped detail. If a pocket is unavoidable, make sure the flange is welded and probed, the sealer slopes away from the pipe and the pocket is filled above the pipe edge.

8. 8Step 8: Probe, inspect and record the detail

   Probe the flange weld around its full circumference and any vertical seam. Check the clamp, the sealant bead and the pipe for movement. A boot that is stretched tight over a pipe that has a bend or sway will crack at the top.

   Penetrations should be noted in the roof plan so they are checked during maintenance visits. Ponding water around a penetration is a warning: a pipe should not be the low point, so see how to plan flat roof drainage.

Spacing and Layout Around Penetrations

When the Penetration Is a Roof Vent or Skylight

Common Mistakes

• A boot that is too large for the pipe. A loose boot leaves a gap at the clamp, the clamp cannot pull the boot evenly tight and water runs down the pipe.
• Clamp left off. Friction alone does not hold a boot through temperature swings, and it slides down and opens.
• Sealant instead of a weld. Caulk around a flange fails within a few seasons. The flange must be fused to the field sheet.
• Scorching the flange. Holding the nozzle still around a curve burns the membrane and weakens the weld.
• Skipping the T-joint patch on a split boot. The junction of the vertical seam and base weld is left as an open channel.
• Flashing too close to a wall or another pipe. There is no room to weld or probe, and the detail is impossible to repair later.