Installation of Fatrafol PVC Sheet Waterproofing (Flat Roof)

Roofing PVC membrane is made from plasticised PVC. It is usually reinforced with a polyester mesh, which makes it stronger and stops it from shrinking as the roof heats up in summer and freezes in winter. Membranes made for exposed use resist UV light, so they can lie in direct sun. A common thickness is around 1.5 mm. Roofs that take more wear often get a thicker sheet, for example 2 mm.

Your roof will usually be in one of three situations:

• New roof or full replacement: the old build-up is stripped and a new one is laid, including vapour control layer and insulation
• Overlay: the existing roof is dry and sound, so the membrane goes on top over a separation layer
• Local repair: most of the roof is fine and only one detail or damaged area needs work

There are several ways to hold the membrane in place. Mechanical fixing is the most common, with fixings that hold the sheet down to the deck. On a ballasted roof, gravel, paving on pedestals or the layers of a green roof hold it down. Adhered membrane is used where there is nowhere to put fixings. The right choice depends on the roof structure, the height of the building and whether people will walk on the roof.

The process is always similar. A mistake at any step tends to show only at the first heavy rain.

1. Inspect the roof and check the existing layers for moisture, for example with core samples or a moisture meter.
2. Clean the surface and remove sharp bumps, loose material and gravel.
3. Lay a separation and protection layer, most often geotextile.
4. Roll out the membrane according to the layout plan, with the overlap the manufacturer specifies.
5. Fix the membrane with the number and spacing of fixings set by a wind load calculation.
6. Weld the joints with hot air, usually with an automatic welder on long straight runs.
7. Form the details: parapets, outlets, penetrations, corners and abutments with walls.
8. Fit the metal trims, mainly PVC-coated sheet metal on the parapets, which the membrane is welded to.
9. Test every weld and repair any faults.

Why doesn't the membrane go straight onto bitumen or polystyrene? PVC reacts badly with both. The plasticisers slowly migrate out of it, and the membrane hardens and eventually cracks. A separation layer keeps the materials apart.

The shape of the roof drives the workload more than the area does. A large flat roof with no penetrations goes quickly because the automatic welder does most of the work. Every chimney, vent pipe, rooflight or internal corner has to be done by hand.

• The work goes faster on a large open area with good access and a dry, firm existing roof
• Complicated parapets, many penetrations, new outlets and a lot of metal trim add time
• Wet insulation that has to be stripped out and replaced adds a lot of extra work
• Poor access that needs a crane or lift, and tall buildings that need more fixings, also make the job longer

Weather matters too. The membrane has to be welded in dry conditions and within the temperature range the manufacturer allows. Joints do not hold on a wet or frozen surface, so work is often put off in winter or during long wet spells.

Most leaking membrane roofs fail because of workmanship, not the material. These faults come up again and again:

• Cold welds that look sealed but never fused because the temperature was too low or the welder moved too fast
• Membrane laid over a wet build-up, which traps the water and damages the roof from below where nobody sees it
• Too few fixings, especially at corners and edges where wind suction is strongest, so a storm can lift or tear the sheet
• No separation layer between the PVC and bitumen or polystyrene
• Rushed details around outlets and penetrations that leak even when the main area is perfect
• Not enough fall, so water and dirt sit on the roof

The manufacturer's guarantee often depends on the roof being fitted by a contractor they have trained. Before you sign anything, ask whether the installer has that training and what guarantee they give on their own work.

Replacing the waterproofing without changing the shape or height of the roof usually counts as maintenance. Rules differ from place to place, though. If you plan to change the build-up, add thicker insulation, raise the parapets or create a roof terrace, check with your local building authority first. Do not assume you need no approval at all.

In a block of flats, the roof is a shared part of the building. The owners decide on the repair, and the building management or owners' association arranges it. A single flat owner cannot order it alone.

This is work at height with hot-air welding equipment. It belongs with a company that uses fall protection and knows this system, not with a homeowner. If gas or electrical lines pass through the roof, any changes to them must be made by a qualified, registered professional.

Walk the roof with the installer at handover and ask about anything that is unclear:

• The membrane lies flat, with no ripples, blisters or visible damage
• The welds are continuous, with a thin bead of melted material visible along the edge of each seam
• The seams have been tested with a probe, or with a spark test where the build-up allows it, and faults have been repaired
• There are no creases or loose areas around outlets, penetrations or corners
• The parapet trims are firmly fixed and the membrane is welded to them
• Water drains to the outlets after rain without leaving large puddles
• Outlets and downpipes are clear and fitted with leaf guards
• You have received a guarantee for both materials and work, plus a description of the roof build-up