TPO and PVC Roofing Explained: Membranes, Thickness, Welded Seams and Where Each Fits

Step by Step

1. 1Step 1: Understand what the membrane is made of

   Both membranes are built in layers. The top ply carries the UV stabilizers, pigment and flame retardants. The middle layer is a woven polyester scrim that stops the sheet from stretching or shrinking and holds fasteners in a mechanically fastened system. The bottom ply protects the scrim and gives the welder something to melt. Because the plies are thermoplastic, they soften under heat and solidify again when cooled, which is what makes hot-air welding possible.

   Some TPO is made with a fleece backing already laminated to the underside, used when the membrane is adhered over the substrate. PVC and TPO both come in unreinforced versions for flashings and pre-molded accessories. The reinforced sheet is for the field of the roof. The unreinforced flashing is softer and forms around corners.

2. 2Step 2: Choose the thickness in mils

   A 45 mil membrane is the lightest common option and is used on roofs with little traffic. A 60 mil membrane is the everyday choice for commercial and larger residential flat roofs. An 80 mil sheet is chosen where there will be foot traffic, mechanical equipment, sundeck use or the risk of dropped tools, because the extra thickness sits above the scrim as wear layer.

   Thickness is also a warranty issue. Many manufacturers tie the longest warranty periods to 60 mil or 80 mil sheets, so the thickness has to be confirmed on the manufacturer's current specification and matched to what the project is actually being warranted for. Do not assume a thicker sheet can be welded the same way: welder temperature and speed change with thickness.

3. 3Step 3: Pick the sheet width and attachment method

   Mechanically fastened systems use wide sheets, often 10 feet, with fastener plates running down the lap. The plates secure the sheet and the next sheet covers them. This is the fastest and usually the lowest cost method and is covered in how to install mechanically fastened TPO.

   Fully adhered systems bond the whole sheet to the substrate with adhesive. They are chosen where fasteners would be a problem, where the deck cannot be penetrated, on curved or complex roofs and on roofs with lots of detail. See how to install fully adhered TPO. Ballasted systems hold the sheet down with stone or pavers and are rare on wood-framed buildings because of the weight.

4. 4Step 4: Compare TPO and PVC for the building

   TPO has a large share of the flat roof market because it is white, reflective, welds well and is generally priced below PVC. Formulations vary between manufacturers more than they do for PVC, so the product name and manufacturer's track record matter more than the word TPO.

   PVC holds up better in contact with animal fats, cooking oils and many chemicals, which is why it is common over restaurants and industrial buildings. It has good fire performance and is very flexible around details. Its weakness is plasticizer loss over long service, which can leave the sheet stiffer with age, and it is sensitive to contact with asphalt, coal tar and polystyrene insulation. A separator sheet or compatible insulation and cover board is required where contact is possible. TPO and PVC cannot be welded to each other, so a repair must match the existing membrane.

   Tip: When a roof is being repaired or extended, find out whether it is TPO or PVC before ordering anything. A sample cut from a flashing edge can be identified by the manufacturer or supplier.

5. 5Step 5: Check the substrate and slope

   Thermoplastic membranes are installed over rigid insulation, usually polyisocyanurate with a cover board such as high-density polyiso or gypsum-based board, over a structural deck of plywood, steel or concrete. The insulation has to be compatible with the membrane and fastened or adhered to the deck. Wet or damaged insulation must be removed, not covered.

   Slope matters. Membrane manufacturers typically require at least 1/4 inch per foot for warranty, and many buildings have much less than that, which is why tapered insulation and drainage planning come first. Ponding water shortens the life of any membrane and voids many warranties.

6. 6Step 6: Know what a good weld looks like

   A properly made weld shows a slight, even bead of melted material, sometimes called squeeze-out, along the edge of the top sheet. The membrane surface near the weld is smooth, not brown, blistered or glossy. On many products the minimum weld width is 1-1/2 inches, and on most it must be confirmed on the manufacturer's specification. The edge of the top sheet is checked with a rounded probe after it cools, and any spot that lifts is reheated.

   Welding is governed by test welds and peel tests made at the start of each shift and when conditions change. The full method is in how to hot-air weld TPO seams.

7. 7Step 7: Plan the details before the field

   The field of a thermoplastic roof is fast, and the details are slow. Budget time for wall, curb and corner flashing, penetrations and coated edge metal. Use pre-molded accessories from the same manufacturer wherever they exist.

   Also plan drainage: locate each drain, scupper and overflow, and confirm the insulation taper brings water to them. See how to install a flat roof drain.

TPO Versus PVC in Short

Colour and Heat in the Fraser Valley

When a Thermoplastic Roof Is the Wrong Choice

Common Mistakes

• Mixing TPO and PVC. The two materials will not fuse, so a patch or flashing of the wrong type peels off and leaves an open joint.
• Welding without test welds. Heat and speed drift with temperature, wind and the power supply, and a roll of seam made without a test is a roll of guesses.
• Ignoring the slope. A roof that holds water for days will collect dirt and algae, age faster and often falls outside the manufacturer's warranty terms.
• Putting PVC against asphalt or polystyrene. The plasticizer migrates and the membrane goes brittle. A separator or compatible board is required.
• Skipping the cover board. A membrane over soft or crushed insulation is punctured by foot traffic and fails at fastener plates.
• Treating the details as an afterthought. Most leaks on thermoplastic roofs are at corners, penetrations and edges, not in the field.