How to Install Fully Adhered TPO: Adhesive, Flash-Off Time, Rolling and Welded Laps

Step by Step

1. 1Step 1: Check the substrate and the weather

   The substrate is usually a cover board, such as high-density polyiso or gypsum-based board, over insulation and the deck. It must be dry on the surface, free of dust, oil, old adhesive and sharp edges, and firmly attached. Boards that are warped, curled or crushed at the joints will telegraph through the membrane. Fix them now.

   Read the adhesive can and the manufacturer's installation specification for temperature and moisture limits. Many solvent-based adhesives list a minimum of about 4 degrees C, and water-based adhesives are usually more restrictive and must not freeze, but the real limits differ by product and must be confirmed on the label. In the Fraser Valley, fall and winter mornings are damp and cold enough that the substrate can be below the dew point even when the air looks dry.

   Watch out: If the surface is damp, dew covered or colder than the adhesive allows, stop. Adhesive applied to a damp substrate never reaches full strength and the sheet blisters and lifts later.

2. 2Step 2: Lay out the sheets and roll them out dry

   Plan the layout so laps run with the slope where possible and so sheets are not cut into tiny pieces at edges. Fully adhered sheets are commonly 10 or 12 feet wide, which reduces the number of seams.

   Position the sheet, let it relax for a few minutes so wrinkles fall out, and then fold it back on itself to the halfway point, exposing half of the substrate. Work in halves, or in sections the crew can finish before the adhesive skins over.

3. 3Step 3: Apply adhesive to the substrate and the sheet back

   Stir the adhesive, and use a medium-nap roller to lay down an even film with no puddles and no bare patches. Coverage is typically stated in square feet per gallon per surface on the can, so check that you are actually reaching it. Porous substrates may need more than one coat.

   Keep adhesive off the lap edges. A 3 to 4 inch strip along each seam stays bare so the sheets can be welded, because adhesive contamination is the most common reason an adhered sheet will not weld. Mark the strip with a chalk line or tape before starting.

   Some systems use a different method: polyurethane adhesive or low-rise foam is applied to the substrate in ribbons or with a notched tool and the sheet is laid into it without a flash-off time. Follow the manufacturer's method exactly, because the two families behave differently.

4. 4Step 4: Let the adhesive flash off

   Solvent-based bonding adhesive needs time for most of the solvent to leave before the two coated surfaces are joined. Touch a clean, dry knuckle to the film: it should feel sticky but no adhesive should come away on the skin, and it should not pull into strings. Too wet and the solvent gets trapped under the sheet and blisters. Too dry and the film has lost its grab and will not bond.

   Conditions change this window quickly. A warm windy afternoon can skin the adhesive in ten minutes. A cool damp morning can take thirty, or never reach a workable state. Test every section, not just the first.

   Tip: Keep the amount of adhesive applied at any time small enough for the crew to roll in before it dries. A second crew member rolling as the first one applies keeps the timing under control.

5. 5Step 5: Roll the sheet into the adhesive without wrinkles

   Start from the fold and roll the sheet forward onto the substrate while one person holds the sheet off the surface ahead of the contact line. Never let the sheet touch down and then drag it. Once adhesive touches adhesive, it grabs. Broom as you go, from the centre outward, to push air out and press the film together.

   Repeat for the other half of the sheet. Wrinkles are nearly impossible to remove after contact, so keep the sheet aligned with the chalk line as it lays down.

6. 6Step 6: Roll the membrane with a weighted roller

   Broom pressure alone does not set the film. A weighted roller on a stiff handle presses the membrane onto the adhesive across the sheet. Roll in two directions and pay attention to edges and details, where the crew walking the roof has to hand roll with a smaller roller.

   Cold, damp or windy conditions can cause the adhesive to lose its grab before rolling is finished. If so, stop and address the conditions, and do not keep applying adhesive to a roof that is not bonding.

7. 7Step 7: Weld the laps and probe the seams

   Laps on an adhered sheet are commonly 3 to 6 inches and are welded exactly as they are on any other TPO roof. Wipe off any stray adhesive, dust or moisture with cleaner and a clean rag. Weld after a test weld and peel test, following how to hot-air weld TPO seams.

   Probe every seam along its full length when cool. An adhered membrane can hide a seam defect under a flat, smooth surface, so probing is the only way to know the weld is continuous. Mark, reweld and reprobe any gaps.

8. 8Step 8: Finish the details and check for bubbles

   Adhere or weld flashing as the detail calls for. See how to flash TPO walls, curbs and corners and how to flash TPO penetrations. Vertical surfaces are often adhered with contact adhesive and terminated with a bar and sealant.

   A small bubble, one that shows immediately after rolling, can usually be cut, vented and patched with a welded piece. A large area that lifts the next day usually means trapped solvent, moisture or an adhesive applied too thin or too dry. Fix the cause, not just the bubble.

Smooth-Back Versus Fleece-Back TPO

Cold Weather Notes for the Fraser Valley

Common Mistakes

• Applying adhesive to a damp or cold substrate. It never cures properly and the sheet blisters and peels, often months later.
• Joining the surfaces too wet. Trapped solvent forms bubbles under the sheet and weakens the bond.
• Joining the surfaces too dry. The adhesive has lost its tack and the membrane only appears to stick.
• Adhesive on the lap strip. Contamination stops the welder from fusing the sheets, leaving a seam that looks closed but is not welded.
• Skipping the weighted roller. Brooming alone leaves soft spots and voids that open up under foot traffic.
• Using the wrong adhesive for the substrate. Some facers, old asphalt and polystyrene react with solvent adhesives, and each needs an approved system.