Before the membrane
We start by preparing the substrate. Depending on the job, we tear off the old roofing, replace rotten deck boards, and lay a vapour barrier, then the insulation and a cover board.
The surface must be dry, clean and solid. An elastomeric membrane laid on a damp or soft base will age badly, however much care goes into the rest.
The base sheet
The base sheet is rolled out in parallel strips, starting from the low point so the overlaps follow the flow of water. Each strip overlaps its neighbour by a few centimetres.
Depending on the product and the substrate, the base sheet is torched, glued or held with fasteners.
The cap sheet
The cap sheet goes on top, with its seams offset from those of the base sheet.
In a torch installation, we melt the underside of the roll as we unroll it. The liquid bitumen fuses with the base sheet. A thin black bead along the seam shows the weld is complete. In a self-adhesive installation, we peel off the film and press the sheet onto the base sheet.
Torch or self-adhesive
Each of the two methods has its demands.
- Torch applied. This is the classic method. The flame gives a solid weld, even in cool weather. Its drawback is the open flame, working close to dry wood, old insulation and neighbouring walls. The work requires protecting sensitive areas, keeping fire extinguishers at hand and watching the roof after the work ends.
- Self-adhesive. The membrane carries an adhesive protected by a film that is peeled off during installation. No flame is involved. In return, the adhesive needs a very clean substrate and a high enough temperature to bond well.
Ask us which method we plan to use, and for proof of insurance for torch work.
Details at the parapets
The parapet is the low wall that rises around the roof. The membrane leaves the horizontal there, and most leaks on badly done roofs are found at the parapets.
A careful installation includes:
- a cant strip at the base of the parapet, so the membrane does not bend at a right angle
- a base sheet flashing that runs up the wall
- a cap sheet flashing, laid in separate pieces from the roof field
- an upstand high enough to clear the level that water or melting snow can reach
- metal flashing that caps the top of the parapet and keeps water from entering from behind
Frequently asked questions
How long does installation take?
Installation takes a few days for an ordinary plex, depending on the surface, the tear-off and the weather. The deck sometimes holds surprises that lengthen the job.
Is torch installation dangerous for my building?
Torch installation carries a real fire risk, which trained crews manage with known precautions. Ask us which measures are planned and for proof of insurance that covers this work, as you would any contractor. Your own insurer can also tell you what it requires.
Can a membrane be installed in cold weather?
Torch welding tolerates cold better than self-adhesive products. Each manufacturer sets its own temperature limits. If the work is planned late in the season, ask us how we intend to respect those limits.