The slope plan comes before the membrane
Drainage is settled under the membrane, before it goes on. This step decides flat roof drainage for years to come. We face one of two cases.
- The water already drains well. The existing slopes do their job. Insulation of the same thickness everywhere is enough, and the roof keeps its shape.
- The water lingers or the slopes have warped. We plan tapered insulation, over the whole surface or only where the water stays.
An isolated dip does not mean waiting for the roof replacement. We rework it separately.
Sloped framing or cut boards
A flat roof gets its slope from its framing or from its insulation.
The framing. On some older buildings, the roof joists slope slightly down toward the middle of the building. The top-floor ceiling, for its part, hangs lower and stays level. Between the two is an attic space.
The insulation. The deck is roughly level. Rigid boards of varying thickness are laid on it, and their top face forms the slope.
Each tapered insulation board is thicker on one edge than on the other. Laid out according to a plan, they form inclined planes that converge on the drain.
A roof that rises
All added insulation raises the surface of the roof. This rise is what most limits the correction, in three places.
- Around the edges. The membrane has to climb high enough on the parapets. If the roof rises too much, the parapets have to be raised.
- At openings. The low sill of a terrace door or a skylight can end up too close to the surface.
- At the drain. The drain is raised or replaced, otherwise it would no longer be at the low point.
Insulating at the same time
Slopes made of insulation add insulation to the roof.
The thin side. Near the drain, the tapered board has almost no thickness left. For the roof to be well insulated everywhere, a uniform layer is often installed under the tapered boards.
Water vapour. On some old plexes, the original insulation is in the attic space, under the deck. Adding some on top changes how the roof behaves. The position of the vapour barrier and the choice of a ventilated or unventilated attic space are for someone who knows these assemblies. A poorly thought-out roof can let condensation build up in its structure.
The insulation required for your work depends on the building code. Check with the RBQ (Régie du bâtiment du Québec, Quebec's building authority) or with the City of Laval's Service de l'urbanisme (urban planning department), rather than relying on a thickness read somewhere.
Frequently asked questions
Is tapered insulation needed on every flat roof?
No, tapered insulation is only used when water drains poorly. If water stays on yours after rain, tell us in your request, before the roof replacement is planned.
Can the slopes be corrected without a full roof replacement?
Yes for a limited dip. We open the area and rebuild it. For the whole roof, a roof replacement is needed. The insulation is under the membrane.
Can the old insulation stay in place?
Old insulation generally does not stay in place if it is wet. Damp insulation insulates less well and can rot what it touches. Dry and still firm, it is sometimes kept.