Where the slope of a flat roof comes from
There are two ways to give a roof its slope, and both serve flat roof drainage.
Through the framing. In many older plexes, the roof joists are set at a slight tilt toward the centre of the building. The top-floor ceiling hangs level, lower down. The space between the two forms an attic.
Through the insulation. The deck is level, or nearly so. Rigid boards of varying thickness are laid on top, and their surface shapes the slope.
Tapered insulation
A tapered insulation board is thicker on one side than the other. Assembled according to a plan, the boards form sloped surfaces that meet at the drain.
Correcting the slopes at replacement
Good drainage is decided before the membrane goes down.
The approach depends on the existing slopes.
- The existing slopes are good. Water already drains well. Insulation of even thickness can go down and the roof keeps its shape.
- The slopes are weak or distorted. Tapered insulation goes down, over the whole roof or over the problem areas.
Height constraints
Adding insulation raises the roof surface. Height is the main limit of the exercise.
- The parapets. The membrane must run high enough up the surrounding walls. If the roof rises too much, the parapets have to be raised.
- The sills. A patio door or a skylight with a low sill can end up too close to the surface.
- The drain. It must be raised or replaced to stay at the low point.
The link with insulation
Redoing the slopes with insulation also means insulating.
Minimum thickness and moisture call for attention.
Minimum thickness. Tapered insulation is thin near the drain. For the whole roof to be well insulated, a layer of even thickness often goes under the tapered boards.
Moisture. In many older plexes, the insulation sits in the attic, under the deck. Adding more on top changes how the roof behaves. A well placed vapour barrier and the question of the attic, vented or not, must be settled by someone who knows these assemblies. A badly designed roof can build up condensation in its structure.
The insulation requirements that apply to your work fall under the construction code. Check them with the RBQ or your borough rather than relying on a thickness you read elsewhere.
Frequently asked questions
Does my roof need tapered insulation?
A roof needs tapered insulation only if water drains poorly on it. If puddles sit on it, tell us before the roof is replaced.
Can the slopes be corrected without redoing the whole roof?
Yes, for a localized dip. We open the area and rebuild it. For a whole roof, the correction goes through a roof replacement, since the insulation sits under the membrane.
Should the old insulation be removed?
Wet old insulation is removed without exception. Damp insulation no longer insulates and rots what surrounds it. Dry, firm old insulation can sometimes be kept.