Renovation & Design

On the level

Building a deck atop a sloping roof is laborious, but the results are worth it


After completion, the outdoor space seemed to look bigger with the composite top-decking when peering through the glass patio doors from the second-floor master bedroom, the roll-roofing that channels rainwater towards a downspout, now hidden underneath.

Every joist is custom cut to match the taper of the roll-roofing grade below, shimmed to allow for proper water flow and drainage towards the downspout.

During a summer building season filled with exterior upgrades, a client project that had been postponed the year before also made the roster. After years of using a 14-by-eight-foot second-floor balcony without any top-decking, this cosy little outdoor space finally received some much-needed attention.

Through sliding patio doors from the second level master bedroom, a quaint little outdoor area sheltered by enormous trees strewn throughout my client’s backyard is just barely visible. Once outside, the necessity for a balcony upgrade becomes clear — potted plants and chairs simply rest on roll roofing atop this hidden gem, graded to funnel rainwater towards a downspout roughly three-quarters of the way on one side, from the patio door wall. Needless to say, the ‘balcony’ experience is somewhat hindered by the uneven and unpleasant surface.

To remedy this unfinished area, while maintaining its water-evacuating purposes, a framework of graded joists upon which a proper top-decking surface can be fastened, would be required before any top-decking could be installed. Now, this is not as simple as it sounds. Due to the multi-tapered nature of the roofline, each joist must have a custom bottom-side cut, to match the roof taper of that specific location.

To determine the precise shape of the bottom side of every joist, a level-line is first indicated along all four perimeter walls and edges. The two outermost joists are then measured, precut and fastened to each end of the framing area. A long board spanning the entire length of the balcony is then balanced atop these two prefastened outer joists every 12 inches, allowing careful measurements to be taken for each of the inner joists, to be set on 16-inch centres.

Once the gap lengths between the bottom side of the straight board and the roll roofing below are recorded, the measurements are then systematically transcribed to each respective joist. The taper-matching cut along the bottom of each joist allows for shims — it is imperative that rainwater be allowed to flow towards the downspout, without pooling. If the joists rested directly on the roll roofing, the cavities between the joists would hold water, creating drainage issues and the potential for leaks over time, into the interior living space below. One by one, each joist was carefully set into position and shimmed accordingly to ensure the topside of the joists met the level lines along the perimeter walls and edges. Once the entire framework was confirmed level, installation of the top decking could begin.


A light-grey Trex product was chosen for this project, primarily because composite is virtually maintenance-free once installed. Starting along one long side, the first 14-foot two-by-six Trex board was set into position, perpendicular to the joists. Trex clips used to affix the boards to every joist are first loosely fastened, until the next board is aligned adjacent the previous. Then, the clips are tightened to hold the board firmly in place. This process continues using full boards until the space between the opposing wall and the second-to-last board reveals a gap that is less than the width of a board. This final board is then scribed to match the idiosyncrasies of the exterior wall and cut in that fashion along the outer-facing long side. The true potential of this outdoor space is finally realized once the composite top-decking installation process is completed — a hidden second-storey gem, nestled in the confines of my client’s backyard.

It is a laborious process to tailor every joist to an uneven surface below, simply to achieve a level top surface. And it is just as important to consider how these joists will affect water drainage. In the end, a good design will produce the desired effects, functionally and esthetically. Peering through the sliding glass patio doors after completion, I couldn’t help but notice the area seemed bigger, for some reason. Maybe it’s because this exterior space finally looked like a proper balcony — top-decking will do that.


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