Composite Decking Roof Terrace in Dore Sheffield

Why Composite Decking?

The client wanted an attractive, modern surface for the roof of their extension to make it a usable space.  There needed to be as seamless as possible transition from the living room, through the sliding doors onto the extension roof.  The two key problems to overcome were; 1.) the GRP roof could not have anything physically attached to it in order to retain it’s integrity, and 2.) the balustrades had been installed before the finished surface was decided upon, leaving only 100mm of height clearance for the finished surface in order that the height of the balustrade still met building regulations.

The Roof Terrace Construction:

Due to the low profile nature of the aluminium joists they needed supporting on pedestals every 400mm.  The composite decking boards also require that the joists are spaced at a maximum of 400mm intervals.  This required us to space the pedestals in a 400mm grid pattern across the roof, taking great care to ensure an even level running perpendicular to the decking, and an accurate and even fall running parallel to the decking. 

The standard fall to achieve water run-off on composite decking is between 1:100 and 1:80, which over a deck this size would have meant a fall of between 43 and 54mm.  However, to meet the design brief of running true with the door sill but still coming in 1.1m below the balustrade height we were required to have a fall of 65mm along the deck, making it slightly steeper than we would normally install, but still a perfectly useable surface.

In order to further protect the GRP roof surface, and also to deaden sound in the games room in the extension below, we placed 3mm rubber matts beneath each pedestal.  In placing the joists we were careful to stagger the joins so as not to create weak ‘seams’ where the joists butted together.  Where the decking boards and the trim pieces had to be cut, eg: at mitred corners or around the balustrade legs, consideration was given to the ambient temperature at the time of installing.  In this case, the temperature was hovering around 2 degrees C, so larger 4mm gaps were left to allow for the boards and trim expanding in warmer temperatures.

The Result:

The client now has an attractive, contemporary roof terrace ready for the glass to be installed in the balustrades, which still meet the required height of 1.1m above the finished surface.  Their terrace is now a perfectly suited and safe space for entertaining, ready for the summer months, with the additional peace of mind that the GRP roof beneath is unaffected and the sound of guests enjoying the terrace is insulated from the games room below.