The door sill interface (the junction between the decking and the door sill) is often the subject of discussion between trades. We have categorised solutions into these options:
The first door sill interface option is utilising a walk on structural support to the door sill avoids the cutting in of decking boards (when butted up to the sill). However, some clients choose to get deck boards cut in to give a more consistent aesthetic appeal.
Such boards can be supported from either a structural door sill or from brackets fixed back to the rear member of the Cassette® frame.
For recesses deeper than a single deck board width (i.e. > 150mm), support is required from the building or façade structure. When supporting it from the building, we can incorporate support from the cast in anchors. Whether or not this is independent of the cast in element this must be thermally broken and properly weatherproofed.
The Approved documents Part M guidance, British standard, NHBC and Lifetime homes guidance all look for buildings to be designed with door sills which allow less able, or wheelchair users to easily access balconies. NHBC guidance, for example, states that “door sills must project a minimum of 45mm along with a maximum upstand of 15mm at the door threshold. “This is to allow minimal obstruction.
Synergistic consideration is needed for deck finishes heights, internal finishes and fenestration. The substrate to which the balcony connection is fixed is typically finished with insulation, floor screeds, etc. along with final finishes, which raise the FFL above the height of the arm. Cassette® balconies have therefore been designed so that the balcony sits at approximately the right height, with the specialist stirrup clamp enabling minor site adjustment to be made.
This can sometimes avoid the need to have a stub with a staggered joint from an anchor (as typical with bolt-on balconies). Staggered joints increase the moment, thus making the balconies deflect more and feel bouncy. A few door sill interface examples can be seen in the door sill interface section.
NHBC Decking Guidance
The NHBC guidance in relation to decking and paving layout and spacing was commonly considered as difficult to understand and could be interpreted in various ways. This confusing has generally been addressed by the new NHBC guidance, which clarifies decking gap restriction in three parts:
- Gaps should be provided between decking and paving at balcony perimeters.
- Minimum 10mm gaps should be provided between individual units of decking or paving, and the threshold sill, perimeter walls, and kerbs.
- Spacers and supports which raise decking or paving should not obstruct the flow of rainwater to outlets.