Decks, Balustrades And Level Thresholds

A door onto a deck seems like a simple detail until it has to be level.

A conventional threshold has a step, and the step is what keeps water out. Remove the step for wheelchair access and the water has nowhere to stop, which is why this detail causes more leaks in accessible housing than anything else.

What a level threshold requires

Four things, all of which have to work together.

**A recessed or low profile sill,** so the frame does not create a lip.

**The external surface falling away** from the opening, continuously, with no flat section against the building.

**A drainage channel immediately outside the door,** grated and connected to a stormwater outlet, so that water driven at the opening is intercepted before it reaches the sill.

**A continuous membrane** turned up behind the door frame and dressed into the channel, so the water that does get in is caught by the waterproofing rather than the carpentry.

Leave out any one of the four and the detail fails. The channel without the fall silts up. The fall without the membrane leaks at the frame. The membrane without the channel is holding back standing water.

The waterproofing principles behind it are the same ones in [waterproofing in a factory built building](/blog/waterproofing-in-a-factory-built-building).

The level has to be set at design stage

The module arrives with its floor at a fixed height. The deck is built on site. The two have to meet.

Which means the finished deck level, the module floor level and the drainage channel invert are all calculated before fabrication, with the thickness of the deck surface, the structure under it and the fall all accounted for.

Getting that wrong on site means either a step the brief said would not exist, or a deck built up against the building with no fall. Both are expensive corrections, and both come from the levels not being resolved in the design stages described in [the eight design stages explained](/blog/the-eight-design-stages-explained).

It is also why the site survey has to be accurate, because the external levels depend on the ground, as covered in [the site survey and what it must show](/blog/the-site-survey-and-what-it-must-show).

Deck surface and what it has to do

A deck in supported accommodation is a path of travel, which changes the selection.

**Slip resistance when wet,** rated rather than assumed, because the surface will be used by somebody with limited balance or a mobility aid.

**Gaps narrow enough** that a castor or a walking frame foot cannot drop in. Standard decking gaps are too wide for a wheelchair to cross comfortably.

**Flat and even,** without the cupping and movement that timber develops, because a raised board edge is a trip hazard and a wheel obstruction.

**A turning circle** large enough for a wheelchair to turn and for the door to be opened from a seated position, which is more area than most decks are given.

Balustrades

A balustrade is a structural element and a safety one, and it carries a horizontal load applied at the top rail.

Which means the fixing matters more than the balustrade. On a modular deck, the posts are fixed to the deck structure rather than to the module, and that structure has to be designed for the load rather than built to a standard detail.

Three things decide the design: the height required for the drop below, the opening sizes so nothing can be climbed or passed through, and whether the infill is transparent or solid. Transparent gives a seated resident a view out; solid gives privacy. The choice usually comes down to whether the deck is overlooked.

Where the balustrade also acts as a handrail, the profile has to be graspable for the whole length rather than a flat capping.

Covered or open

A deck that is exposed is unused for most of the year.

Which makes a roof or an awning over at least part of it a functional decision rather than an upgrade, particularly for a resident who spends time outdoors and cannot move quickly indoors when weather changes.

A covered deck also protects the threshold, which reduces how hard the drainage detail has to work.

It is site work, so it is sequenced

The module is installed, then the deck is built to it, then the thresholds are completed and tested.

Which means the deck sits in the site programme rather than the factory one, alongside the work in [landscaping and external works](/blog/landscaping-and-external-works), and the water test on each threshold happens before the handover rather than at the first storm.

Our standards. Discuss a project.

MADEmodular / MADEbetter.