Balcony drainage is one of the key areas to consider when designing balconies, however there are a lot of key terms used, and a lot of cross over of terms. The key drainage types to now know are 'Drained Through', 'Pipe Drainage' and 'Edge Drainage' as this is how it's categorised in the new BS 8579. However below are a number of the other terms which you may hear.
Controlled Drainage: Where incident water is collected and disposed of in one of two ways, with edge drainage, or piped drainage.
Edge Drainage: Incident water is collected on a surface and directed to an edge where it is
thrown clear of the building.
Piped Drainage: Incident water is collected on a surface and directed via gutters and/or outlets
to pipes in the building drainage system.
Drained Through: Essentially no drainage, so liquid on the balcony simple falls down to the one below. This would be achieved with no soffit, or with perforated soffits. Drained through is recommended against in BS 8579.
Positive Drainage: Incident water is drained via a RWP, usually the water is brought back towards the building the RWP running down the façade.
Free Drainage: Anything that's not positive drainage e.g Drainage through or pipe drainage.
Historically with the absence of anything else, NHBC's 6m2 rule for pitched rooves was often applied to balconies which is that 'greater than 6m2 in area should be provided with rainwater gutters and downpipes'. On the other hand, NHBC's clause 7.1 states 'balconies shall have adequate rainwater disposal to suitable outfall' indicating NHBC do not expect all drainage to be via RWP's. We have had our edge drainage design approved multiple times by NHBC on balconies over 6m2.
Additionally the new BS 8579 puts no area limitations on edge drained balconies, with the only limitation being that balconies with a greater plan depth (projection) than 2500mm should be pipe drained. BS 8579 also the highest point of the principle water collecting surface should be at least 75mm below the ingress level.
Our two key recommendations when it comes to drainage is 1. Take the water away from the building, and 2. drain to a linear edge, as opposed to one point.
How our system works is the soffit is also the drainage tray, draining the water, within the 400mm modules, to the front edge of the balcony.
From the cost point of view, adding a soffit and edge drainage is cost neutral with having no soffit and drainage through as without a soffit we then have to power coat all visual parts within the underside of the cassette. Piped drainage is significantly more expensive than controlled drainage, especially since all pipes now need to be Class A material.