Different balcony structures have different drainage requirements. Concrete balconies, for example, tend to be treated like a flat roof and are normally finished with a waterproof membrane, drainage outlet or overflow pipe. When using a metal balcony structure, which will often have a permeable deck, drainage options are interrelated to the use of fascias and soffits.
Fascia boards ensure a neat finish to a residential balcony and protect exposed edges from weathering. Soffit boards are fitted to the underside of the fascia to complete the clean-edged look and provide a weatherproofing seal. Typically, balcony fascias and soffits are manufactured from polyester powder coated aluminium for a durable, low maintenance solution.
Typical Soffit Options
PPC aluminium sheet: this is the most popular option for balcony soffits. Usually the fixings are concealed and the soffit is broken up into panels 400mm wide with neat butt joints, although this can vary.
Open soffit: this is a cost saving option which is not as neat in appearance and is only suitable for free draining balconies. With open soffits the balcony structure can be left as mill finish, or powder coated in a darker colour to make it less noticeable.
Decking soffit: some clients choose to clad the underside of the balcony with decking to match the top surface. This would normally be in a free draining situation where water falls between the deck boards.
Any water collecting on a balcony deck, either from rainfall or from watering container plants, should be addressed in the design to ensure it doesn’t drip onto the balcony below. Drainage of balconies is covered by Building Regulations and industry guidance as follows:-
The legal requirement under the Building Regulations is this:
Adequate provision shall be made for rainwater to be carried from the roof of the building
Approved Document H3 does not clearly state whether balconies are considered to be part of ‘the roof’, so this question is therefore down to individual interpretation. Different clients and different Building Control inspectors have their own views on what is required to comply with the regulation.
Section 1 has a table showing that roof areas under 6m2 do not require drainage, whereas larger roof areas do. This is interpreted by some to mean that balconies over 6m2 require drainage to an outlet. Others interpret this as applying only to roofs, not to projecting balconies.
Section 1.13 describes ‘Eaves Drop Systems’ where water is dripped from an edge, subject to certain considerations which are listed, primarily the need to protect persons using doorways below, and the need to protect the fabric of the building from water ingress.
NHBC guidance documents contain section 7.1 which is entitled ‘Flat roofs and balconies’. This guidance is a bit more specific than the Approved Document, as it uses the word balcony in several places, but it is still open to different interpretations.
In clauses such as 7.1D8, the guidance makes a distinction of ‘balconies functioning as roofs’. By inference, balconies not functioning as roofs do not have the same requirements, but this is not clearly stated.
Clause 7.1 D9 states that ‘balconies shall have adequate rainwater disposal to a suitable outfall’. It does not go into detail of what constitutes a suitable outfall. It then speaks of the ‘cumulative effect of water discharging from multiple balconies in vertical alignment’. This clearly indicates that NHBC expect that in some cases water will be discharged into the open, not into a RWP.
The guidance also contains a comment that ‘Open slatted balcony decking should drain away from the dwelling’. This is open to interpretation.
Appendix 7.1-D focuses on balcony drainage, and shows 3 typical sections. However, these are all concrete balconies with upstands which could trap water and form a pond, which is not relevant for metal balconies. None of the sections address how metal balconies should be drained. An NHBC publication in February 2010 explains that the drawings show concrete as ‘these are potentially more difficult to meet with the principles, rather than freestanding balcony structures (typically constructed from steel)’. This infers that requirements for metal balconies are less onerous as there is less risk of water penetration, but no detail is given.
Section 7.2 – D15 says ‘Roofs greater than 6m2 in area should be provided with rainwater gutters and downpipes’. It does not mention balconies.
Main Factors In Making a Decision
Cost – free draining balconies are the lowest in cost. The addition of a drip tray typically adds around 10%. Incorporating positive drainage to a Rain Water Pipe (RWP) typically adds a further 25% and also makes the construction significantly more complex.
Appearance – free draining balconies tend to look quite industrial, as the structure and decking are usually visible when looking up at the building. The addition of a soffit/drip tray greatly enhances the appearance. The addition of a RWP is typically a visual detriment, unless it can be incorporated within the façade, which adds complexity. Also, balconies draining to a RWP need to be significantly deeper to accommodate the pipework, which makes them look more chunky.
Safety – with free draining balconies, there is a risk of hot liquids being spilt on a balcony and falling through gaps onto residents below. A drip tray reduces this risk by draining water to the edge of the balcony. A RWP eliminates the risk altogether.
Façade staining – every situation will be different, but with free draining balconies, there is an increased risk that water will track along the support arms and run down the façade, giving a greater likelihood of staining. Positive drainage is likely to reduce this risk.
Other Points to Consider
If there are no balconies, rainwater falls directly to the ground, or runs down the façade. Adding free draining projecting balconies to a building does not increase the amount of rain which reaches the ground.
Some residents like to think of a balcony as a covered storage space where items do not get too wet. Other residents like to think of a balcony as an outdoor garden where they can grow plants, and rainwater is welcomed.
Grooved decking boards are commonly used as a surface. Experience has shown that the majority of rainwater falling on a decking board will run along the grooves and drip off the end of the board. For this reason it is generally better to run decking parallel with the building line, otherwise there can be a substantial amount of water dripping of adjacent to the façade, giving an increased risk of staining or water penetration.
A stack of balconies can be considered somewhat like a tree. When it starts to rain, the balconies (like leaves and branches) trap and collect water, so you would get less wet standing underneath than you would when standing in the open. However, if rain persists, water will start to drip in some places, not a very even pattern. After the rain has stopped, water will continue to drip for a while, like it does from a tree.
Taking water from projecting balconies back inside the building envelope to a RWP generates the risk of water ingress if anything goes wrong or gets blocked. Some clients believe drainage to a drip edge is therefore a more satisfactory long term solution than draining to a RWP.