Building a balcony

All the layers that make up our balcony

We’ve been building our balcony today, and that involves a number of layers of various materials that all come together to make a nice “whole”.

Our timber frame started off with a number of joists supporting an 18mm OSB board. We painted this with bitumen paint to attempt to stop the constant rain drips from the Cornish summer (which mostly worked).

Before building up the layers, we bolted large blocks of timber around the three outside edges of the balcony to provide the border, which will also stop water running off the end of the balcony.

To make sure the water runs off our balcony rather than pooling there, we started the build-up by installing firrings, which are basically timber wedges. The fall isn’t much, but it makes sure that water runs down. The firrings are attached through the bitumen-painted OSB into the joists below.

On top of this, we installed 18mm WBP plywood. This is a waterproof board designed for outside use (we also used this as the base layer of our roof) and this provides the main strength to everything on top of it. Again like the roof, the plywood is topped with breather membrane which stops most water but allows the layers underneath to breathe and stops condensation.

Next up is two layers of insulation. This brings up the level and also stops cold bridging, where cold materials meet warm materials and form condensation in between. The insulation sits in-between and stops this interaction.

Finally, we have 18mm tongue-and-groove OSB layed on top of the insulation, and screwed through it into the plywood below. This is the ideal base to fibreglass onto. The fibreglassing will finally make the balcony waterproof and give us some time to work out our next steps, such as what balustrades we want to have.

We decided to keep the fascias clean by going for a hidden gutter. That means making space inside the balcony itself for the water to run down and off. We cut back the top two layers by about 15cm to create a channel, and then installed a couple of long firrings running from side-to-side, to create a gentle slope to the right-hand side of the balcony. Here, we have a gap in the edge block which leads to the outside edge of the balcony, where a hopper will catch the water.

We’re on track to have all of this covered in fibreglass in the next few weeks, and we’ll post again once it’s done with the result!

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