Garden Office Pt 2 - Walls & Roof

This is the second post in a series about the design and construction of my garden office. You can read part one here.


Wall composition

Just like the floor frame, we made the walls out of sections with each section comprising:

We spaced the wall studs on 600mm centres to ensure attaching the plasterboard would be straightforward later.

Rear walls

We started with the "rear walls", one long side and one short side, that run along the fence. On the one hand, these walls were relatively simple as they didn't contain doors or windows. However, due to their proximity to the fence, we knew that we wouldn't be able to do any work on the outside of the walls once they were erected -- this meant we had to build these walls "complete" and then erect them.

Building the walls completely before erecting them introduced challenges:

One option we considered was to build the corner (two wall sections) ex-situ and then slide it into position. By building it ex-situ we would have access to both sides of the walls and could ensure that it was perfect and sealed before sliding it into position. However, a couple of points prevented us from taking this approach -- the weight and the overhang. The external OSB and cladding needed to extend below the floor frame to prevent water ingress, this compounded with the weight meant it would be basically impossible to slide/move it without damaging the overhang.

In the end, we decided to erect the two sides of the corner and seal it later.

We applied a generous amount of silicon to each wall section join before tightening the screws as an extra precaution against water ingress.


As the rear walls were not visible, I chose to use cheaper plastic cladding which saved a fair amount of money.

I learned that you shouldn't fit cladding directly onto the outside of a wall. Instead, you should first attach battening, and attach to the cladding to that -- this is to allow air to flow behind the cladding.

As this cladding was designed to be fitted horizontally, the batten had to be fitted vertically. Given that we wanted vertical airflow, this was ideal.

Front walls

Even though the front walls would contain windows and doors, they were a whole lot easier than the rear walls as we could easily access both sides.

We added additional timbers to frame the door openings. We didn't do the same for the windows at this time as I wasn't sure exactly where I wanted the windows to be and this was a decision we could delay easily.

It didn't take us long to get these walls built and joined because we could add the wrap after they had been erected, plus we didn't need to clad these walls yet -- this was a job for much later.


Roof frame

A key decision that remained was how to slope the roof. Ultimately, one side needed to be higher than the opposite, but there were a few things to consider that had both practical and aesthetic ramifications.

One common approach is to build one wall higher than the other and then attach a roof that is uniformly deep on top. But, when looking at the roof slope, it would appear wonky relative to the other straight lines. In my case, this would mean the bottom of the roof and the top of the door wouldn't be parallel, I didn't like that. Plus, the interior ceiling would slope too, I didn't like that either. We could have found a way around the interior sloping ceiling but we would be wasting valuable interior height.

We opted for a more complicated solution that would give me what wanted, a flat interior ceiling and a roof that was parallel to the door frame at the bottom. To achieve this, the walls would be the same height but the roof joists had to slope instead.

One way to slope the joists is by attaching firring strips (long wedges that attach to the top of the joists). This option is expensive, but more importantly, it wasn't possible due to the height limitations of the building. We built the walls with a specific height factoring in all of the roof specifics, assuming that we would use 145mm joists. Adding firring strips would have made the building too tall.

We opted to cut the joist timbers, sloping them from 145mm to 100mm. The offcuts from the joist timbers were themselves firring strips (45mm to zero) that we later sold.

Before installing the OSB on top of the frame, we drilled holes in each of the joists to allow for airflow between the OSB and the insulation. In the end, air will be able to flow up the wall behind the cladding, through the soffit into the roof cavity, and then along the roof through the joist holes and out the other side.

We used a thicker 18mm OSB for added rigidity.


We used an EPDM rubber roof, basically a huge single sheet of rubber that covers the entire roof. It is tough, easy to fit, and as a single piece there is a reduced chance of leakage (except in case of it being pierced).

It was extremely heavy and we strugged to lift it onto the roof. We decided to take it upstairs in the house, make a bridge between the house and the office and slide it over.

We set it out and let it set for a couple of days before glueing it down.

Soffit, fascia, guttering

Fitting the soffit on the long wall was quite tricky, it came in 5m lengths and was extremely bendy. We used plasterboard stands to hold it up whilst we nailed it on. Once the plastic trimmings were attached we glued the edge of the EPDM down with a strong glue and then added the final EPDM plastic upstand on the outside, sealing the EPDM behind it.

We added all the joiners and corner pieces and then the roof was complete.

The final step was to add guttering along the rear length. We fitted each bracket a few mm lower than the previous to create a gradual slope to ensure water would flow towards the drainpipe.