By Martin Hill

I have recently undertaken destructive investigations of two very similar buildings that are located either side of the Auckland Harbour Bridge. These buildings shared the same designer and builder and were both primarily single storey buildings. Both buildings had been clad with a fibre cement panelised system incorporating vertical and horizontal expressed joints.

The similarity of the buildings does not stop there; both buildings had an array of building deficiencies that were allowing moisture entry behind the cladding. These included joinery, wall cladding and roofing defects that had been caused by a combination of poor design and workmanship. Furthermore, the wall framing used to form the exterior walls to both buildings comprised kiln dried untreated Radiata pine.

But this is where the similarities end.

One of the buildings investigated requires extensive remediation, including removal of significant areas of structural framing decay. So severe is the framing decay in some areas that temporary supports to walls and roofs have been required to ensure the immediate safety of the building.

The other building will require remediation to correct building deficiencies, however framing decay is minimal and timber replacement will not be significant. In both buildings we will treat sound in-situ framing with Boron preservative.

Both of these buildings leak, so why does one building need extensive and costly framing replacement and the other does not?

The simple answer to that question lies in the only significant difference in the construction of the buildings. This being that only one of the buildings has the cladding installed over a drained cavity. This was achieved by installing vertical timber battens between the wall framing and cladding. Consequently, although the cladding construction was defective, the cavity has prevented moisture from accessing the framing timbers.

The photograph over page shows a section of wall framing beneath a critical roof and cladding junction.  The termination of the apron flashing was poorly formed and allows moisture behind the cladding. The cladding has been fixed over a cavity, and although the building paper was wet to touch, moisture was able to drain away.

(Photo Caption – Cladding fixed to cavity battens beneath poorly formed and leaking roof and wall apron flashing termination.)

The next photograph, on the other hand, shows the effect of moisture penetration upon wall framing beneath an equally poorly formed roof and cladding junction, where the cladding has been directly fixed to the framing timbers. Moisture is unable to drain away once it penetrates behind the cladding and is absorbed by the framing timbers, resulting in significant decay.  

(Photo Caption – Direct fixed cladding beneath poorly formed and leaking roof and wall apron flashing termination.)

The advantages of installing cladding on a correctly formed cavity, in my opinion, outweigh any additional financial cost providing a degree of peace of mind to the designer, builder and the end user of the building. We must be mindful that we can still compromise the ability of a cavity to function adequately with poor detailing and construction. I leave you with the last photograph, which shows that even with a cavity, poor construction can remove any benefit that cavity would have provided.  Treated timber then provides an additional degree of robustness most of us expect from our buildings.

(Photo Caption – Solid horizontal battens and cladding embedment below ground level.