Advanced Structural Designs

ACN 097 789 87 92 Vasey Cres CAMPBELL ACT 2612

Telephone 61612171 Facsimile 61612170

Email mal@structuraldesigns.com.au

What should I know about using Compressed FC Sheeting in Canberra?

Canberra has not had a happy history with this product with two problems arising over the years. The most common problem was cracking at the sheet corners and the other problem was bowing out of the sheets due to moisture variations across the thickness of the sheets. Neither of these problems occurred on every project but they were prevalent enough for some designers to start taking additional precautions.

Cracking at Sheet Corners

Compared with coastal capital cities Canberra has a relatively dry atmosphere and very large seasonal temperature variations. For this reason the stresses built up in FC sheeting were at times large enough to crack the corners of the sheeting. It was becoming common practice over the last 4 years or so for local contractors to use double top hats at the edges of the sheeting to increase the edge distance of the fixings and this seemed to address the problem.

James Hardie have now come up with a new fixing system (D3 – 1000), which enables screws to be placed much further from the edges. Reversing the top hats to have their legs out also affords some flexibility to the connection. We strongly suggest following this system and updating your old details.

Bowing out of the Sheets

This problem was particularly prevalent when waterproof products like Vitrathane were used on only one side of the panel and occurred to a lesser extent with more breathable paint systems. The problem is best addressed by sealing both sides of all sheets with a suitable paint system. In the case of the Vitrathane, a coat of Vitrathane is also applied to the inside face, while for other paint systems only an undercoat may need to be applied to the inside face. We understand that this is now a standing recommendation from Hardies when using FC in Canberra.

Below is a photo of the Novel on Northbourne which is having a subtle problem, most of the more extreme examples have now had the panels replaced.

Thermal Bridging

We occasionally come across projects where top hats are fastened directly to large steel studs and we recommend the use of thermal bridging foam between the outer panel and its top hat in these cases, especially if plasterboard is fastened to the inside face. If you don’t do this you will often see the stud line showing through the plasterboard after a couple of years.

Following JH’s recommendations on sarking is also important if any guarantees are going to stand.

Please note that the above advice on cracking and bowing relates to 9mm HardiPanel Compressed Panels rather than say the D3-Comtex Panels which are less compressed and less rigid.

If you require advice on building practices from an experienced structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs on (02) 61612171

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