How to fix vertical cracks in fibre cement board?

[colsi281]

 Hi. We have a second story put on in the 90's by previous owners. Large sheet fibre cement board cladding. In a number of places (mostly below window frames at joins) there are vertical lines/cracks appearing. I am worried about water tightness, what do I do to fill/repair this? Thanks. 

[JacobZ]

Hi @colsi281,

A warm welcome to the Bunnings Workshop community and thank you for your question.

I believe you're seeing a previous attempt to cover up a crack that failed due to the use of incorrect materials. You see paper tape that has been covered with a plaster compound. Water has gotten into the paper tape causing it to swell and crack the previous repair.

I'd suggest using a Kango 100mm Stainless Joint Knife to remove the paper tape and scrape the surface back.

Once prepared, apply Builders Edge 50mm x 20m Fibreglass Builders Joint Tape across any cracks as it will not swell in the same way paper will.

You can then apply a smooth coat of Polyfilla Exterior Brick and Render Filler over the tape, spreading it around 100mm wider than the tape on each side to give yourself room to taper the filler so it transitions seamlessly into its surroundings. 

You can then use a sanding sponge to smooth and prepare the patch before painting.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob