Your camera has a rolling shutter. This is not a physical mechanism but rather the pixels on your cameras sensor are activated in descending rows, think of it a bit like a narrow slit moving down the sensor at a speed determined by whatever you set your FPS as which is the same as the sampling frequency.
Now the horizontal lines on the roof caused by the corrugated construction will appear to interact with the speed of the rolling shutter which results in the formation of the Moiré pattern. As you move further up the corrugated effect on the roof becomes narrower, meaning more horizontal lines will be sampled by the opening in the rolling shutter. This is a form of aliasing, or distortion.
Aliasing is a result of the device doing the sampling, in this case the rolling shutter, sampling at a frequency that is less than twice the frequency of the object being sampled, in this case the relative separation of the corrugated construction of the roof.
It’s much easier to demonstrate this effect with audio. Consider a commercial CD. The highest expected frequency is 22KHz so to reproduce this recording digitally it needs to be sampled at 44KHz or higher. If the sampling rate is less than this the result is that the higher frequencies, such as say snare drums, will sound distorted and unnatural. The same applies to video sampling but in this case patterns within the frame will appear distorted. A very basic example of this is when you see something like the wheel of a car on video appearing to rotate backwards when the car is actually moving forward. This is because the shutter speed of the camera is less than twice the rotational speed of the camera shutter, i.e. the wheel is being under-sampled.
A clever bloke called Nyquist, along with another Tefal Toaster Head called Shannon, produced a simple equation, known as the Nyquist-Shannon theorem whereby to truly reproduce a waveform (audio, video) it has to be sampled at a minimum of twice the highest frequency. The basic representation of this theorem is below.
N(number of samples) = 2 x hf(highest frequency to be sampled)
Nidge, whom is still at a loss as to why I don’t get invited to parties.