What is the best way to attach heat cables to wood shake shingles ?

The clips supplied with new cables will still hold to shakes,tucking them either between the shakes or if the shake is #1 smooth(starter style) there should be enough tight space under them.

Thanks for your response
Have you installed them yourself and do you think there are any concerns with the fact that the roof is wood

Not sure what brand you have, but attached is the manual for a popular brand:

http://www.easyheat.com/Content1/PDF_Files/_files/11629_001_r2_ADKS_ownersman_web.pdf

See Section III beginning on Page 6, does not allow for installation on wood.

I have used most if not all brands on the market, and I am sure if yours is not this, it still would have the same warning.

Something to look for.

Thanks for your response
Have you installed them yourself and do you think there are any concerns with the fact that the roof is wood

I have installed many heating cables,but never on wood shake.

Logic tells me that using them will only be in the winter and to melt ice/snow, this would make the shake wet and would not pose a fire problem,, some shakes are treated for fire retention.

Are these?
Did you install them?

As Tk stated,the MFGR listing might clear up the use of them on shakes.

No I did not install the roof I think it is about 45 years old
I just read the manual above and found this so I am not so sure if this is a good idea

Keep all combustible materials away from the cable, such
As leaves, pine needles, seeds or windblown trash. Do not attach
The cable to a combustible material, such as any wooden parts of
The eave.