One thing to think of too is your bracing inside of your attic.  There is nothing wrong with going above and beyond the code when it comes to bracing your roof.  If you have noticed some sag spots in your roof you might want to think about this as well.  Also, have the roofers use aluminum on your vallies.  (if you have any) Many leaks happen in these areas because of poor application of tar paper before
the shingles are put on.

How to plan for the future when installing a new roof?
I am not sure I understand what the short-hand anti-symmetrization means. I.e. I know that 
$$\delta_{cd}^{[ab]} ~=~ \frac{1}{2}(\delta_{c}^{a}\delta_{d}^{b} - \delta_{c}^{b}\delta_{d}^{a})$$
but how do we expand "bigger" analogues? I.e.
$$\delta_{b_1 b_2 b_3 b_4}^{[a_1 a_2 a_3 a_4]} ~=~ ?$$

What is the order of permutation in this case?
