Question: It's well known that if E is a vector bundle with Chern roots $a\_1,\ldots, a\_r$,
then the Chern roots of the $p$th exterior power of E consist of all sums of $k$ distinct $a\_i$'s. I would like to say the same is true if E is just a torsion-free coherent sheaf on $P^n$. It seems non-obvious, though, maybe because an exterior power isn't generally an additive functor. 

Presumably this is either false or also well known, but I can't find a reference.

Answer: 
My guess would be that the formula you want does not extend to the case of coherent sheaves. As indicated in Mariano and David answers (which has unfortunately been deleted), the best hope to compute is via a resolution $\mathcal F$ of $E$ by vector bundles. In general, for 2 perfect complexes $\mathcal F, \mathcal G$ of vector bundles, there is a formula for the localized chern classes 
$$ch\_{Y\cap Z}(\mathcal F \otimes \mathcal G) = ch\_Y(\mathcal F)ch\_Z(\mathcal G)$$, with $Y,Z$ being the respective support. Unfortunately, this only gives the right formula for the "derived tensor product".

So to mess up the formula, one can pick $E$ such that $Tor^i(E,E)$ are non-trivial. I think an ideal sheaf of codimension at least 2 would be your best bet for computation purpose.
This is really a reference for the problem in Graham's comment, rather than an answer to the question. See

* Tchernev, Alexandre B. Acyclicity of symmetric and exterior powers of complexes. J. Algebra 184 (1996), no. 3, 1113--1135. [MR1407888](http://www.ams.org/mathscinet-getitem?mr=MR1407888)
* Weyman, Jerzy. Resolutions of the exterior and symmetric powers of a module. J. Algebra 58 (1979), no. 2, 333--341. [MR0540642](http://www.ams.org/mathscinet-getitem?mr=MR0540642)

where the symmetric and exterior powers of finite free resolutions of modules are considered.
Under some conditions on the determinantal ideals determined by the maps in the reslutions, the complexes obtained are resolutions of the symmetric and exterior powers of the modules.