Simple roots of E6 root system

Question

The simple roots of root system E6 are six 8-dimensional vetors. As I understand, simple roots should form a basis for the underlying vector space(in this case it is $\mathbb{R}^8$). For example, in root system E8, of which E6 is a subset, it does hold that simple roots are 8 eight-dimensional pairwise orthogonal vectors. Can someone explain how does it hold that in E6 we only have 6 eight-dimensional vectors? Do simple roots not form a basis then? Thank you!