Carhartt Men's Storm Defender® Waterproof Loose Fit Lightweight Hooded Jacket

Is this jacket actually waterproof, or just water-resistant?

It is fully waterproof. It features Carhartt's Storm Defender® technology and fully seam-sealed construction, meaning it is designed to block water entry during heavy, sustained rain while allowing sweat vapor to escape through the breathable membrane.

Is it roomy enough to wear over a hoodie or a heavy work shirt?

Yes, it features a "Loose Fit" design, which provides ample room in the chest and shoulders. This specific fit is optimized for layering over bulky sweatshirts, hoodies, or other work gear without restricting your movement.