100% Width, But Apply Margins To Certain Elements

I want the vast majority of my site to be 100% width in design, so given that's the greatest common denominator, I changed my CSS to look like so:

.site-inner,
.wrap {
	margin: 0 auto;
	max-width: 100%;
}

That's great, everything is 100%, easy enough. Now I want various elements (fewer than not) to have a margin so that they're indented like a "normal" non-100% width site. I wrote a class called "common-margin", which applies this. What I'm stuck on, is how to "apply" or "inject" that class to say my header, x, and y.

Furthermore, if there's a better way to accomplish this, I'm all ears.