Sidebar CSS positioning glitch

Help – this is going to sound newbie. Normally I can figure these things out, but I’m adjusting the default theme on a charity site and I need a fresh pair of eyes.

I’ve set up my header to float:left (never done this before) and the sidebar to float:right. I’ve adjusted the templates so that the sidebar php appears in before the content area php (which I’ve done before creating themes with two sidebars). I’ve done the math over and over and I can’t get the sidebar to be flush to the top of the window. Can somebody please have a quick look and let me know if you can spot the error? Does the sidebar always have to be under the header area?

https://www.writerscentre.ca.php5-8.dfw1-2.websitetestlink.com/

Thanks in advance.

Al