CSS: Page Layout explodes when Header Logo is 100px heigh

What I do:
I add a Custom Logo with height 100px. Default height of Header is 100px (Default).

Result: The layout explodes (CSS problem).

1) When my site layout has Left Sidebar (L) and Content (C), then L is above C - like this:
Observed: L\nC
Expected: LC (The LC-Blocks should be next to each other).

2) When my site layout has Left Sidebar (L), Content (C) and Right Sidebar (R):
Observed: LC\nR
Expected: LCR.

(\n = newline)

The layout is correct when I set Height of Header to 103px (/admin/build/themes/settings/sky) - but then space is shown between header and navigation (div#header and div#navigation).