Unnecessary space under ther header, can't make it overlay the content

The issue may look as follows:

First of all it should be understood that the header by default is actually always over the content. If any unnecessary space appears before the content of the page, two options are possible:

1. Most probably there is top padding set for the first row on your page. To remove it:

a) go to Row options:

save image

b) set Top padding to 0px:

save image

2. Top padding for the page content may be set to other than 0px.

save image