I am not very sure what you mean by "radGrid scrolls up beyond the radMenu". However, two things come to my mind with regard to your scenario:
RadMenu is a floated element by default, so you have to clear its float below the control. This can be done with the following element placed right after the menu:
<div style="clear:both"><!-- --></div>
If you have two elements in a single parent and one of the elements is 100% high, this leads to content overflowing. It is advisable that you place the RadMenu and RadGrid in separate containers, e.g. two RadPanes.
Let us know if you need further assistance. If this is the case, please send your RadSplitter declaration including the controls inside it.
the Telerik team