The best solutions for something like this would be to use AltContent1. Example:
<div class="main-content grid-container">
<div class="toprow">
<portal:LayoutPanel ID="divAlt1" runat="server" SkinID="TopPane">
<asp:ContentPlaceHolder ID="altContent1" runat="server"></asp:ContentPlaceHolder>
</portal:LayoutPanel>
</div>
<div class="middlerow">
<portal:LayoutPanel id="divLeft" runat="server" SkinID="LeftPane">
<asp:contentplaceholder ID="leftContent" runat="server"></asp:contentplaceholder>
</portal:LayoutPanel>
<portal:LayoutPanel id="divCenter" runat="server" SkinID="CenterPane">
<asp:ContentPlaceHolder ID="mainContent" runat="server"></asp:ContentPlaceHolder>
</portal:LayoutPanel>
<portal:LayoutPanel id="divRight" runat="server" SkinID="RightPane">
<asp:contentplaceholder ID="rightContent" runat="server"></asp:contentplaceholder>
</portal:LayoutPanel>
</div>
<div class="bottomrow">
<portal:LayoutPanel ID="divAltContent2" runat="server" SkinID="BottomPane">
<asp:ContentPlaceHolder ID="altContent2" runat="server"></asp:ContentPlaceHolder>
</portal:LayoutPanel>
</div>
</div>
The div with the class "middlerow" could be equivalent to your "sheet." The important part is that the "AltContent1" or "TopPane" panel is outside of the "sheet" div so it's not constrained by it.
HTH,
Joe D.