RowSpan "Auto" makes two rows merge for WP8.1 - xaml

Question

I've got a Grid where I've created three rows using column definition. In the first row I've placed a grid with a rectangle and set the Rowspan = 2. In the second I've also got a grid with a rectangle. Instead of the first row just overlapping the second a bit, which is what I wish, They totally overlap, and seems that there is just one row.
Te RowDefinition Height I've set to Auto, because I wish to change the Heights of the rows by clicking a button later on.
Is there a way to avoid the two rows to merge using the Auto Height for RowDefinition?
The Code I've got so far is:
<Page
x:Class="GridRetreatTest.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:GridRetreatTest"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid Grid.Row="0" Height="100" Grid.RowSpan="2">
<Rectangle Fill="Blue"/>
</Grid>
<Grid Grid.Row="1" Height="100">
<Rectangle Fill="Red"/>
</Grid>
<Grid Grid.Row="2" Height="100">
<Rectangle Fill="Green"/>
</Grid>
</Grid>

Answer 1

try this:
XAML:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid Grid.Row="0" Height="100" >
<Rectangle Fill="Blue"/>
</Grid>
<Grid x:Name="grd1" Grid.Row="1" Height="100" Margin="0,-50,0,0">
<Rectangle Fill="Red"/>
</Grid>
<Grid Grid.Row="2" Height="100">
<Rectangle Fill="Green"/>
</Grid>
</Grid>
<StackPanel Grid.Row="4">
<Button Content="click" Click="Button_Click"></Button>
</StackPanel>
CS:
private void Button_Click(object sender, RoutedEventArgs e)
{
grd1.Margin = new Thickness(0, 0, 0, 0);
grd1.Height = 300;
}