Setting Elevation in XML on AppCompat CardView on Android 5.0

It looks like a margin/padding problem, try to set the cardUseCompatPadding attribute to true. E.g.:

<android.support.v7.widget.CardView 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="6dp"
    card_view:cardUseCompatPadding="true"
    card_view:cardElevation="4dp"
    card_view:cardCornerRadius="3dp">

Explanation from Android doc :

CardView adds additional padding to draw shadows on platforms before
L.

This may cause Cards to have different sizes between L and before L.
If you need to align CardView with other Views, you may need api
version specific dimension resources to account for the changes. As an
alternative, you can set cardUseCompatPadding flag to true and CardView will add the same padding values on platforms L and after.

Since setting cardUseCompatPadding flag to true adds unnecessary gaps in the UI, default value is false.