unchecked checkboxes are not returned to the form processor so there is little point in conditionaly setting the checkbox's value. just do:
<input name="tiled_roof" type="checkbox" id="tiled_roof" value="1">
then in your form processor:
if (isset($_POST['tiled_roof']))
{
// yes, it was checked
}
else
{
// no, it was not checked
}