checkout_order_weight filter

function order_weight($weight = null, $co_id = null, $items = null) {

    //do something with the weight here
    //for example:...

    $weight = 0;
    foreach ($items as $item) {
        $weight += $item -> product -> weight;
    }

    return $weight;
}

add_filter('checkout_order_weight', 'order_weight', 10, 3);