We riveted the stiffner, skin doubler and rib to the top skin. This was not as simple as it appeared. I was able to squeeze some rivets with the 4" yoke but the rest had to be bucked. The top skin is flimsy so we ended up laying it between 2 tables (top side up). I used a variety of bucking bars underneath while Mike used the 3x Rivet gun with the mushroom set from the top.