Zavim 3:3-4:1

Zavim 3:3

If a zav and a ritually clean person sat together in a large boat – which, according to Rabbi Yehuda means one that doesn’t rock when a person gets on – or if they sat on a plank, a bench, a bed frame or a beam that was fixed securely, or if they both climbed a strong tree, a strong branch, a Tyrian ladder or an Egyptian ladder that was connected with a nail, or if they sat on a bridge, rafter or door that was affixed with clay, even if at just one end – in all of these cases, the ritually clean person remains clean. If the clean person struck the zav, he remains clean but if the zav struck the clean person, he is rendered unclean. This is because, if the ritually clean person had stepped back, the zav would have fallen (so the clean person is supporting him).

Zavim 4:1

Rabbi Yehoshua said that if a niddah sat on a bed with a ritually clean woman, (even) the hat on her head contracts midras impurity. If she sat in a boat, (even) the utensils on top of the mast contract midras impurity. If she took a tub full of clothes and they weigh a lot, they are rendered unclean (because they weigh her down); if their weight is light, they remain ritually clean. If a zav banged against a balcony, causing a loaf of trumah to fall, it remains clean (because the balcony is steady, the loaf didn't fall as a direct result of the zav shaking it, but because of subsequent vibration).