Chapter 41: Carrying items on head
- It is forbidden to carry any item on one’s head while he is wearing his head Tefillin if the item is not a clothing, and is not commonly worn on the head.
- Light items: This prohibition applies even against carrying a light item on one’s head, such as a handkerchief.
- Common items: However, items which are commonly placed on one’s head, such as a hat or scarf, may be worn on the head while wearing Tefillin, even if they rest on the actual Tefillin.
- Heavy common items: Nonetheless, if the item is a weight of four Kavin [6.4 kilo] or more, and is weighing down the Tefillin, then it must be removed.
 See “The Laws of Tefillin” Chapter 5 Halacha 3