The weir over flow is usually installed on box or gully gutters designed for roofs that require a large capacity of rain water flow, the weir acts as an over flow primarily if there is a blockage in the gutter to down pipe outlet that prevent normal drainage. If the gutter does not have an overflow point then rain water is likely to breach the levels or capacity of the gutter and flood internally into the building. The over flow weir should be positioned at a level that prevents the water level breaching the top of the gutter or flashing points dependant on the type of box or gully gutter. It should also be designed to be able to allow flow of water to drain adequately enough to ensure water does not back up.
Its is also recommended to position the overflow in an area that is visible so that occupants of the building are aware of an issue with the gutter when they see water come from the overflow.

A wier overflow works by releasing the water at a certain point of the gutter system when the level gets to high therefore preventing it from overflowing. It will not push the water in any direction other than stopping it at that point.

Hi to who this may concern roof drainage system is an important aspect of the overall roof design. This is particularly the case for parapet and valley gutters which are
over the building where any overflow into the building would be unacceptable. Eaves gutters are normally positioned outside the building where occasional overflow would not be harmful.
Eaves guttering is generally fixed to the fascia or rafters of the building. Most eaves gutters are laid level or nominally level. Any falls should be laid in the direction of the outlets which discharge rainwater via downpipes to surface or underground drainage points. hope this helps..👍