Floor joists

When terraced houses were constructed the loft space was not constructed for everyday use, so the joists in the loft are not strong enough. They are really only strong enough to carry the ceiling board below, and allow some occasional access across the loft without too much bowing.

New joists, thick enough to carry furniture, flooring and people, will be installed.

The existing joists which carry the ceiling don't need to be removed.
They are left in place (so no disruption to the rooms below), the new joists are installed between them and flooring is laid on top of the new joists.

To the front, a beam is installed across the tops of the existing joists and carried by the partywalls. The new joists then hang from this new beam.

To the rear, the new joists are usually taken over the external wall, just as the existing joists typically do, and the ends shaped to fit the angle of the rafters.

As there will now be more weight bearing down onto that rear wall, lintels above windows on the first floor will be assessed and strengthened if needed.