sagging plywood floor

[confused-one]

A small part of the upstairs flooring on a house I bought a little while ago is sagging when stepped upon.

The house is a former local-authority house, build 23 years ago.
The flooring looks like it is large sheets of plywood, which are nailed down every 2ft or so.

The central heating has obviously been added post-construction, and it looks like a strips of the flooring have been cut to enable the piping to be laid.

It is some of these strips which have been cut which are sagging. Each strip is (very approximately) 4 inches wide and 4ft long.

The amount of sag is difficult to judge but it is very noticeable and is probably more than 1/2".

Obviously I am concerend about the sagging - and the potential for me to arrive downstairs via the ceiling! Should I be looking to replace the individual strips or do I have a bigger problem on my hands?

 

[TexMex]

Sounds like the central heating fitter was a bit of a cowboy. Remedial treatment depends on which way the joists go.

If the strips run across the joists fit noggins in between the joists so that the edges of the strips, and adjacent boards are supported.

If the strips run parallel with the joists, cut the adjoining boards back to the centre of the nearest joist. Replace missing section with a single board right across. If one of the edges of the strip is sitting beside a joist you can just screw a batton to the joist to give that edge support.

Basically, all you need to do is get the edges of all boards supported.

Finally screw everything down.