Chimney venting - problems

[Tumbleweed101]

So, I had someone come to vent a chimney breast today as it seems this hasn't been done since the chimney was removed and capped.

The actual fireplace hasn't been open since I've lived here (over 20 years) but there has been damp stains appearing on the upstairs chimney breast and also downstairs about head height.

When he opened it a lot of small grey stones started to come out. He didn't know how far up they went so has resealed for now until they can find out more about the history of previous works.

This is a 1930/40 council property.

Does anyone have any idea what the stones could be and how high up the stack they might go? I've heard debris fall down through the chimney during works about 10 years ago but not sure if this was deliberate or not.

Builders often throw rubble into fireplaces and disused chimneys to save themselves the trouble of carting it away. They encourage damp by blocking ventilation and, especially on the ground floor, absorb damp from the hearthstone or ground, which does not have a DPM in old houses.

I don't know what your stones are. It would normally best to shovel them out and dispose of properly.

The stones may be loose rubble, but I wouldn’t have them all removed on guesswork until someone has established whether there is any proper masonry closure or support above. If the chimney is redundant, it normally needs controlled ventilation at the old fireplace/opening (or another suitable low-level vent), rather than being sealed completely; the arrangement should be checked against how the stack was altered.

The damp pattern is also worth investigating from outside: the stack/cap, flashing where it meets the roof, pointing and any bridging at the hearth can all contribute. Once the cause is dealt with, salts and staining can take quite a while to dry out. A builder or surveyor who is able to inspect both the chimney and the roof junction should be able to confirm what the stones are before any more is opened up.

Is it possible the 'small grey stones' are vermiculite from behind the old fireback, which might not be there now?