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Chasing out under floorboards!

Former Community Member
Former Community Member
Yes, I've never seen or done this before.

Wing added to a mansion house in 1875 and on lifting floorboards off 8" x 2.5" joists found an intermediate layer between floorboards and lath and plaster ceiling below.

This layer consisted of tile batten nailed to sides of joists about 2,5" from the bottom, usual lathes between joists which have approximately 1.5" plaster with fine straw mixed in and a second layer on top of harder plaster approximately 3/4" deep. with a fairly smooth finish.

This leaves approximately 1" void below boards and as described earlier, about 2.5" void above ceiling.

So to be able to drill holes in the joists entails chasing out this layer,........... Easiest way I found is to use multicutter and accept their wear. Fortunately I can regrind them to get second and third life out of the blades.

Right, the question for you is why this construction? Fire protection or sound transmission or some other reason? I know the Victorian houses of this status with 1500+ acres did suffer catastrophic fires and were also very mindful of the servants listening in!


Regards


BOD
  • 0
    perhaps the original design was to have exposed beams giving that rustic look. Then someone decided that a flat ceiling was required.
  • 0
    Not seen straw with plaster, but certainly in old substantial houses in Glasgow and many old tenements there was the exact same arrangement,between floorboars of the upper level and lathe and plaster ceiling of room below, we called then "Dwarf Floorboards" as they were of timber construction and laid on top was maybe four five inches of ashes, horrible black stuff, it was put in and must have been tons of the stuff and it acted as deafning, or so I was told as an apprentice to act as a sound transfer barrier, certainly worked.


    Cheers GTB
  • 0
    I came across a very similar thing once many years ago - in that case it was more like floorboards on battens between the joists as as far as I recall a single layer of fairly weak lime mortar (I don't recall any straw in it but there might have been - there was a huge amount of horse hair in the wall plaster) - but just the same in terms of just a small gap below the boards. In those days we were still allowed to run cables in notches in the top of joists - so avoided the need to cut through the layer other then where the cable needed to reach the room below. I guess you could still go for that option if you used BS 8436 cable or similar.


    As for why - I could only guess it was either for sound or fire proofing - in my case it was in a Parsonage (as in Vicarage/Manse) - probably of a similar era as yours - that had half the downstairs as the school room/church hall - so this layer was between the ceiling of the school room and the bedroom floors above. Nothing equivalent in the rest of the building where the rooms both above and below belonged to the Parsonage proper.


      - Andy.

  • 0
    well it is no worse than rockwool and FR membrane. May also be a draught proofing as well as sound.  However, tricky in your case. Probably not worth re-instating with more than perhaps pink foam.

    Mike
  • Former Community Member
    0 Former Community Member
    I've come across it more than once in "grander" houses in south west Wales. Boards on battens at the lower edge of the joist and the void almost fully filled with cockle shells (presumably as a readily available waste material from local estuaries)


    I've seen similar with lime and (probably) straw in Warwickshire


    I'd always assumed a combination of thermal insulation, sound absorption and a degree of fire resistance


    Regards


    OMS

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