knocking on suspended floor - advise pelase

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hi

our lounge is carpeted and we are looking at nailing an oak floor down however before i do that i want to remedy a problem with the springy / knocking sound i get in certain areas when the floor is walked on.

the room is 8 mtrs long x 4 mtrs wide. the floorboards run widthways. i took a few of them up to check on how things are underneath and all looks dry and in good condition. no rot that i could see.

the floorboards are nailed to joists that run lengthways not sure if they run the full 8mtr length or if they will be joined at some point?.

these length joists are then resting on to some mortar and another joist that runs widthways and these in turn sit on some damp proof material more mortar and a small 2 (visible) brick wall which runs the width of the room. these dwarf walls are spaced at approx 2 mtrs apart before the next one.

the dwarf wall with the joist on it appears to be level across the 4mtr span however the lenghtway joists that sit on top of it show a gap in the middle area of about 5mm which when walked on causes the floorboards and the top joist to knock onto the widthways joist.

when you see this transferred onto the floorboards above it does make everything wobble as it is moveed around on.

sorry to go on but hard to explain!

my question is can i put some packing into the gap to stop the knock or should the joists be taken up and reset in mortar? to the required level.(major job)

its just that once i nail the oak floor down i waont want to be lifting it!

it seems worse at one end of the room but there is also a knock exactly in the middle.

if packing it out is the answer, what sort of material would be good. i thought maybe some lengths of the damp prrof material which looks like the sort of stuff you put on flat roofs. as this could be slid into the gap and then nailed to the joist to stop it moving? and has a small amount of cushioning.

anyway over to the experts
 
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if im reading this right and you definatley have no rot on the wall plates then it leads me to think that through natural movement and shrinkage the floor is now bouncing,there are 2 ways i would deal with this,

no1:you can lift quite a few boards,obtain a couple of car jacks and some lengths of 4x2 and jack the floor up and clear out of mortar under the wall plate and rebed,let it set and then take the jacks out.

no2:again lift quite a few boards,prise the joists off of the wall plate,and install a load of folding wedges under the joists to take out the bounce then refix the joists back to the wall plate,

this is if im reading your post correctly,if not then see if you can post some pictures of the problem.hth.
 
doo - the traditional fix for this problem is to use slate packing driven into the gaps between the top of the dwalf wall and the underside of the joists. These can go in dry or a 'belt & braces' way is to bond them, either with sand/cement mortar or a building adhesive (something like Gripfill).
 
excellent that sounds like a fairly simple job , would i just buy roof slates ? or would they be too brittle ?
 
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doo - yeh, roof slate. They're thin so bits can be added (stacked) to make-up the gaps; place in position and tap in with a hammer on the edges. Tip: keep your eyes open when driving to work for any roofing work going on and ask for a couple of old slates (freebies) or delve into a skip.
 

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