Firming up a 'bouncy' floor

[Jthomas]

Our living room has approx 17' span, don't know what size the joists are but the floor does sag, or at least it did - it was levelled with shims in order to lay a laminate floor a couple of years ago. It's level now but there is a bounce when you walk across the floor.

My question is, can I get rid of the bounce by supporting the underneath with a cross beam - ie running a cross beam along the ceiling of the room beneath our living room. The span of this beam would be 20' - assuming I get a steel beam, what dimension would be required in order to provide a firmer feel to the floor above?

thanks

J

 

[OwainDIYer]

What's under the living room floor.

If it's a void then a concrete raft some intermediate dwarf walls would be adequate.

 

[ollieman]

You might want to check what is causing the floor to bounce as it could be unstable. Can you easily lift the floorboards at the edges where the joists are supported at the walls?

You should check with your local building control dpeartment as you might need an engineers certificate to put a beam in. A beam can be brought in in two pieces and spliced together.

 

[RonnyRaygun]

So your room is 17 feet wide by 20 feet long?

You are going to need something like a 254x146x43 UB, or a 203x203x46 UC.

For a 17 foot span you would need 50x250mm joists if spaced at 400mm centres. If it's always been like this there's a good chance they are undersized...

 

[Jthomas]

Thanks for the replies.

There is a room underneath that is used.

A builder that we use did previously comment on the bounce and he suggested that the joists should be thicker but the house is 25 years old and he didn't think there was any fundamental issue with the floor.

RonnyRaygun said
"You are going to need something like a 254x146x43 UB, or a 203x203x46 UC."

I'm not sure what this means, can you give me an idea of the cross section dimensions of these beams so that I can judge the visual impact on the room beneath (I'm sure there's a clue in what you posted!)

thanks

J

 

[RonnyRaygun]

RonnyRaygun said
"You are going to need something like a 254x146x43 UB, or a 203x203x46 UC."

I'm not sure what this means, can you give me an idea of the cross section dimensions of these beams so that I can judge the visual impact on the room beneath (I'm sure there's a clue in what you posted!)

thanks

J

Depth x Width x Weight per metre in kg
UB is Universal Beam
UC is Universal Column

Actual depth of a 254x146x43 is nearly 260mm, which is 56mm deeper than the 203 UC. Plus you've got to allow for your fire protection...

 

[JohnD]

good idea to lift floorboards so you can look at the ends of the joists where they go into the walls. Make sure they are all firmly supported at the same level. Also measure the width and depth of the joists, for an unusually large upstairs room like your they should be deeper than normal, as RonnyRaygun says, and maybe yours are too small; or maybe there used to be a support underneath like a dividing wall that has been removed. You might see traces of this on the plaster ceiling of the room below, or under the floor of your bouncy room.

was the upstairs room ever divided into two smaller rooms? there might have been a trussed partition.

 

[Jthomas]

I'm pretty sure that that the downstairs room was never divided. It used to be a double garage (as in a proper double garage, not two singles separated by a wall etc).

I have probably got some photos of the joists before we had the floor put down and could get a reasonable idea of dimension - what should they be for that size span? (ie 17')

If they are too small, wouldn't a cross beam underneath solve the problem? Seems its probably the cheapest way of fixing the problem.

If they're not fixed properly at the wall then I guess that's something that would need to be addressed but wouldn't there be more obvious problems after 25 years?

thanks

J

 

[Jthomas]

Just re-read RonnyRaygun - so ignore the question about what size the joists should be.

 

[RonnyRaygun]

I have probably got some photos of the joists before we had the floor put down and could get a reasonable idea of dimension - what should they be for that size span? (ie 17')

For a 17 foot span you would need 50x250mm joists if spaced at 400mm centres.

If they are too small, wouldn't a cross beam underneath solve the problem?

Yes, it would

Alternative would be to double up (sister) the joists. You don't necessarily have to take the new joists into the wall if the bearings of the current joists are OK.
Means taking up your floorboards, but wouldn't need steelwork, and no downstand in the room below. Can be difficult if electric cables or plumbing run through the existing joists.

Something else you could check for is excessive notching if pipes are running under the floor.

 

[JohnD]

joists do sometimes come loose if they are just mortared into the brickwork. Movement, vibration, crumbly mortar, occasionally a soft or cracked brick can cause it.

though yours might have galvanised joist hangers. It is worth having a look, taking up some floorboards at the edges of the room will be cheaper than going into structural re-engineering.

Even a 18mm ply floor, made of large sheets screwed tightly down, will add rigidity by becoming a deck

good idea about the notching, btw

 

[Jthomas]

Thanks for all the replies - very helpful

I like the idea of sistering the joists but there are a set of 8 downlighters used for the room beneath and not sure how much room I have (I dont really want them too close to the joists) -s but definitely worth a check.

There's very little notching from memory (if by that you are referring to cuts in the joists in which to run wires and pipes) - I cannot imagine that they would be causing any problems.

If I decide to go for the supporting cross beam, does this necessarily require building control, structural engineer etc as in theory it's just cosmetic work isn't it ie I'm fixing a bounce?

Also - would anyone have a ballpark figures for how much a cross beam like this would cost to install?

thanks again

J

 

[OwainDIYer]

You need building control for the new beam, if for no other reason to make sure it is itself suppoted sufficiently and won't fall down.