Floors bouncing from one bedroom to the other

[Foofoothesnoo]

Hi, I'm posting hoping someone can advise me the best way to fix my problem.

I have a typical 1930s detached house with equal sized front room and back rooms with bedrooms above. Each room is 3.65m wide by 3.85m long. Joists run front to back (I.e the 3.85 measurement) and are 7" by 2" with original t&g floorboards. From the boards we've had up I haven't seen any cross-nogins.

The living rooms are through rooms, from what we can see it's original as the original skirting and floorboards are undisturbed. The I shaped steel beam is 8" by 5 1/4" I think, with approx 13" piers each side and the beam sits at least 9" each side. The brick wall above the beam continues into the loft where it is one of three supports for the hipped roof.

Anyway, the bedroom floors are extremely bouncy, so much so that if the kids are jumping around in the front bedroom the floor shakes in the back. If my son gets out of bed heavily I can feel the shake as I lay in my bed! If my husband is walking around the bedroom the living room door rattles.

So my first question is, is the beam sufficient for the span and load? If so, what could be causing so much bounce/vibration and how can we fix it?

Thankfully the living room isn't renovated yet so it wouldn't be impossible to drop the plasterboard ceilings and work from below. That would give us opportunity to add sound insulation as the acoustics are shocking (I can be in the living room and have a conversation with my son in his bedroom just by raising my voice). Accessing the joists from above isn't really possible as we're living in the house and the rooms are decorated/carpeted and have built in furniture.

I've seen on other threads about herringbone straps but we wouldn't be able to put those in unless they could fix to the sides (rather than tops) and undersides of the joists. I also read about adding slate to the ends of the joists, but would that be to stop sideways movement or to prop them up at the ends?

I hope I've given enough details and someone has some suggestions, let me know if I've missed anything (apologies for the mix of metric and imperial!).

Thanks.

 

[Deleted member 174758]

Sadly it wasn't uncommon for houses in the cash-strapped 1930s, wartime-1940s or early post war period to have under specified joists and timbers (e.g. ungraded or insufficiently seasoned timber was sometimes used). The results can be as you describe, but a 7 x 2 C16 joist should be good for at least a 14 feet span (c.4.2m) unless an internal wall has been removed downstairs. I still wouldn't expect to see solid strutting (presumably what you refer to as "cross noggins") in such a small room, though.

Having said that, solid strutting across each room (with the strutting at least 70% of the height of the original joists and of the same thickness) should make the floor a lot less bouncy. Timber herringbone strutting could be retro fitted instead and whilst slightly lighter and slightly more rigid than solid strutting it takes a lot more accuracy in cutting and installation and it isn't an easy retro fit with an existing floor or ceiling at all. Metal herringboine strutting is designed to be nailed over the top and bottom of the joists during installation - it requires the floor and ceiling both out and your joists may not be the right distance apart in any case so it's probably a non-starter. In addition I'd also consider sistering some of the existing joists with graded softwood, such as C24 which is very stiff

For soundproofing you might want to consider replacing the PB with 15mm soundblock (mauve) boards screwed onto resillient bar which in turn is fixed across the joists at right angles. Should make some difference at relatively low cost