Best way to strengthen upstairs floor

jblakes

Evening all, 

We have a dinning room of 5m x 4m that is supported by three walls and an RJS to an extension. Upstair we have a bedroom and walk way, the studd wall between bedroom and walkway is across the joists. The issue we have is where you enter the room from the walkway, you are walking the down the length of one of the joist and because it's an old house the floor has slightly bowed and I believe the joist size is of an old standard (not seem the joists size, but the owner said it was done 20odd years ago). He said its always been like that, so I can only assume it's fine but I am wanting to strengthen it, so you cant feel the floor bounce when walked upon.

What is the best way to go about this, I thought an rsj running underneath the studd wall would be ideal because is would split the 4metre joist length into, but could you just stick it under the floor or would you need to cut joist to attach to the rsj?

Or I thought of doubling up the joists, but the ceiling in the dinning room would definitely have to come down.

What is the standard procedure for this situation and ball park cost involved?

Kind regards
James

ThisIsWeird

Hi. Any chance of floor plans, so it's perfectly clear, please?

Section62

jblakes said:

We have a dinning room of 5m x 4m that is supported by three walls and an RJS to an extension. Upstair we have a bedroom and walk way, the studd wall between bedroom and walkway is across the joists. The issue we have is where you enter the room from the walkway, you are walking the down the length of one of the joist and because it's an old house the floor has slightly bowed and I believe the joist size is of an old standard (not seem the joists size, but the owner said it was done 20odd years ago). He said its always been like that, so I can only assume it's fine but I am wanting to strengthen it, so you cant feel the floor bounce when walked upon.

What is the best way to go about this, I thought an rsj running underneath the studd wall would be ideal because is would split the 4metre joist length into, but could you just stick it under the floor or would you need to cut joist to attach to the rsj?

Or I thought of doubling up the joists, but the ceiling in the dinning room would definitely have to come down.

What is the standard procedure for this situation and ball park cost involved?

You need to get a structural engineer to advise you about the options and costs.  It won't be as simple as just adding a steel beam somewhere, because the beam itself will need to be supported somehow.

The methods used for sizing timber joists haven't changed much in the last 40 years, so if the owner is saying the joist(s) were put in20 years ago (not clear if this is what you mean) then they should have been sized correctly to avoid floor bounce.  I would be concerned that the bounce is due to one or more of 1) the joists being undersized, 2) someone has incorrectly notched or drilled them for plumbing/electrical runs, 3) the joist(s) aren't correctly supported at one or both ends.

There are different methods for fixing each of those issues, and a structural engineer would be the right person to come up with the best solution, as well as producing the drawings and calculations needed for building control signoff (if needed).

jblakes

Hi, my very quick drawing on a app, the stairs are on the left, the beams are the red/pink lines. Maybe it wasn't 20 agos then, they are around 100-120mm deep, they are def not current code.

As you can see from the upstairs basic plan, the joist runs exactly where we walk into the room. 

I've had a few builders out who have suggested a few different suggestions, two said RJS, and the other suggested doubling up joists/add bracers. They all said structural engineer would not be required, the house has been like that years so safe, I just don't want to bounce.

Section62

jblakes said:

I've had a few builders out who have suggested a few different suggestions, two said RJS, and the other suggested doubling up joists/add bracers. They all said structural engineer would not be required, the house has been like that years so safe, I just don't want to bounce.

Some builders always say that.

If you need to add a beam of 4 or 5m length then it is difficult to see how that isn't structural work that requires BC signoff.

The amount of 'flex' in a joist or beam (one of the things that causes bounce) depends partly on the 'span/depth ratio'.  The longer the joist, the deeper (i.e. taller) it needs to be.  Doubing up a 100mm 'joist' won't improve the span/depth ratio.  You either need to increase the depth, or else reduce the span.

Working this out properly is what structural engineers do.

weeg

Putting in a beam is going to be more intrusive and expensive than strengthening the joists, and also more effective. Without an engineer, how are you going to know how big a beam?

Bounce is partly about span/ depth ratio and partly about resonant frequency and adding additional stiffness (more joists) will address the latter. 

I think one of the most expensive sentences I know is "my builder said it would be fine". Fwiw the design standards haven't changed meaningfully since at least the 80s (at least the results haven't - the actual codes have, but the answers are still the same). 

This isn't going to be cheap.

GDB2222

Can I ask a question here, just to show my ignorance.

I assume that doubling up the joists would roughly double their strength, and that might halve the bounciness, say.  Is one problem that just halving the bounce won’t really solve the problem? The OP wants the bounce to go away more or less completely, which might require increasing the strength of the joists by a factor of 10 say? 

stuart45

There's always a certain amount of deflection in a timber floor. It's just a question of whether it's in the acceptable limits. They can also get a bit bouncy when the bearing ends start to rot.

pieroabcd

I would be very wary of a builder or a tradesman saying that the structural enginer or a builder control signoff are not required when evidently they are.
In the very short time that I've tried to get some work done i've heard a huge amount of evident insanity.

Eldi_Dos

@jblakes
In the downstairs dining room how far is the joist marked in pink from where the RSJ was placed for extension.

jblakes

4m, it was placed where the wall was.

Surely you don't need a structural engineer or BC to strengthen the floor, it's perfectly fine just abit bouncy. I'd like to stop as much bounce as possible.