Reinforcing a 1st floor timber floor by adding additional joists

[TimL]

Hello all,

 

Our current 1st floor consists of 40x210mm joists (unstamped but assumed to be C16) spaced at nominally 600mm centres. Based on the increased dead load from removing the old paramount partitions, replacing with 38x89mm CLS studwork with some acoustic insulation, replacing the chipboard subfloor with this floating acoustic subfloor and adding an overlay underfloor heating system, a structural engineer has recommended reinforcing the floor by adding additional joists in between the existing joists by building into the external walls to bring the nominal spacing down to 300mm centres. What would be the best way of installing these new joists?

 

The existing joists are either built into the external walls or rest on masonry joist hangers on the party wall. I was initially considering raking out the mortar joints, filling with fresh mortar with a mortar gun and installing masonry joist hangers to match the existing joist hangers but a civil engineer work colleague recommended against this. He was uneasy about there not being adequate compression of the mortar if retrofit this way.

I really want to avoid building these new joists into the wall as I doubt I would be able to cut the holes at the right height and size without making a mess of it and I expect it would take a really long time. I wouldn't want to do this to the party wall anyway as it would compromise the acoustic performance of the party wall.

The other solution I'm considering (pictured below) is installing ledger boards between the existing joists and installing joist hangers off those, but I'm not sure if that would be a good idea since the external walls consist of hollow concrete blockwork. From my online searches I can only seem to find American examples of fixing ledger boards to cinder blocks to build outdoor decking.

Does anyone have any views on this?

 

 

 

 

Edited February 7 by TimL

[Russell griffiths]

More context needed

is this a full renovation ?

ceilings down, no floors upstairs, no electrics no plumbing. 
this will be such an awkward job. 
 

any pictures 

[saveasteading]

There are other ways. Your loading is only increasing slightly, isn't it? So doesn't need doubling the joists?

1. Bolt a wallplate to the wall and rest the new joists on it.

2. Sister the existing joists. They shouldn't need to go all the way into the wall, as bending is in the middle.

3. Fix plywood on top and/ of bottom of joists thus turning ghe whole floor into a very strong structure.

 

These are practical suggestions, not mainstream,  that you must run past the SE who knows the full context.

[TimL]

10 hours ago, Russell griffiths said:

More context needed

is this a full renovation ?

ceilings down, no floors upstairs, no electrics no plumbing. 
this will be such an awkward job. 
 

any pictures 

Yep, full renovation. We're going after the EnerPHit retrofit standard which has a target airtightness of 1 air change per hour.

Almost all the ceilings are down except for in the living room, though that's coming down too so we can seal the bottom edge of the joists that are against the party wall and external wall with airtight tape.

Upstairs only has the chipboard subfloor left but that will be removed to be replaced with the floating acoustic subfloor.

Planning on doing a full rewire when it comes to reinforcing as the wires going through all the joists will naturally need to come out.

Radiator plumbing is all being removed as we're planning on using an overlay UFH system upstairs, the remaining plumbing we are planning on refreshing whilst we're at it. 

It will indeed be a very awkward job.

[TimL]

7 hours ago, saveasteading said:

There are other ways. Your loading is only increasing slightly, isn't it? So doesn't need doubling the joists?

1. Bolt a wallplate to the wall and rest the new joists on it.

2. Sister the existing joists. They shouldn't need to go all the way into the wall, as bending is in the middle.

3. Fix plywood on top and/ of bottom of joists thus turning ghe whole floor into a very strong structure.

 

These are practical suggestions, not mainstream,  that you must run past the SE who knows the full context.

That's what we were hoping but the SE mentioned it would be necessary for us given the additional dead load of the upgraded stud walls and acoustic subfloor, let alone the UFH system. They also seemed to suggest that 600mm centres for C16 joists was apparently on the high end and somewhat uncommon though I'm not sure about that. 

We were considering the first option you suggested but given (based on a hole left in the wall from a previous owner) the external walls are hollow concrete blockwork, we're not sure whether you can get a reliable fixing into the hollow blockwork. I've seen mentions of resin based fixings but haven't found much detail on it.

[ETC]

Trimmers between the existing joists against the walls. New joists supported on the trimmers between the existing joists.

[TimL]

9 minutes ago, ETC said:

Trimmers between the existing joists against the walls. New joists supported on the trimmers between the existing joists.

Do you mean to support the trimmers on hangers to be attached to the existing joists?

[ETC]

Yes. Trimmers between the existing joists and supported on the existing joists with joist hangers as tight as you can get them against the wall at each end. Then new joists supported on the trimmers with joist hangers half way between the existing joists. You might need to double up the trimmers between the existing joists and you should see if your SE is ok with this.

 

Saves having to bolt anything to the existing walls.

[Gus Potter]

22 hours ago, TimL said:

I really want to avoid building these new joists into the wall

Sometimes there is an easy way around this. Below is a screen shot of part of one of my designs where I'm strengthening a floor.

 

 

The additional joists are shown in green hatch.

 

You'll notice that at the top of the drawing that the sistering joists don't extend to the wall head. The roof on this job is a pole plate roof so there were issues around extending the joist to the wall head that I needed to avoid.

 

The sistered joists are well fixed to the existing. The objective is:

 

1/ The big bending forces near the middle of the joists are resisted by the double timbers, old and new.

 

2/ A lot of the deflection is reduced by the doubled timbers where the bending stresses (near the middle of the span) are high.

 

To prove this works you:

 

A/ Check the double timbers for strength so they don't snap near the middle of the span.

 

B/ That the deflection is ok.. how much they bend down by in the middle.

 

C/ That where you only have the existing single part of the joist bearing onto the wall that is can take the extra shear loads arising form the extra load you are adding to the floor.

 

D/ That the bearing strength of the existing timber on the wall plate is ok in terms of transferring the extra load to the wall head.

 

E/ That any door / window lintels etc under the wall plate / wall under the joist ends can take the extra loading.

 

You may find this approach works so long as you are not adding too much extra load to the floor... say by using a concrete screed for the UF.. a lightweight overlay UF should be possible.

 

I would ask an SE say to have a proper look at this as they could easily save you more money and hassle than the amount you need to pay them.

 

Given the size of joists you have (40x210mm) you may find the above works. You may need some extra noggings (dwangs) to stop the joists twisting at the ends.

 

The above is one solution but you can often tweek it to avoid having to but a ledger piece to the wall  or form extra pockes in the masonry.

 

 

 

 

 

 

 

 

 

 

[Russell griffiths]

I would grab a chainsaw and cut every single one of those joist out. 
fix a ledger board around the whole perimeter of the blockwork, fixed using resin anchors with a mesh sleeve into the holes to allow for the dodgy blocks. 
then fit new metal web joists to the whole area. 
 

far easier to get all your new services in, will be flat and level. 
 

all depends on how far you want to go. 
 

or alternatively remove all services. 
 

cut existing joists out from the pockets in the wall, fit a ledger board along that wall and fit the old joist to the new ledger board with hangers, changing the spacing to 400centres, add new joists to make up the short fall. 
 

or something like that. 

[ETC]

A bit drastic.

[joe90]

You appear to be leaving the existing flooring in place? If so and you’re ok with the flooring on 600mm spacing the easiest solution is the sistering by @Gus Potter which will take any bowing from the floor. The party wall appears to be concrete block (so not donkeys years old crumbling brick wall!!). If you prefer 300mm joist spacing then noggins close to the party wall and extra joists on hangers on that wall and resting on the internal wall the other end.

[saveasteading]

The joists are stabilising the wall that holds up the roof. Not high science, just the way traditional buildings work.

I wouldn't dream of removing them.

[TimL]

On 07/02/2024 at 22:43, Gus Potter said:

Sometimes there is an easy way around this. Below is a screen shot of part of one of my designs where I'm strengthening a floor.

 

 

The additional joists are shown in green hatch.

 

You'll notice that at the top of the drawing that the sistering joists don't extend to the wall head. The roof on this job is a pole plate roof so there were issues around extending the joist to the wall head that I needed to avoid.

 

The sistered joists are well fixed to the existing. The objective is:

 

1/ The big bending forces near the middle of the joists are resisted by the double timbers, old and new.

 

2/ A lot of the deflection is reduced by the doubled timbers where the bending stresses (near the middle of the span) are high.

 

To prove this works you:

 

A/ Check the double timbers for strength so they don't snap near the middle of the span.

 

B/ That the deflection is ok.. how much they bend down by in the middle.

 

C/ That where you only have the existing single part of the joist bearing onto the wall that is can take the extra shear loads arising form the extra load you are adding to the floor.

 

D/ That the bearing strength of the existing timber on the wall plate is ok in terms of transferring the extra load to the wall head.

 

E/ That any door / window lintels etc under the wall plate / wall under the joist ends can take the extra loading.

 

You may find this approach works so long as you are not adding too much extra load to the floor... say by using a concrete screed for the UF.. a lightweight overlay UF should be possible.

 

I would ask an SE say to have a proper look at this as they could easily save you more money and hassle than the amount you need to pay them.

 

Given the size of joists you have (40x210mm) you may find the above works. You may need some extra noggings (dwangs) to stop the joists twisting at the ends.

 

The above is one solution but you can often tweek it to avoid having to but a ledger piece to the wall  or form extra pockes in the masonry.

 

 

 

 

 

 

 

 

 

 

Thanks for the suggestion. I think our first preference would be to take the joist centres down to 300mm as this would also allow us to use a slightly thinner (and marginally lighter) subfloor, but this sounds like a reasonable alternative that we might end up using.