replace floor/ceiling joists -am I insane?

[Crowbar hero]

I live in a 1930's built house with some pretty skinny (4X3") floor joists on the first floor spanning approx 3.5m at 400mm (ish) centres.

For several reasons* this is not satisfactory.  
Part of the long term renovation plan is to remove a chimney stack in the wall supporting some of these joists, thus I suspect once it's gone the joists will need to be lengthened -or rather replaced.  

Would I be insane for considering replacing *all* the joists in the affected room(s) with something deeper like 8x2"  Obviously this would lower the ceilings some, which is no problem (they're at 2.5m + already).  This would be quite disruptive, however the longer term benefits are pretty obvious.

 

* We plan to install underfloor heating, between joists rather than an overlay, to keep the floor heights consistent.

* we can hear every thump, bump and fart from the bedroom above the lounge.

* given the state of some of the plumbing and DIY work I've encountered, I'm fairly sure some of these joists have been notched to death in some locations too.

[Bonner]

Not insane at all, it will be worth it if you’re planning to stay in the house a long time. Consider using engineered joists but keep in mind that you can’t notch them.

[Crowbar hero]

13 hours ago, Bonner said:

Not insane at all, it will be worth it if you’re planning to stay in the house a long time. Consider using engineered joists but keep in mind that you can’t notch them.

I'll probably stick with traditional timber, as the tops will need notching for UFH pipework, and I'm not sure what else engineered joists will bring to the party. 

[Annker]

On 26/06/2022 at 21:53, Crowbar hero said:

I'll probably stick with traditional timber, as the tops will need notching for UFH pipework, and I'm not sure what else engineered joists will bring to the party. 

Hi Crowbar, did yo go ahead with replacing all the joists?

I have an upcoming project where I am considering doing the same.

[Crowbar hero]

13 minutes ago, Annker said:

Hi Crowbar, did yo go ahead with replacing all the joists?

I have an upcoming project where I am considering doing the same.

Not yet, we're still extending, and haven't gotten around to the renovation side.  I like to think of the next phase when I'm designing the current as I don't end up routing linking services the wrong way..

 

[Annker]

9 minutes ago, Crowbar hero said:

Not yet, we're still extending, and haven't gotten around to the renovation side.  I like to think of the next phase when I'm designing the current as I don't end up routing linking services the wrong way..

 

One concern I'd have is that removal of the existing joist deck could take away any lateral restraint they are providing the wall on which they bear.

In my situation I want to raise the floor levels of some rooms above ~340mm above their existing level. So the procedure I would use would be to firstly install/fix the replacement joist deck prior to removal of the existing joist deck.

I believe you will be reinstalling the new floor at the same level as the existing, therefore you wouldn't be able to do that, but I think you should think about installing some manner of temporary shoring bracing until the new joist desk can provide it.

 

[Gus Potter]

 

8 hours ago, Annker said:

One concern I'd have is that removal of the existing joist deck could take away any lateral restraint they are providing the wall on which they bear.

Great point @Annker

 

On 25/06/2022 at 13:30, Crowbar hero said:

I live in a 1930's built house with some pretty skinny (4X3") floor joists on the first floor spanning approx 3.5m at 400mm (ish) centres.

Something looks a bit odd here. From the info you give I have understood this is definitly the first floor.

 

On these 1930's houses you often find that the first floor joists are 1 1/2 " thick ~ 38mm. The old rule of thumb is when working on a 2" thick joist is to take the span in feet( 3.5m ~ 11.5 feet) divide 11.5 by two .. 11.5 /2 = 5.7 and add two inches for joists at 2' centres. Thus 5.7 + 2 = 7.7.. about 8". By inspection you can then say.. a thinner 1 1/2" x  8" deep joist looks about right at 18" centres. But you may ask.. why did they go to the bother of putting in thinner joists at closer centres. One reason is that the closer centre joists lets you make the floor boards thinner. In todays money we know that for domestic floor loading you need 22m thick chipboard for 600mm spaced joists and 18mm thick for 400mm spaced joists. Measure you floor board thickness and let us know.

 

But you mention you have 4 x 3 floor joists. Yes the width is a lot more but I suspect you have a steel transfer beam hidden in the floor that you have not found yet. Even though you have much thicker joists I doubt they are spanning the full 3.5m. You may find you have an old rolled steel joist in the floor depth acting as a transfer beam.

 

On 25/06/2022 at 13:30, Crowbar hero said:

Obviously this would lower the ceilings some, which is no problem (they're at 2.5m + already). 

If you are able to drop the ceilings then there are a few things you can do. But you need to know about this transfer beam.

 

For all. Lets just say you don't have a transfer beam. You want to add UFH,  beef up the sound proofing, make the floor less bouncy and say make the ceiling more level as it may have sagged over time. The easy way is to just bolt and glue deeper timbers to the sides of the old timbers. This stiffens and strengthens the timber between the walls. But at the ends the new timbers don't get embeded into the walls.. you leave the walls, bearing and restraint as @Annker alludes to. But now you have created a stronger timber with a big nasty notch at the end where it bears onto the walls. When you check this you'll often find that the timbers now fail in shear at the ends and the grain wants to delaminate / split not least.

 

You can get round this sometimes by using steel straps at the joist ends .. like stirrups. You can see this with Engineered joists where the top flange is the bearing part on the wall head, they configure the nail plates to act as stirrups.

 

@Crowbar hero

 

If you want post more info, a few sketches would help a lot.

 

 

 

 

 

[saveasteading]

You can fix a plywood layer on top of the joists and massively improve the strength with minimal disruption. It needs proper approval and spec by SE.

[Crowbar hero]

16 hours ago, Gus Potter said:

 

Great point @Annker

 

Something looks a bit odd here. From the info you give I have understood this is definitly the first floor.

 

On these 1930's houses you often find that the first floor joists are 1 1/2 " thick ~ 38mm. The old rule of thumb is when working on a 2" thick joist is to take the span in feet( 3.5m ~ 11.5 feet) divide 11.5 by two .. 11.5 /2 = 5.7 and add two inches for joists at 2' centres. Thus 5.7 + 2 = 7.7.. about 8". By inspection you can then say.. a thinner 1 1/2" x  8" deep joist looks about right at 18" centres. But you may ask.. why did they go to the bother of putting in thinner joists at closer centres. One reason is that the closer centre joists lets you make the floor boards thinner. In todays money we know that for domestic floor loading you need 22m thick chipboard for 600mm spaced joists and 18mm thick for 400mm spaced joists. Measure you floor board thickness and let us know.

 

But you mention you have 4 x 3 floor joists. Yes the width is a lot more but I suspect you have a steel transfer beam hidden in the floor that you have not found yet. Even though you have much thicker joists I doubt they are spanning the full 3.5m. You may find you have an old rolled steel joist in the floor depth acting as a transfer beam.

 

If you are able to drop the ceilings then there are a few things you can do. But you need to know about this transfer beam.

 

For all. Lets just say you don't have a transfer beam. You want to add UFH,  beef up the sound proofing, make the floor less bouncy and say make the ceiling more level as it may have sagged over time. The easy way is to just bolt and glue deeper timbers to the sides of the old timbers. This stiffens and strengthens the timber between the walls. But at the ends the new timbers don't get embeded into the walls.. you leave the walls, bearing and restraint as @Annker alludes to. But now you have created a stronger timber with a big nasty notch at the end where it bears onto the walls. When you check this you'll often find that the timbers now fail in shear at the ends and the grain wants to delaminate / split not least.

 

You can get round this sometimes by using steel straps at the joist ends .. like stirrups. You can see this with Engineered joists where the top flange is the bearing part on the wall head, they configure the nail plates to act as stirrups.

 

@Crowbar hero

 

If you want post more info, a few sketches would help a lot.

 

 

 

 

 

You're quite right about the transfer beam, though I'm not convinced it's steel (the only serious steel I've found so far is a lintel over an opening which turns out to be a bit of railway track),  these transfer beams runs perpendicular to the obvious joists, and thus parallel to the floor-boards (which are 22mm ish).

 

Attached are a couple of CAD sketches illustrating the estimated layout of the joists and transfer beam(s) which are about 4" deep (first floor external walls removed for clarity). Given the size and orientation of the timbers, I'm not convinced they're adding much to the party as far a lateral restraint is concerned.  The structure to the rear is the extension, and obviously not part of my current speculations.

One approach I'm considering is to add more timber parallel to and of a similar depth to the transfer beams underneath the existing joists.  This will hopefully reduce the spring somewhat, yet allow mechanical and electrical services to pass over/under the timber without the need for notching. Added depth will allow for UFH between existing upper timber and insulation between lower (new) timbers.  They can be attached to the wall with joist hangars rather than digging out fresh/deeper pockets.

Edited December 28, 2022 by Crowbar hero

[saveasteading]

2 hours ago, saveasteading said:

You can fix a plywood layer on top of the joists and massively improve the strength with minimal disruption. It needs proper approval and spec by SE

Anything about my suggestion that doesn't resolve your issues at lower cost and inconvenience?

[Crowbar hero]

1 minute ago, saveasteading said:

Anything about my suggestion that doesn't resolve your issues at lower cost and inconvenience?

Sadly it doesn't allow any greater floor void for services routing (electrical and plumbing) or noise insulation, once the UFH goes in there'll be very little space left in the remaining void.

[saveasteading]

Good point. Counterbatten underneath?  Will still be easier I'd guess.

Getting crazier here.......plywood on the bottom, which may even work better, and your floor boards provide even more stiffening, then batten underneath at plasterboard centres,   thus creating a service void.

Or your way and drill holes.

[Gus Potter]

On 28/12/2022 at 16:54, saveasteading said:

Getting crazier here.......plywood on the bottom

No that is far from crazy.. more a good understanding of how things work and the theory.

 

Ok SE wise @saveasteading is bang on so not crazy at all. The idea is that you make the floor beam deeper so it acts like a deeper timber. Yes you can do this on the top or the bottom of a beam..great idea from @saveasteading it's these small ideas on BH that can save you loads of cash.

 

To make it work you need to make sure that what to add to the top or bottom of the beam is connected to it in such a way that is acts like a solid timber. The best way to do this is to use a structural glue and really clean the mating surfaces. Any dirt is a no no as the glue does not work as intended. Modern structural glues are stroger than  a typical softwood.. D4 glue is NOT a structural glue specification. The D stands for DURABILITY.. not structural STRENGTH.. hey folks the clue is in the prefix.. it's a D so don't use it in a structural application and expect your SE to sign off!

 

No point in me pointing out just the problems.. solution is use say Cascamite or similar glue.. this is a STRUCTURAL resin glue and by default meets the durability requirements. But both D4 and resin glues are very hard to clean off your hands so wear gloves!

 

In my day job I'm reluctant to do this plating of timbers unless I really know who the builder is, that they will do what the are told and that the Client is willing to pay the builder to do what is quite a technical job. Alternatively a Client can pay me to supervise the job.. but often they don't want to pay for that last and most crucial part..I have probably saved them thousands by being an old sckool SE.. such is life.

 

But as we are BH folk we will spend that time and attention to detail.. you are more inclinded to do it right! That lifts my spirits as it does (I'm sure) for the MODS and all the other folk that post on BH.

 

Where the ply comes in is that you can also use ply to act like a wider flange like on an enginnered joist and that can give a lot more bang to for your buck. But you like an engineered joist you can't then go cutting service slots and  holes in it.

 

It a long storey but @saveasteading has put forward a nugget / great solution.. all you need to do is think if it suits your application.

 

it sounds complcated but if unsure post loads of info on you project / ideas and folk will chip in on how it all could work for you.

 

 

 

 

 

 

 

 

 

 

[George]

You would ideally prop/jack up the joist and plywood combination until the glue and fixings are in place. Else you're adding dead weight without getting the section strengthened.