Floor Joist Design

[saveasteading]

  On 13/03/2025 at 14:42, Iceverge said:

PIR between timber is only a good idea if you're selling

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"Exit interview" with the family.  What would you do differently? "No pir between rafters, only mineral wool squeezed in tight."

We had already iterated to half and half because 100pir was hopeless. 

Better 100mm of well fitting fibre than 100pir with lots of gaps.

[Iceverge]

Ok.....

 

 

Here's my proposal. 

 

1. Install a timber ledger beam bolted to the existing walls at the entire perimeter of the build to take the joists on hangers. Pozi/I beams for the long spans, c24 for the short spans to save cash. 

2. Behind this ledger include a membrane. ( Something tough like protect VP400 ) Fold it over the top of the ledger at the gables. 

3. Drop a 75*100 wall plate on top of these ledgers sandwiching the membrane. 

4. Build a gluelam beam into the gables as a ridge beam. 

5. Use 220x44 mm c24 cut rafters at 600cc to frame the roof. Sitting on the lower wall plate and joist hangers from the ridge. To be insulated with blown cellulose. 

6. Apply airtight membrane below all joists and seal to walls and membrane sandwiched at eaves. 

7. Counter batten with 45*45mm  timbers to give an insulated service cavity. Plasterboard below. 

8. Above the rafters use 11mm OSB racking. Breather membrane, battens up the rafters line, tile battens, tiles.

 

U value about 0.14. 

 

Excellent performance in airtightness, thermal bridging, noise and fire and overheating. 

 

Mostly off the shelf materials. ( Cellulose and pozi joists being the exceptions) 

 

No crane needed. 

 

All "nailgunable" timber sizes. 

 

Stick framed is adaptable to old wonky building.

 

Will do a sketch later. 

 

 

 

 

[Iceverge]

  On 13/03/2025 at 15:25, saveasteading said:

"Exit interview" with the family.  What would you do differently? "No pir between rafters, only mineral wool squeezed in tight."

We had already iterated to half and half because 100pir was hopeless. 

Better 100mm of well fitting fibre than 100pir with lots of gaps.

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RIGID INSULATION BOARDS ARE NOT SUITABLE BETWEEN TIMBERS OR IN CAVITY WALLS!!!!!!!!

 

I must have said this maybe a bizzallion times !!!

 

Hard to fight a billion pound industry, even if they do lie and burn people to death in their sleep. 

[ProDave]

  On 13/03/2025 at 14:58, Pappa said:

Sorry, could you clarify what this means.
Do you mean putting a steel beam in to split the span? That way the joists will be half the size?

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Each piece of the roof is a single bit of timber (or engineered joist) assembled on site and cut to final length on site.  so the roof is assembled one stick at a time all by hand.  The original roof will almost certainly be like that but supported on purlins rather than the ridge beam.  In our case the ridge beam is a timber Kerto beam, think an overgrown bit of plywood.  Stronger for a given size than a Glue Laminated beam.

 

some pictures of our roof being assembled:

 

 

The last picture in particular, you can see the big ridge beam, the individually cut rafters and how a dormer was formed (one on the front, 2 on the rear)

 

In our case the 11 metre long ridge beam was lifted using a digger with a boom extender. the rest were all done by hand.  At your size it should all be possible by hand.

[Tony L]

  On 13/03/2025 at 15:33, Iceverge said:

 

RIGID INSULATION BOARDS ARE NOT SUITABLE BETWEEN TIMBERS OR IN CAVITY WALLS!!!!!!!!

 

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I agree with your point re cavity walls, & having learnt so much from your other posts (thank you again @Iceverge ), I was able to tell my arch tech why he needed to change his drawings/notes to show blown EPS beads in my block cavity walls.

 

I was thinking I'd be putting PIR boards between my rafters, with Gapotape around the sides.  I'm planning to do this myself, to ensure good attention to detail & also save paying somebody else to do something that looks like it would be easy to do & also probably save money over blown cellulose (I haven't priced these two alternative methods yet).   The rafters will be a mix of 150 x 50 & 200 x 50 C24 & there are no valleys, so there won't be any awkward compound cuts.  Will PIR + Gapotape be a good solution for me, or should I specify blown cellulose?

 

Video here, for anyone who hasn't seen Gapotape:

 

[Iceverge]

  On 13/03/2025 at 17:41, Tony L said:

Will PIR + Gapotape be a good solution for me

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It'll work ok if you're very careful. 

 

It doesn't do much for protecting from noise, airtighess and fire and you'll have waste PIR that won't fit in anywhere. 

 

Robin Clevetts levels of workmanship are excellent, however he's sponcered by them and the process is still very very slow. Don't undervalue your time on a self build. 

 

The there's the cost. .....

 

Oh my lord the cost.......

 

100mm gapotape is £2/m. 

 

For 600mm cc 200mm deep rafters the actual best case scenario with zero waste  is about £17/M2. 

 

With funny angles and ends to trim etc you'll be still short at £20/m² on gapotape alone. 

 

 

 

[Roger440]

I bought some gapo tape for a small area in the loft where i need access to the water tank.

 

As above, the cost of the tape is simply eyewatering. It might be good, but to use this on any serious quantity is nuts.

 

And its still a bugger to do, though marginally less of a bugger than without.

 

[Tony L]

Thanks, @Iceverge & @Roger440.  I've spent some more time on this this evening, including looking at some of your older posts, Iceverge, & I am convinced.

 

Is Warmcel my only option or are there other suppliers I should consider?  

[Nickfromwales]

  On 13/03/2025 at 21:05, Roger440 said:

I bought some gapo tape for a small area in the loft where i need access to the water tank.

 

As above, the cost of the tape is simply eyewatering. It might be good, but to use this on any serious quantity is nuts.

 

And its still a bugger to do, though marginally less of a bugger than without.

 

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Yup, the sales patter and videos done by paid actors makes you drool for this..... I reckon you'd be regretting it in the first 2 hours or so.

 

I just used to slash cut PIR so it had a wedge shape to it, then it would fit tight in between the timbers and leave a gap towards the front face. I'd just then foam between the timber and the cut edge of the PIR, let it cure, and trim back. Then just foil tape from PIR to timber back to PIR so zero infiltration can occur. As good as it's getting afaic.

 

[Pappa]

Hi Guys

 

I've spoken to some joist companies, and the smallest they'll make posi joists etc is 225mm.

Is it worth splitting the span with an RSJ? that way, I should be able to design the floor in C24.

Can an RSJ be fastened a single brick party wall?

 

Here is the diagram again:

 

 

I'll start a new thread when I'm ready to talk about insulation. Your concerns are noted in the meantime,

 

Thanks again.

 

P

[saveasteading]

  On 14/03/2025 at 14:14, Pappa said:

splitting the span with an RSJ

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That would be pretty standard, but the walls need to support the steel. Padstones. Possibly wall reinforcement. Possibly more foundation. 

Are these existing walls? 

[Pappa]

  On 14/03/2025 at 14:59, saveasteading said:

That would be pretty standard, but the walls need to support the steel. Padstones. Possibly wall reinforcement. Possibly more foundation. 

Are these existing walls? 

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Thanks for the reply.

Yes, these are existing walls that run all the way down to the 120 year old clay foundation

[saveasteading]

  On 14/03/2025 at 15:18, Pappa said:

the 120 year old clay 

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My misreading was that  clay is rather older than that.

 

120 year old foundations though. they tend to be about 2 ft deep at most. 

By the time you do trial pits and check the wall out too, then new foundations etc, I think you might just go with timber or engineered joists.

 

This needs building regulations.  With timber onto wall plates the bco might accept it by inspection, plus the load tables for the joist spec.

If you have an Engineer then he could look at adding plywood top or bottom to make the floor into one huge slab....and the joists depth would reduce.

 

 

 

 

[ProDave]

Most terrace houses seem to span joists front to back, which often makes the span longer but avoids the party wall issued.

 

Your front room is pretty square at just under 6 metres so the span of that is the same whichever way you go.  so size jousts for 6 metre span and go front to back with a joist supporting over the door opening from that front room.  But I suspect whatever you choose you are looking at 300mm deep joists for that span.  Our posi joists for 5 metre span are 300mm deep.

[Pappa]

 

  On 14/03/2025 at 17:40, ProDave said:

I suspect whatever you choose you are looking at 300mm deep joists for that span.

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The problem I have is that I need to keep the joist size below 200mm otherwise I lose too much head height. I might be able to tolerate this, but the other problem is that I'm going to have to start shifting the staircase towards the top of the plan so I have the appropriate head height at the top of the stairs to  satisfy building regs.
If I move the stairs too far up, It means I have to start chopping in to the existing staircase coming up from the ground floor (not depicted on my plan).

 

In a nutshell, I solve all of my problems if I can keep joist size below 200mm.

Do you know if there are any ways this can be done without an RSJ?

 

If we definitely need an RSJ. It's going to be quite big (according to Deepseek and ChatGPT) - 356 x 171. This means the RSJ will dip in to the 1st floor room(s) below.

Do you know if an RSJ can be fastened on to a single skin party wall?

 

Thanks again

[saveasteading]

  On 14/03/2025 at 22:56, Pappa said:

if there are any ways this can be done without an RSJ?

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As I described above, using joists as the webs of a beam, and ply above and below as flanges. Needs an SE.

I've done that to upgrade an old floor that was bouncy,

  On 14/03/2025 at 22:56, Pappa said:

if an RSJ can be fastened on to a single skin party wall?

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Not very likely. The load is very concentrated so you need a brick pier or steel column, plus extra footing.

 

before going too far...can you get 6m joists in there and up into place?

[Pappa]

Thanks buddy

 

  On 14/03/2025 at 23:03, saveasteading said:

As I described above, using joists as the webs of a beam, and ply above and below as flanges

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I've spoken to two Posi joist companies and the thinnest they can go is 220mm. Do you know anybody who can make thinner joists?

 

  On 14/03/2025 at 23:03, saveasteading said:

before going too far...can you get 6m joists in there and up into place?

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Good question, Yes! I've got two very good windows; some strong lads to pull the joists up; and plenty of clearance to pull them all the way in to the house.

 

Just a thought about how I can keep 195mm x 45mm C24 Joists.
According to my back-of-the-envelope calculations... If I go with 215mm centers or less... It is technically possible.
(Lets round it down to 200mm centers for a nice even number when installing chipboard/plasterboard).

I believe the extra materials/labour will still be cheaper than an RSJ.

 

Is this practical however?

It does mean I don't need to get someone in to install the RSJ (this is outside my competence/confidence).

It also means I don't have an RSJ in the middle of the span which makes routing soil pipe and other services awkward.

 

Thanks again

Edited March 14 by Pappa

[Gone West]

  On 14/03/2025 at 23:14, Pappa said:

Do you know anybody who can make thinner joists?

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All my flooring joists were 5.9m long and 254mm deep, but depending on position, had flange widths of 72mm, 97mm or 122mm. The flange width is as important as the joist depth, but I haven't seen the flange width mentioned, although I must admit I've only skim read this thread. I would ask whether it's possible to use a less deep joist with a wider flange. I used Scotts, https://www.scottste.co.uk/

 

[saveasteading]

  On 14/03/2025 at 23:14, Pappa said:

Is this practical however?

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It is almost solid timber! 

[Pappa]

  On 15/03/2025 at 08:36, Gone West said:

All my flooring joists were 5.9m long and 254mm deep, but depending on position, had flange widths of 72mm, 97mm or 122mm. The flange width is as important as the joist depth, but I haven't seen the flange width mentioned, although I must admit I've only skim read this thread. I would ask whether it's possible to use a less deep joist with a wider flange. I used Scotts, https://www.scottste.co.uk/

 

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Thanks.

I've sent them an enquiry.

The metal web companies i spoke to did use varying joist widths. However the depth was 220mm or greater.

 

  On 15/03/2025 at 11:01, saveasteading said:

It is almost solid timber! 

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Yeah, I got the idea from someone above who suggested a system closer to a solid slab!

 

I'll get structural engineer to review my calculations, but tell me from a practical standpoint, in terms of installation and routing services in a narrower joist space....

Will this unorthodox installation work?

For example, I'll try to notch where possible, but there'll be reduced clearance for my drill if making holes in joists with 200 centers. So either I drill diagonally or buy a new compact tool to drill in tight spaces.

 

Thanks again. 

[torre]

What's the ceiling height below? Could you drop the ceiling to give more space for a deeper section joist? Then run them side to side off a ledger board.

 

200 centres is getting a bit extreme, even a compact drill plus a bit at least 50mm long will be tight.

 

Have any neighbours already done anything similar? Perhaps they've already found a good solution

 

 

[saveasteading]

245 x 97 @ 300mm cc from john james. They don't show any closer cc.

[Mr Punter]

I have had an engineer specify joists at 200 centres for part of a floor, so it is not unheard of.  We ended up splitting the span with a flitch beam so we could do 400 ctrs.

[ETC]

https://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletD2012.pdf

[Nickfromwales]

  On 15/03/2025 at 12:58, Pappa said:

I'll try to notch where possible, but there'll be reduced clearance for my drill if making holes in joists with 200 centers. So either I drill diagonally or buy a new compact tool to drill in tight spaces.

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Not a chance, matey. You would set all the joists out on the floor, twang a string line down each 1/3 point and then pre-drill for small bore services BEFORE installing . 3x 25mm holes down one end for electrics, 4x down the other for plumbing / other electrical / miscellaneous.

 

Oh, and yes, fit the solid timbers in at 200mm oc and move on with life. It's not going to go anywhere, and as you say, it's perfectly DIY'able.