Timber frame allowing for close proximity to neighbours….

[G and J]

We will be building well within a metre of our boundary and literally just under a meter wall to wall from neighbours house.

 

We’ve been told we will need to use something like cement board for the sheathing if we go stick built, rather than OSB3.  I now realise that that will also mean we cannot have PIR outside of the sheathing board - please correct me if I’m wrong.

 

So I think our build up might look something like….

 

1. 3mm ish plaster.

2. 12.5mm plasterboard

3. OSB/ply to stiffen, solidify the walls and enable stuff to be hung wherever….

4. 25mm service void for cables only. 
5. Plastic sheet stuff of some kind.

6. 50mm PIR.

7. 140mm timber, completely filled with insulation, unsure what type but maybe rockwool.

8. Cement board

9. Water vapour permeable membrane.

10. Vertical battens (25mm)

11. Horizontal counter battens (25mm).

12. Vertical larch cladding (20mm)

 

All adding up to circa 330mm, which is a bit more than target but on reflection and applying the stuff I’ve picked up from working out what a SIP wall build up would look like I think this maybe a good starting point for analysis.

 

So folks, does the above look sensible?  How can I improve it either from a buildability, insulation, cost or any other point of view?

[joe90]

I would be suprised if you can put any timber outside the cement board (only because of regs I had to adhere too with a roof dormer on a terraced house which required fire proof cladding as the final finish.)

[kandgmitchell]

Well the larch cladding will need to be no worse than ClassB-s3 d2 and the whole wall construction should have a fire resistance of 30 minutes (which with the cement board at 12mm if I recall correctly you will probably get).

[ETC]

1HRFR external wall required plus Class 0 spread of flame for the cladding - you’d need that spec looked at by a specialist supplier.

[Alan Ambrose]

I wonder whether you can use something like a fibre cement board for that part of the build only e.g.:

 

https://www.cladcodecking.co.uk/3-66m-fibre-cement-exterior-wall-cladding

 

It may also be possible to use say steel or aluminium square tube 'battens' instead of, say, spruce. Then you would have:

 

...

cement board

membrane

V steel 'batten'

H steel 'batten'

cement board planks

 

.. which looks properly fireproof - to me, at least.

 

 

[nod]

2 hours ago, joe90 said:

I would be suprised if you can put any timber outside the cement board (only because of regs I had to adhere too with a roof dormer on a terraced house which required fire proof cladding as the final finish.)

Plus 1

[JohnMo]

Was looking through some old drawings for our house and one iterations was timber frame 1.1 U value, build was 

 

Change the osb for cement board. And build fire resistance outwards.

 

 

 

[LnP]

I've just run across this as well and realised I'll need extra money in the budget for it. Even if you're planning to clad the building in brick, you have to consider a fire during construction. The STA have published guidance, "Design guide to separating distances during construction. For timber frame buildings. Version 3.3", and "The 16 Steps to Fire Safety", available here.

I've learned I'll have to do an off site fire risk assessment. I'm still considering timber frame quotes but Potton have quoted £1,188 to arrange one. Haven't shopped around yet. I don't know yet what the cost of the mitigation will be. One timber frame supplier said they would expect to simply supply the outer sheathing on the side closest to the neighbour (and 2.4 m round the corners) in cement board instead of OSB which wouldn't cost that much more.

[G and J]

8 minutes ago, LnP said:

I've just run across this as well and realised I'll need extra money in the budget for it. Even if you're planning to clad the building in brick, you have to consider a fire during construction. The STA have published guidance, "Design guide to separating distances during construction. For timber frame buildings. Version 3.3", and "The 16 Steps to Fire Safety", available here.

I've learned I'll have to do an off site fire risk assessment. I'm still considering timber frame quotes but Potton have quoted £1,188 to arrange one. Haven't shopped around yet. I don't know yet what the cost of the mitigation will be. One timber frame supplier said they would expect to simply supply the outer sheathing on the side closest to the neighbour (and 2.4 m round the corners) in cement board instead of OSB which wouldn't cost that much more.

Our understanding from our architect is that we will have to sheath in cement board if we timber frame and have the larch cladding treated.  I stupidly assumed that as I received enthusiastic quotes from SIP companies for builds sheathed in osb that SIPs were somehow different.  That was dumb of me.  

[ETC]

Build a brick wall just inside your boundary.

[Bournbrook]

At what distance from boundary/other properties does this apply?

[G and J]

1m from the boundary I believe.  

[G and J]

10 hours ago, ETC said:

Build a brick wall just inside your boundary.

That’s not the look we are going for and it would reduce room sizes by a fair bit too.  And it wouldn’t help construction fire risk.  

[Crofter]

14 hours ago, G and J said:

 

10. Vertical battens (25mm)

11. Horizontal counter battens (25mm).

12. Vertical larch cladding (20mm)

 

You may find that the 25mm counter battens flex too much.

Is the cladding itself a T&G flush finish? If not, you may need to allow for another 20mm layer for overlaps.

[G and J]

9 minutes ago, Crofter said:

 

You may find that the 25mm counter battens flex too much.

Is the cladding itself a T&G flush finish? If not, you may need to allow for another 20mm layer for overlaps.

Oh crumbs good point.  Are you thinking flex too much at fixing or otherwise?   As I’m fixing the vertical battens to the cement board presumably I could double them up to reduce the span for the horizontal battens.

 

This is the profile we are looking at….

[LnP]

2 hours ago, Bournbrook said:

At what distance from boundary/other properties does this apply?

The STA guidance has tables of separation distances. It depends on the classification and size of the timber frame.

[G and J]

One thing I have been pondering.  Lots of peeps appear to include service voids - I’m looking at 25mm and it will have cables only in the main.  But no one talks about stuffing insulation in those voids.   
 

I think the cable ratings will be fine even allowing for insulation, but is there another reason why it’s not done? Perhaps it’s just not worth it (I can’t work out how to drive ubakus yet but I’ll will!). 

[JohnMo]

28 minutes ago, G and J said:

I’m looking at 25mm and it will have cables only in the main.  But no one talks about stuffing insulation in those voids.

It's been discussed before,

 

Would be worth doing a sanity check on the maths

 

 

[G and J]

20 minutes ago, JohnMo said:

It's been discussed before,

 

Would be worth doing a sanity check on the maths

 

 

Thank you JohnMo, will read up.

[Crofter]

8 hours ago, G and J said:

Oh crumbs good point.  Are you thinking flex too much at fixing or otherwise?   As I’m fixing the vertical battens to the cement board presumably I could double them up to reduce the span for the horizontal battens.

 

This is the profile we are looking at….

If you can put your battens closer together that will help a lot. I had 600mm centres and there was no way 25mm counter battens would have worked, they would have bounced too much to drive the nails in. But halving that span should work pretty well I think. 

 

If you're including a cement board layer anyway, could you go frame-membrane-void and then use cement board cladding? It would give a more fire proof and lower maintenance alternative to larch.

[Crofter]

2 hours ago, G and J said:

One thing I have been pondering.  Lots of peeps appear to include service voids - I’m looking at 25mm and it will have cables only in the main.  But no one talks about stuffing insulation in those voids.   
 

I think the cable ratings will be fine even allowing for insulation, but is there another reason why it’s not done? Perhaps it’s just not worth it (I can’t work out how to drive ubakus yet but I’ll will!). 

It's basically not worth the hassle. Lots of thermal bridges and gaps where your cables have to run so you wouldn't really get a full 25mm insulation layer.

[G and J]

1 hour ago, Crofter said:

If you can put your battens closer together that will help a lot. I had 600mm centres and there was no way 25mm counter battens would have worked, they would have bounced too much to drive the nails in. But halving that span should work pretty well I think. 

 

If you're including a cement board layer anyway, could you go frame-membrane-void and then use cement board cladding? It would give a more fire proof and lower maintenance alternative to larch.

Reduced batten spacing makes perfect sense.  
 

As for the latch, we are thinking we’ll get it delivered treated for both fire resistance and this sacrificial coat stuff that helps it age prematurely (much like the self build process does to us all lol). 
 

We are of the perhaps naive belief that it will be maintenance free thereafter.  No high level painting either, I’ve done enough of that over the last third of a century.  

[G and J]

1 hour ago, Crofter said:

It's basically not worth the hassle. Lots of thermal bridges and gaps where your cables have to run so you wouldn't really get a full 25mm insulation layer.

Noted.  Makes perfect sense. 

[JackOfNoTrades]

On 11/06/2024 at 18:21, Alan Ambrose said:

I wonder whether you can use something like a fibre cement board for that part of the build only e.g.:

 

This is precisely what I'm doing. The wall facing the boundary has fibre cement board cladding, and I'm having treated larch elsewhere. 

 

 

On 11/06/2024 at 17:26, G and J said:

We’ve been told we will need to use something like cement board for the sheathing if we go stick built, rather than OSB3.  I now realise that that will also mean we cannot have PIR outside of the sheathing board - please correct me if I’m wrong.

 

Double check your SE is happy with this too. I wanted to replace my external OSB3 with cement board sheathing, but have been told I need to add it in addition to the OSB as they couldn't find the details on the cement board to do accurate racking calcs. 

[Bozza]

Be very careful.  A few years ago neighbour in my parents street built a modest two storey TF extension, maybes 2-3 metres wide. It ended up uncomfortably close to the neighbours boundary / house than permitted.    I think the reason was the extension was stepped in from the front elevation but for structural  reasons when building they had to move if forward a bit more than expected and because the two houses were at an angle the extension front corner ended closer than planned.  By about an inch or two.
 

my understanding is that is was resolved with planning dept as an amended or retrospective application that was granted on the basis that the “victim” hadn’t objected.  This was because the applicant had compensated them with an huge amount of money comparative to the cost of the extension. The applicant has no choice because if the neighbour had objected they would have had to demo it, and probably abandoned the project.  I believe the error and compensation costs double the cost of the extension.

 

it does look bad TBH as on the eye it’s is uncomfortably close.