300mm timber rafters

[G and J]

11 minutes ago, joe90 said:

And the council will never measure

We are advised that our planners will, rightly methinks, come out and measure if there is a complaint.

[SBMS]

On 07/07/2024 at 23:36, ETC said:

Post a section.

Posted a section. 

[joe90]

45 minutes ago, G and J said:

if there is a complaint.

What oversize are we talking about? Mine was going to be 150mm above planned height, that would be difficult to see on a two story build. Mine was a hipped roof so difficult to measure ridge height anyway. As stated I reduced the slope by a couple of degrees which was not noticeable but reduced the ridge by that required.

[G and J]

Architect just had a case where a 100mm over height was complained about and they had to re work..........one of our neighbours to be would notice.....they did a lot of measurement and comparisons in their objection and are peeved their objections were all ignored.......we may have this issue as alternative to a slight kick in the upstairs or lower ceilings in which case we'd do what @joe90 did and lower pitch very slightly .......for running of mvhr and cost we want to use 304mm posi joist, architect had assumed 200mm and we didn't check...... ho hum

[SBMS]

I wonder whether the valleys and complexity of our roof are making blown cellulose a difficult or suboptimal option? I keep

trying to work out whether there are going to be a lot of ‘nooks and crannies’ to fill with cellulose?

[Iceverge]

1 hour ago, SBMS said:

cant really go much more than 300mm rafter depth

 

Rather than rafter thickness do you have an overall thickness of the roof please?

 

When considering 0.12 Vs 0.13 might be worth sitting down and doing some calcs to really get a handle on the little difference it will make. 

 

0.01w/m²K  will really pale into insignificance Vs a little better airtighess or improved COP on a heat pump. 

 

Say 100m² of roof, a pessimistic average delta T of 15deg for the heating season of 4 months. About 2900hrs x 0.01w/m²K  x 15⁰ = 435Wh or 0.435kWh.

 

Run it through a heat pump at a COP of 4 and it'll cost you about 3p per year. 

 

 

 

 

 

 

 

 

[SBMS]

1 minute ago, Iceverge said:

 

Rather than rafter thickness do you have an overall thickness of the roof please?

 

When considering 0.12 Vs 0.13 might be worth sitting down and doing some calcs to really get a handle on the little difference it will make. 

 

0.01w/m²K  will really pale into insignificance Vs a little better airtighess or improved COP on a heat pump. 

 

Say 100m² of roof, a pessimistic average delta T of 15deg for the heating season of 4 months. About 2900hrs x 0.01w/m²K  x 15⁰ = 435Wh or 0.435kWh.

 

Run it through a heat pump at a COP of 4 and it'll cost you about 3p per year. 

 

 

 

 

 

 

 

 

Hi @Iceverge

 

I have attached full roof makeup showing depth below. I suppose I felt that 0.1 would be a great target and so every point I move away from that is psychologically difficult! But your logic makes sense.

 

[Iceverge]

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. 

 

How about this. 

 

 

 

 

Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

 

 

 

 

[SBMS]

36 minutes ago, Iceverge said:

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. 

 

How about this. 

 

 

 

 

Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

 

 

 

 

The first option is interesting - the staggered timbers looks good for thermal bridging. How would you go about connecting the 97x45mm To the 220s above?

[saveasteading]

I once had a tentative complaint that a building was too high. I asked what their datum was and they had no idea. Sorted.

[Iceverge]

3 hours ago, SBMS said:

The first option is interesting - the staggered timbers looks good for thermal bridging. How would you go about connecting the 97x45mm To the 220s above?

 

Toe nail them to get them in place,  then 150mm screws would be my idea but I'm not a carpenter. I would run them at 90 Deg to the rafters. 

 

 

Alternatively you could just nail gun  45mm service cavity battens. And extend the 220mm rafter by the same on the other side of the membrane and use more cellulose and less mineral wool. 

 

 

 

Edited July 25, 2024 by Iceverge

[Rishard]

On 25/07/2024 at 14:23, Iceverge said:

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. 

 

How about this. 

 

 

 

 

Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

 

 

 

 

While researching for my roof build up I came across these. I’m in a similar situation, considering heights and insulation / timber build up. Can I ask, how does it work with the pir option with mineral below? I like the idea of full PIR sheets and mineral between the rafters due to it being easy to install. I also like the larger service cavity for some ‘slim’ mvhr ducts in a couple of rooms, which both these examples have. 

[SBMS]

1 hour ago, Rishard said:

While researching for my roof build up I came across these. I’m in a similar situation, considering heights and insulation / timber build up. Can I ask, how does it work with the pir option with mineral below? I like the idea of full PIR sheets and mineral between the rafters due to it being easy to install. I also like the larger service cavity for some ‘slim’ mvhr ducts in a couple of rooms, which both these examples have. 

Our first build we did PIR. I have switched to cellulose and deeper rafters for our second build - again with rooms in roof/attic rooms.  Firstly, cutting and fitting pir is time consuming and difficult to get right. Wood expands and contracts and even with the best fitting there is the potential for gaps to open up. If you’re not fitting it, even less likely to be a good job. PIR also doesn’t have as good decrement delay as cellulose or Rockwool. Factoring labour costs I think blown cellulose was slightly cheaper or maybe a wash - but you’ll get a better end result and won’t risk your thermal performance on installation. For me, I have tried to engineer out as much reliance on skilled labour when it comes to the superstructure as possible. So engineered floor joists, blown eps beads in cavity, posi rafters roof, blown cellulose in roof, aerobarrier for air tightness. 

[Iceverge]

3 hours ago, Rishard said:

Can I ask, how does it work with the pir option with mineral below?

 

A hybrid warm roof is an option not loved by many on here. 

 

However if installed by someone who really has an excellent grasp of airtighess then it's totally fine. 

 

In the lower drawing you could omit the top layer of OSB and I would be happy to push it to a 1:1 ratio mineral wool to PIR. 

 

Lots of this is dictated by what depth of structural timer you require but it can be a good choice for complex roofs as it moves the airtighess layer above the rafters. 

 

 

 

[Rishard]

I have a similar calc done with pir with 120mm above and 100mm between on 120mm rafters. I would need to use purlins to get my rafters down to 120mm which I’d rather not. I reality I need 175mm rafters. 
 

Does the 18mm osb between the pir and wool need to be there? Or is that for a bit of racking strength? I’ve seen roofs with pir over rafters without a deck below on pitched roofs.
 

Are you using the PIR as the air tightness layer in this lower drawing? Or is the osb part of the airtightness layer? 

[Iceverge]

Don't put PIR between the rafters unless you're a sadist. 

 

Osb is the airtight layer. 

[Rishard]

Would you use something like propassive osb for that osb layer or another vcl laid over the osb? 
 

I’ve cut too much PIR. I’d love to not do it for my house. 

[Iceverge]

I wouldn't bother.

 

Well taped normal OSB is fine. 

[SBMS]

30 minutes ago, Iceverge said:

I wouldn't bother.

 

Well taped normal OSB is fine. 

Not if it’s cellulose filled though? OSB on top and underneath I think produced a condensation risk the manufacturer said. They recommended something vapour permeable like propassiv or a vcl. 

[Rishard]

On 25/07/2024 at 14:23, Iceverge said:

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. 

 

How about this. 

 

 

 

 

Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

 

 

 

 

Would there be much of an issue with the vcl moving to below the mineral wool? It would make joining with my airtightness layer internally much easier. 

[Iceverge]

On 08/04/2025 at 09:59, SBMS said:

Not if it’s cellulose filled though? OSB on top and underneath I think produced a condensation risk the manufacturer said. They recommended something vapour permeable like propassiv or a vcl. 

 

Nonsense. Confusing vapour driven moisture with moisture carried by crap airtightness as usual. Drafts carry often thousands of more undesirable moisture into the wall than vapour diffusion. 

 

If you put a cracker Airtightness layer somewhere/anywhere then you'll be fine. 

 

On 08/04/2025 at 16:32, Rishard said:

Would there be much of an issue with the vcl moving to below the mineral wool? It would make joining with my airtightness layer internally much easier. 

 

Which diagram are you thinking about?

 

No issue with moving the airtighess layer in board. Will it not be very tricky to detail around the roof structure though? 

 

Edited April 9 by Iceverge

[SBMS]

2 hours ago, Iceverge said:

Nonsense. Confusing vapour driven moisture with moisture carried by crap airtightness as usual. Drafts carry often thousands of more undesirable moisture into the wall than vapour diffusion.

This was the recommendation from the manufacturer who I assume I need to comply with in order to secure the BBA certificate:

 

”Using a 22mm OSB internally and a 12mm OSB externally won’t work, as there isn’t enough differential in vapour resistivity of the 2 layers (it doesn’t meet the 5:1 ratio). A vcl would need to be fitted to the 22mm OSB to ensure safety.”

 

Is that not correct?

[Rishard]

2 hours ago, Iceverge said:

 

Nonsense. Confusing vapour driven moisture with moisture carried by crap airtightness as usual. Drafts carry often thousands of more undesirable moisture into the wall than vapour diffusion. 

 

If you put a cracker Airtightness layer somewhere/anywhere then you'll be fine. 

 

 

Which diagram are you thinking about?

 

No issue with moving the airtighess layer in board. Will it not be very tricky to detail around the roof structure though? 

 

I’m doing a cut roof so I was going to bring vcl from ceiling down to meet internal plaster. At the ridge I’m using glue lam which are on show. I was going to drape a metre strip of vcl over the top of the glue lam sandwiched with a 6x2 timber plate which the rafters would fly over. This could then be taped into the ceiling vcl when roofing done. 
 

Im using 175mm rafters. My latest plan was to fill them with 180mm of mineral wool at 0.034 with 100mm pir boarded directly below the rafters taped, an additional vcl and then a 25mm service void. 
 

I’ve looked at the above make up and wonder if I could flip them, pir on the outside as you say. My concern is that if the vcl isn’t 100% then I may run into issues… thoughts? 

[Rishard]

Option 1 

 

Option 2 

[Rishard]

Would an intelligent membrane work well as the vcl in option 2?