300mm timber rafters

[SBMS]

Has anyone any experience of a loose cut roof with 300mm deep rafters? We were looking to use posi rafters at 304mm to get the depth for blown cellulose but it doesn’t appear to be a good fit as we’ve got two roofs running into each other with rooms in the roof. It’s looking like attic trusses are out for the same reason. 
 

I believe we can get 300mm rafters which might work, or could potentially look for an engineered I-beams. Any gotchas with this approach?

[joe90]

I used 300mm I beams for my warm hipped roof, they came with galvanised brackets for the hip joints. 

 

[ETC]

I’ve a suggestion - use a different insulation.

[SBMS]

1 minute ago, ETC said:

I’ve a suggestion - use a different insulation.

The only option for a shallower rafter is PIR. We did that on our previous build which also had attic trusses/ rooms in roof. Whilst the h value is okay the top floor gets very warm in warm weather due to the lower thermal mass (or whatever you call it) of the rigid insulation. I also don’t like the fact that timber shrinks, PIR then isnt tight and thermal bridging increases and performance is affected.  
 

if there’s another solution that gives high mass and low thermal decay in a smaller rafter am all ears?

[SBMS]

7 minutes ago, joe90 said:

I used 300mm I beams for my warm hipped roof, they came with galvanised brackets for the hip joints. 

 

@joe90what did you insulate with?

[ETC]

Post a section.

[SBMS]

10 minutes ago, ETC said:

Post a section.

Thanks @ETC will do once I’ve got the section drawings back. Only got elevations and planning drawings at the moment, unless you can work it out from these and floor plan. 

Edited July 7 by SBMS

[joe90]

30 minutes ago, SBMS said:

@joe90what did you insulate with?

Rockwall batts 

[SBMS]

3 minutes ago, joe90 said:

Rockwall batts 

Did you full fill or manage to create a cavity somehow? What’s the performance like?

[joe90]

Just now, SBMS said:

Did you full fill or manage to create a cavity somehow? What’s the performance like?

Full fill, not passive but very effective.

[IanR]

2 hours ago, SBMS said:

I believe we can get 300mm rafters which might work, or could potentially look for an engineered I-beams. Any gotchas with this approach?

My roof is simpler in some respects, but successfully done from 350mm I Joists. 300mm would have been fine but with a large roof area it improved the overall performance quite a bit going with the extra 50mm. Filled with cellulose and have no issue with sun loading.

 

 

Don't know if it would be value for money for just the roof, but if you need an I-Joist roof designed and engineered I'm sure Cullen Timber Design could do it for you and provide a full cutting list.

Edited July 7 by IanR

[Iceverge]

9 hours ago, SBMS said:

Has anyone any experience of a loose cut roof with 300mm deep rafters? We were looking to use posi rafters at 304mm to get the depth for blown cellulose but it doesn’t appear to be a good fit as we’ve got two roofs running into each other with rooms in the roof. It’s looking like attic trusses are out for the same reason. 
 

I believe we can get 300mm rafters which might work, or could potentially look for an engineered I-beams. Any gotchas with this approach?

 

300 mm rafters are hard to find and expensive. 

 

220mm should cover most bases structurally and an insulated service cavity or external layer of insulation should see you OK.

[SBMS]

1 hour ago, Iceverge said:

 

300 mm rafters are hard to find and expensive. 

 

220mm should cover most bases structurally and an insulated service cavity or external layer of insulation should see you OK.

Thanks @Iceverge - I'm trying to minimise installation effort so was trying to get a rafter depth that meant I could just use blown in cellulose.  Whilst I could extend the depth of the rafters internally, with the additional labour and timber, I wonder if I'd be better off just going for deeper rafters or I-Joists like @joe90 mentioned - even if there's additional cost?

[Dave Jones]

lot of money and hassle just to avoid using celotex and gapotape.

[JohnMo]

16 hours ago, SBMS said:

posi rafters at 304mm to get the depth for blown cellulose but it doesn’t appear to be a good fit as we’ve got two roofs running into each other with rooms in the roof.

Why is that a good fit. We have a 45 degree slope in one direction and 12 degrees in another direction, with pozi rafters, there were zero issues with construction.

 

We spray foamed 350mm deep. Done in a day and a half to complete 250m² give or take. All cut back ready for VCL.

[SBMS]

3 hours ago, JohnMo said:

Why is that a good fit. We have a 45 degree slope in one direction and 12 degrees in another direction, with pozi rafters, there were zero issues with construction.

 

We spray foamed 350mm deep. Done in a day and a half to complete 250m² give or take. All cut back ready for VCL.

@JohnMo we were told that where the rafters hit the valley they'd need cutting at angles and not ideal for pozi rafters? We've got two 45 deg roofs connecting at 90 degrees to each other - all vaulted. You got a picture of your roof? I've heard issues with spray foam and mortgages as well?

[SBMS]

4 hours ago, Dave Jones said:

lot of money and hassle just to avoid using celotex and gapotape.

I think I priced up the celotex to be more than the cellulose? Granted I could have shallower rafters with celotex but then id need insulated PB to hit the same u values?

 

maybe I’m over engineering / thinking it

Edited July 8 by SBMS

[joe90]

4 minutes ago, SBMS said:

think I priced up the celotex to be more than the cellulose?

But they have other values, cellulose is better at sound insulation and decrement delay (if I remember correctly). Cellulose would be my choice.

[JohnMo]

35 minutes ago, SBMS said:

spray foam and mortgages as well?

Not really comparing apples with apples. The mortgage issue is retrofit applied to non vapour open membrane and generally using closed cell foam. Once moisture is in the structure there is no way fir it to get out 

 

Ours is vapour open, breathing membrane for upwards transmission of vapour, the vapour shield below to stop moisture entering.

 

My roof in the photos 

 

 

[Iceverge]

17 hours ago, SBMS said:

Thanks @Iceverge - I'm trying to minimise installation effort so was trying to get a rafter depth that meant I could just use blown in cellulose.  Whilst I could extend the depth of the rafters internally, with the additional labour and timber, I wonder if I'd be better off just going for deeper rafters or I-Joists like @joe90 mentioned - even if there's additional cost?

 

 

Maybe post some sections and we can compete for your affections!

 

Personally I would try to construct something that can be made from off the shelf materials at your local BM combined with blown cellulose. 

 

Just because it's "low tech", low cost, and low risk doesn't mean it'll be low performance. 

 

 

 

 

[Iceverge]

 

The only thing I would change here is I would add a layer of OSB sheathing above the rafters. 

 

For extra noise and vermin protection and added roof strength. 

 

Or alternatively a 22mm layer of T&G woodfiber like Steico Universal. You could omit the breather membrane then.

 

 

Edited July 8 by Iceverge

[SBMS]

On 09/07/2024 at 02:44, Iceverge said:

 

 

Maybe post some sections and we can compete for your affections!

 

Personally I would try to construct something that can be made from off the shelf materials at your local BM combined with blown cellulose. 

 

Just because it's "low tech", low cost, and low risk doesn't mean it'll be low performance. 

 

 

 

 

Hi all

 

@Iceverge as requested - posting up some section drawings here. We’ve had a number of different options in terms of engineered roof including an attic truss with quite a lot of loose rafters around the valleys and a posi rafter design but now they are having to double check if it’s possible due to the way the posi rafters connect into the valley steels. 

 

We need a 300mm rafter depth for blown cellulose and would prefer not to have to extend the rafters to get this depth. 
 

we could do it all in loose cut timbers but we don’t quite hit the u value I wanted with 45mm solid timber so an I beam or posi rafter design would have been ideal. 
 

 

 

 

Edited July 25 by SBMS

[Iceverge]

What U value do you need and what's the motivation for that particular number? 

 

How thick a roof build up can you afford? 

 

It sounds like you have some tricky junctions.  As there'll be inevitably plenty of onsite carpentry a cut roof from off the shelf lumber would have lots of advantages. Time, material availability, cost and workability. 

 

However you'll have more significant thermal bridging (especially that steel) so a layer of continuous insulation somewhere would be helpful. What type is the wall construction and what stage is the build at currently? 

 

 

[SBMS]

15 minutes ago, Iceverge said:

What U value do you need and what's the motivation for that particular number? 

 

How thick a roof build up can you afford? 

 

It sounds like you have some tricky junctions.  As there'll be inevitably plenty of onsite carpentry a cut roof from off the shelf lumber would have lots of advantages. Time, material availability, cost and workability. 

 

However you'll have more significant thermal bridging (especially that steel) so a layer of continuous insulation somewhere would be helpful. What type is the wall construction and what stage is the build at currently? 

 

 

Architects are currently doing full drawings. 200mm cavity wall with platinum eco beads. 
We cant really go much more than 300mm rafter depth for planning ridge height. I had planned to put wood fibre Steico board on top of the rafters which helps a bit with bridging. This makeup, with a 300mm cellulose fill based on an I beam rafter yields about 0.12 u value for the roof which I was trying to hit. I think solid rafters increase that to about 0.13 so that might be the route we go. We could also do a hybrid - loose rafters round the valleys and then post rafters for the simple sections?
 

I was trying to avoid insulated plasterboard internal and was going to with propassiv on the internal side of the rafters with a cavity then plasterboard and skim. 

[joe90]

12 minutes ago, SBMS said:

We cant really go much more than 300mm rafter depth for planning ridge height.

I had this problem and reducing the slope by a few degrees got over the problem and was not noticeable. (And the council will never measure the actual height anyway 🤷‍♂️) my neighbour built 1200mm above his planning permission height and it was never noticed by planning.