HELP---Attic truss or Cut Rafters?????

[Renegade105]

Hi, 

 

I am in a dilemma, I need to decide which way the roof will be designed.

 

Either Attic Truss which will mean appx £30k worth of Attic Trusses, or I go Cut Rafters on site using large amount of steel work in the roof.

 

The home will be made in ICF.

 

What would you guys suggest?

 

Thanks team.

 

 

[Chanmenie]

I have decided upon attic trusses, the cost of the steels and then paying chippies to make the cut roof came to a very similar price, but would take much longer.

My chalet bungalow not as big as your house, but the principle will be similar.

Edited July 9, 2022 by Chanmenie

[Crowbar hero]

Just had attic trusses installed on my extension - it's a raised tie design to match the existing 30's cut roof lines with raised tie.
We were almost scuppered when it became apparent we couldn't get a crane onsite to lift the trusses - not a problem with conventional timber truss, but these were chonky 8" deep timber. there was a lot of swearing that morning as the builders had to lift the trusses up the scaffold by hand.

this may not be relevant to you however an attic truss means a heavy single lift, whereas cut roof tends to be in component form so easier to lift.

[Thorfun]

How are you and who is insulating the roof? If you’re doing it I highly recommend cut rafters! Having almost finished installing 190mm mineral wool between the rafters and cutting PIR to go underneath the rafters I can honestly say I wish I had a cut roof! 
 

but, if I was to do it again I’d go for really wide cut rafters ( or I-joists) and find someone to fill with blown cellulose. 

[Nickfromwales]

30 minutes ago, Thorfun said:

but, if I was to do it again I’d go for really wide cut rafters ( or I-joists) and find someone to fill with blown cellulose. 

Amen.

Blown cellulose is just too good to not use for these situations. I would not, however, go for I-joists, but would instead go for posi-joists eg so the cold bridging was as low as possible. Cellulose fills all the voids when pumped ( blown ) in, so no probs there.  Also allows a place to hide MVHR ducts in vaulted ceilings , just remember to keep the ducts tight to the upper side of the inner chord.

[JohnMo]

Our original plans were cut joists, but the joiner recommended posi-joists/rafters.  Came cut to the correct length and shaped ends, no trimming on site.  Longest rafter circa 9m, all manual handled in to place.

 

I then added counter battons to give 350mm cavity.  Mine was spray foamed; as I couldn't find a cellulose installer anywhere close to me.

[Renegade105]

hi @JohnMo, Regarding the bottom cords. I was just going to plaster board direct onto it to create the ceiling for the 1st floor.... And run all services like MVHR etc in the attic voids and just drop down where I need to.

 

Is that not something that sounds right? What was the need for the counter battons?

 

Thanks again

[JohnMo]

I counter battened to increase insulation depth, from 250mm to 350mm.  But also get rid repeat thermal bridging.

[deuce22]

I am in the process of framing my roof on an ICF construction. I used UB steels for the ridges and 10mm flitch beams for the valleys and hip. The rafters will be 8x2s filled with mineral wool and 150mm PIR on top to create a warm roof.

It will be slightly cheaper than raised tie trusses because I'll be doing it with a chippie friend. Loft trusses lost me 2m on the width of the rooms and raised tie trusses lost head height.

If the cost is roughly the same, I'd go with steels and a cut roof, you can then maximise the area of the loft. 

[Renegade105]

Hi @Nickfromwales,

 

I was planning attic trusses. Bottom cord is just timber about 225 depth. 
I am a total noob, but what exactly is the most efficient way to build a comfortable passive roof here. It will be habitable and I plan to have my office here too so want it comfortable.

 

I thought about running hvac ducts in the eaves.

in any case, if I insulate with cellulose, will the house be mortgageable in the future?

 

What would a typical design look like or where can I get this info from.?

 

thanks