Increasing depth of existing rafters

[ab12]

Dear All

 

Can you please advise on the following scenario.

Old property with partly vaulted ceiling has rafter depth of about 3 inches. Can this depth be somehow supplemented by adding extra timbers to the underside of existing rafters to make allowance for maximum of insulation that can be fitted between the rafters.

 

Early next year the currently worn bitumen felt will be lifted and replaced with breathable membrane by replacing all the rafters is going to be a major headache and expense as its a very complicated roof structure. 

 

Ideally I want to add extra timbers to the exposed vaulted parts of the ceiling so can pack insulating between the rafters?

[Crofter]

If the goal is to add extra insulation, could you use rigid insulation boards that go across the rafters? This would also reduce thermal bridging.

[Redbeard]

Look up 'Larsen Trusses'. Lightweight 'separating rafters' attached by plywood or softwood 'separators' to the rafters. They are most often used on the outside walls of timber framed buildings to add lots of space for lots of insulation without adding lots of weight.

 

My refurb of my room-in-roof space will use them (effectively a new, thinner, rafter 'dangling' below the existing). I have already drawn it and had it appraised by a structural eng'r to make sure the exg structure will not be overloaded, and it has proved OK. It allows me to use 'friendly' materials (weight included in the submission to SE, of course) rather than PIR. Note that unless you can get solid mountings for your 'under-rafter' in masonry elements all the weight will be on the rafters and any purlins they sit on, so the appraisal is essential. Even if you can get a mounting on masonry it is still a structural alteration so still needs SE 'blessing'.

[gravelld]

@Redbeard I'm interested in how these work when used internally - I've only seen them hung on outside walls. Can you upload your drawings?

[JohnMo]

Pretty easy, add counter battens. These will jive you a thermal repeat break as well. On mine I did 50mm packers then 50mm counter battens on that to give an additional 100mm.

 

[Redbeard]

Just now, gravelld said:

 I'm interested in how these work when used internally - I've only seen them hung on outside walls. Can you upload your drawings

 

Too late! @JohnMo has pipped me to it. They could also have been in line with the rafters but the principle is exactly the same. Thanks @JohnMo!

[Oz07]

21 minutes ago, JohnMo said:

Pretty easy, add counter battens. These will jive you a thermal repeat break as well. On mine I did 50mm packers then 50mm counter battens on that to give an additional 100mm.

 

Some big screws used there i assume? What are the packers just an offcut of same section timber?

[JohnMo]

Ring groove nails, put in at different angles so they can't pull out.

 

Packets are same material as battens. 

[Roundtuit]

Is the 3 inches the full depth of the rafter?  If so, I'd be cautious of hanging any additional weight off 75mm rafters, and be looking for a solution that added strength not just extra load I think.

[ab12]

Thank you all for the replies

I've attached pictures of the proposed rafter depth for extension.

Essentially extension will comprise two lengths of 1 metre timber attachments to the rafters and will be separated by purlin (photo shows proposed location for the timber attachments as white ellipses)

 

a) 1st segment will run from the brick wall/wall plate to the 1st purlin

b) 2nd segment will run from 1st purlin to 2nd purlin

 

 

 

Old rafters have been sistererd/doubled with adjacently placed  new rafters as shown in the photo (old dark brown, new light brown). Now I'm unsure where to attach the extra timber for  depth extensions to the old rafters or the the new sister rafters.

 

I want to extend the timber depth as much as practicably possibly but would be happy with even say an extra 10cm depth,  current old rafters are about 7 cm in depth giving me space of 17 cm which will be insulated minus 2.5 cm allowance of gap between the insulation and breathable membrane.

 

Currently there are about 10 sistered rafters and the widht of cavity space for insulation is going to be about 45cm and there will be about 9 cavity spaces to fill up

 

Rest of room has essentialy horizontal ceiling and this section of the room will be insulated between and above joists quite easily.

 

However of the opposite side of the room there is a similar sloped section of ceiling.

 

Sorry for the extra details but I thought it will provide a more clearer picture of the ambitious target.

Thanks

Edited 1 hour ago by ab12

[ab12]

[crispy_wafer]

I'm looking at this and thinking if it would it be too simple to just forget the rafter extensions, insulate with PIR Board under the rafters then screw 3*1 or 4*1 timber through to the rafters above to accept plasterboard?

[Crofter]

53 minutes ago, crispy_wafer said:

I'm looking at this and thinking if it would it be too simple to just forget the rafter extensions, insulate with PIR Board under the rafters then screw 3*1 or 4*1 timber through to the rafters above to accept plasterboard?

That's basically what I was suggesting, and how I built my own house. Except I used 2x1 battens on top of the PIR. Make sure you mark the line of the rafters before you cover them up.