Rafter depth increase- is it feasible?

[mh12]

Question: Is it possible to increase depth of of old rafters (3 inch depth) but adding/screwing in another say 2 inch deep timber into the rafter effectively to create an extra depth for fitting flexi woodfibre type insulation between the rafters?

Aim is to maximise on depth of insulation to mimimise heat loss through roof space.

 

[Kearn]

Yes, if you’re willing to lose internal volume.

We needed to deal with some inadequate rafters on what was originally a hayloft over a stable, now kitchen plus bedroom above.

Way too skinny and the run was a bit too long without being tied, so there was a reasonable amount of thrust on the walls.

We sistered the centre four commons with deeper timber to increase stiffness, and then battened out the rest of the commons to match the depth.
Woodfibre going between, but with a 50mm air gap above, flush with the underside and then using woodfibre boards under, plastered in lime.

You’ll want to make sure then existing rafters can cope with the added weight of timber, boards and lime though. It will be a decent amount more than the likely lath and thin lime skim…

Also, the battens will need fixing very frequently along the existing rafters.

Then screw the boards all the way through to original.

 

[Stevers]

Back in 1997 we did that with our ceiling drops in the three back rooms and used 80mm CeloTex between the thickened rafters. We haven't noticed less warm spots where the rafters are but no doubt a thermal camera would be able to see them. Nowadays I'd expect to have to put thin insulation between the rafters and then insulation that isn't squashy across the rafters in the other direction with some long screws.

 

[Kearn]

Back in 1997 we did that with our ceiling drops in the three back rooms and used 80mm CeloTex between the thickened rafters. We haven't noticed less warm spots where the rafters are but no doubt a thermal camera would be able to see them. Nowadays I'd expect to have to put thin insulation between the rafters and then insulation that isn't squashy across the rafters in the other direction with some long screws.

Even with modern PIR which gives better bang for your buck, insulation wise anyway, to sheepswool/woodfibre, it’s still 75mm between and 75mm over or under that’s seen as the norm now so you’re right, the covering of rafters is necessary for a good layer of total insulation as long as feasible to do, plus the concern of cold bridging, but as you say, I don’t know how much this is theory vs actual issue.

That said, I take the approach that some insulation is better than none, and I wouldn’t remove and lift an old roof or unnecessarily reduce limited head space internally.

 

[Alex_]

Question: Is it possible to increase depth of of old rafters (3 inch depth) but adding/screwing in another say 2 inch deep timber into the rafter effectively to create an extra depth for fitting flexi woodfibre type insulation between the rafters?

Aim is to maximise on depth of insulation to mimimise heat loss through roof space.

The advice we received when asking a similar question from a structural surveyor was along the lines of possibly but the old timbers in the roof likely wouldn't take it and the only way for me to do my calculations on this is if you pull down your 200+ year old lath and lime plaster over the pitched part of the bedrooms so you might as well replace the whole roof. Wasn't going to happen. We're going to put in what we can when replacing the roof tiles. Building control are on board with this and recognise that insisting on modern standards in old properties isn't always practical.

 

[mh12]

Thank you all for the replies. I thought it best to attach some pictures of the proposed rafter depth extension.

The extension will comprise two lengths of 1 metre timber attachments to the rafters and will be separated by purlin (photo shows the proposed place for these 1m length 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 newer rafters as shown in the photo (old dark brown, new light brown). Now I'm unsure where to attach the additional timber attachments, 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 width 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.

Hope I'm making sense.

 

[Kearn]

Your roof doesn’t look too old judging by the purlins, so Hopefully timber is ok. Odd arrangement for the purlin / common interface to be the aris, what’s the contact situation?
To be fair, I wouldn’t go out if my way to leave them exposed, you could gain some extra depth easily if need be… but appreciate that may not be ideal for various reasons.

Our room in question is similar, except head room is more limited, especially along the flanks. Plus we have no purlins, inadequate collars, and could ideally have used a central tie, but that wasn’t feasible to achieve, hence the sistering in this area.

I’m about to start this room soon, frankly I’ve been ignoring it for the last year or so!

I forget the dimensions, think the original rafters were similar at 70mm deep, they’ve all come out to 150mm, so 100+ woodfibre insulation and tbc whether woodfibre boards or simple woodwool below. Probably the latter, easier to plaster onto and lighter, but less insulation.

It’s obviously specific to my roof, but the arrangement I agreed with our SE:

additional 150mmx50mm softwood rafters sistered to central 4 existing, screwed to the existing using 5mm diameter x 70mm long woodscrews @ 100mm centres ensuring adequate edge distance of minimum 25mm.

Other existing rafters are to have a 50mmx50mm softwood batten fixing to the underside using 5mm diameter x 70mm long woodscrews @ 100mm centres located along the centreline of the rafter and batten.

The additional rafters and battens should be of sufficient length to extend at least 600mm above the collar and within 100mm of the wall plate.

We needed to add some rigidity to our roof primarily, the bonus was extra depth for insulation.

 

[Lacan07]

Question: Is it possible to increase depth of of old rafters (3 inch depth) but adding/screwing in another say 2 inch deep timber into the rafter effectively to create an extra depth for fitting flexi woodfibre type insulation between the rafters?

Aim is to maximise on depth of insulation to mimimise heat loss through roof space.

the short answer is 'yes'