JohnMo Posted March 10 Posted March 10 On 10/03/2025 at 22:34, Nickfromwales said: Did you install any below grade? Expand No, mainly because at time of design I didn't realise you could. Plus I didn't order enough to do it as a change. A guy down the road did his with Durisol below dpc, full filled with concrete. He even did all the internal walls with Durisol.
Nickfromwales Posted March 10 Posted March 10 On 10/03/2025 at 23:00, JohnMo said: No, mainly because at time of design I didn't realise you could. Plus I didn't order enough to do it as a change. A guy down the road did his with Durisol below dpc, full filled with concrete. He even did all the internal walls with Durisol. Expand Ok, thanks. Do they stipulate waterproof concrete below DPC like Nudura did with us?
JohnMo Posted March 11 Posted March 11 On 10/03/2025 at 23:02, Nickfromwales said: Do they stipulate waterproof concrete below DPC like Nudura did with us? Expand Didn't see anything, so presume nothing
lizzieuk1 Posted March 11 Author Posted March 11 On 01/03/2025 at 19:33, Iceverge said: Something like. Tiles Tile battens Counter battens Breather membrane 11mm OSB 350 mm I joists with Cellulose. Membrane 20*70mm battened service cavity Plasterboard Skim. U value about 0.11. Expand Would it be an option to use rockwool slabs between the I joists if we couldn't do the blown cellulose. Looking at thermal values they look very similar. No idea on cost comparison though so, any info on that would be helpful.
Iceverge Posted March 11 Posted March 11 Cellulose is dearer than Rockwool but that includes fitting. About €150/m3 plus VAT near me. Not sure of UK prices. There's a notable difference between the cheap mineral wool "loft roll" and higher density batts like Rockwool so ensure you're comparing like for like. Cellulose has the advantage that it really helps the airtighess too when it's properly installed..
lizzieuk1 Posted March 11 Author Posted March 11 On 11/03/2025 at 17:53, Iceverge said: Cellulose is dearer than Rockwool but that includes fitting. About €150/m3 plus VAT near me. Not sure of UK prices. There's a notable difference between the cheap mineral wool "loft roll" and higher density batts like Rockwool so ensure you're comparing like for like. Cellulose has the advantage that it really helps the airtighess too when it's properly installed.. Expand Thanks Iceverge, food for thought then.
Russell griffiths Posted March 11 Posted March 11 (edited) On 11/03/2025 at 15:04, lizzieuk1 said: Would it be an option to use rockwool slabs between the I joists if we couldn't do the blown cellulose. Looking at thermal values they look very similar. No idea on cost comparison though so, any info on that would be helpful. Expand I used a product called knauf omni fit. far nicer to handle than rockwool if installed from underneath. Edited March 11 by Russell griffiths 1
lizzieuk1 Posted March 18 Author Posted March 18 On 01/03/2025 at 19:33, Iceverge said: Yup, they create closed "boxes" which is easier to get a good densepack of the cellulose. There's the square root of zero thermal bridging through the OSB webs of an I joist. I would counter batten underneath for a service cavity though. Something like. Tiles Tile battens Counter battens Breather membrane 11mm OSB 350 mm I joists with Cellulose. Membrane 20*70mm battened service cavity Plasterboard Skim. U value about 0.11. Alternatively Tiles Tile battens Counter battens Breather membrane 11mm OSB 220mm sawn joists with Cellulose. Membrane 95*45 mm battened service cavity with mineral wool or cellulose Plasterboard Skim. U value about 0.14 but all off the shelf material from your local merchant and lots cheaper to boot. Expand We've hit a bit of a stumbling block with this! SE has suggested we will likely need a supported ridge beam (span is 14m) to do the I beam detail, prob is our layout on 3rd floor doesn't have any walls directly above the 1st/2nd floor walls so question then is how to provide ridge support. We also have a rear section of the house to tie in (L shaped design) so wondering how/can that work with I beams 🤔 Anyone know if I beams can be made into an attic truss and be 'self' supporting?
JohnMo Posted March 18 Posted March 18 On 18/03/2025 at 14:34, lizzieuk1 said: prob is our layout on 3rd floor doesn't have any walls directly above the 1st/2nd floor walls so question then is how to provide ridge support. Expand Add some walls, move some rooms about, on levels 1&2 or level 3?
lizzieuk1 Posted March 18 Author Posted March 18 On 18/03/2025 at 15:47, JohnMo said: Add some walls, move some rooms about, on levels 1&2 or level 3? Expand Really trying to avoid that option, will have to be a compromise somewhere, just trying to find the best one!
Russell griffiths Posted March 18 Posted March 18 On 18/03/2025 at 16:43, lizzieuk1 said: Really trying to avoid that option, will have to be a compromise somewhere, just trying to find the best one! Expand Are you saying you have a clear span upstairs of 14m without any internal walls, that’s a big area.
lizzieuk1 Posted March 18 Author Posted March 18 On 18/03/2025 at 18:53, Russell griffiths said: Are you saying you have a clear span upstairs of 14m without any internal walls, that’s a big area. Expand The total span is 14m, we do have internal walls but the walls on the 3rd floor aren't above any supporting walls below.
lizzieuk1 Posted March 18 Author Posted March 18 On 18/03/2025 at 18:53, Russell griffiths said: Are you saying you have a clear span upstairs of 14m without any internal walls, that’s a big area. Expand
Nickfromwales Posted March 18 Posted March 18 You’ll have to introduce a steel under one of those upper walls then so you can transfer the ridge down? Or do a steel apex?
Nickfromwales Posted March 18 Posted March 18 Apex would go up with roof timbers to ridge and back down again, profiled to be within the same footprint as the timber. 1
Iceverge Posted March 21 Posted March 21 On 18/03/2025 at 20:18, lizzieuk1 said: The total span is 14m, we do have internal walls but the walls on the 3rd floor aren't above any supporting walls below. Expand Your roof will obviously be supported by a wall plate on the external walls and internally the valley rafters and ridges. ( I like gluelam beams, please use gluelams cause dey so pretty 😍. Hang all the normal rafters from these . Effectively you need a path to transfer the load vertically The external walls are easy peazy. The ridges and valleys (red) you just need to able to draw vertical lines to get the load down. I've made blue dots where I think this could happen. You may need to add a lintel over an internal door but it's not rocket science. DISCLAIMER. I'm not a structural engineer any roof collapsing and tumbling on your bed at 3am instantly pulverising you and your beloved is absolutely someone else's fault . 2
Iceverge Posted March 21 Posted March 21 Playing with the AI (as I do a Saturday evening. A glulam beam 100x320mm should be ok to cover the longest ridge span of 6.7m. A 14m long one for the longest ridge would be about £700-£800. And weight about 200kg. A similar RSJ would be £1000 and weight 3 times as much and have awful thermal bridging and onsite workability charactistics in comparison. However it might be able to be bolted together easier. 225x47mm timber should be fine for the rafters. or you can play with different rafter and service cavities sizes etc . This is a low cost high performance option that i like. You could do a plain I joist with a 22mm uninsulated service cavity too, More material cost but cheaper labour. Or
lizzieuk1 Posted March 22 Author Posted March 22 On 21/03/2025 at 21:34, Iceverge said: Your roof will obviously be supported by a wall plate on the external walls and internally the valley rafters and ridges. ( I like gluelam beams, please use gluelams cause dey so pretty 😍. Hang all the normal rafters from these . Effectively you need a path to transfer the load vertically The external walls are easy peazy. The ridges and valleys (red) you just need to able to draw vertical lines to get the load down. I've made blue dots where I think this could happen. You may need to add a lintel over an internal door but it's not rocket science. DISCLAIMER. I'm not a structural engineer any roof collapsing and tumbling on your bed at 3am instantly pulverising you and your beloved is absolutely someone else's fault . Expand Thankyou 😊 lots of time spent there so really appreciate that. Re the load transfer, the 3rd floor walls around the landing don't sit directly above the 1st floor walls so, is there an issue there transferring the load down? Also, would we need all those internal walls to be solid, load bearing walls? Currently we were intending to do studs on 1st Floor (except stair wall)
lizzieuk1 Posted March 22 Author Posted March 22 (edited) On 18/03/2025 at 20:30, Nickfromwales said: Apex would go up with roof timbers to ridge and back down again, profiled to be within the same footprint as the timber. Expand That's certainly worth thinking about, what would the cold bridging issues be though? I'm wondering if we could add a steel at first floor ceiling that we could take a post up from to support the ridge, do you think that's a viable option? Edited March 22 by lizzieuk1 Altered diagram
Nickfromwales Posted March 22 Posted March 22 On 22/03/2025 at 14:26, lizzieuk1 said: That's certainly worth thinking about, what would the cold bridging issues be though? Expand Can be easily mitigated. 👍
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