Tom Posted January 11, 2022 Share Posted January 11, 2022 Hi all. So, finally, the jokers who put our Nudura walls up have finished and I have a really good builder lined up to start the roof in about 4 weeks. The problem is I need to finalise exactly what structure the roof will be. The building has 3 large elements to it, all single storey with a total of approx 360sqm of roof. It has been designed as a cut roof with insulation of mineral wool, the final covering will be corrugated metal. It is designed with steel ridge beams ( two steel rafters to split the longest of the 3 roofs) and lots of rafters in 8"x2" timber. The reason I'm waivering is over the cost in materials and man hours to do this and of course factor this against achievable U-value, and the builder we have lined up has lots of experience with industrial builds and is recommending going down the composite panel route. The options I can see we have so far are as follows: 1. Cut roof. Costs: approx 900 linear metres of 8x2" at approx £4.50/m (from TP) = £4000 mineral wool - 200mm thick approx £5400 for 360sqm 18mm ply sarking - £30/sheet would be £3,750 battens and counter battens membrane corrugated steel at approx £20/m = £7,200 a lot of man hours to cut and fit joists (total approx 190 timber joists) probably a lot of scaffolding U value? 200mm woool probably not enough 2. Composite panels. Costs: redesign of roofing steels to incorporate a few extra steel rafters and Z purlins composite panel seconds that my builder can get at £15/sqm (120mm thick PIR)-= £5,400 approx, these are in box section, so would need to cover roof with ... battens corrugated steel at approx £20/m = £7,200 fewer man hours, could potentially be done in the building with a scissor lift rather than scaffolding U value: 120mm PIR plus 175mm wool 3. SIPS Same costs for the steels, purlins etc as composite panels, still awaiting a quote for the panels again, much fewer man hours, less scaffolding etc 4. Thermoroof self-supporting EPS panels Same cost for the steels and purlins as the above two options quote received of approx £25k for supply of 360sqm - which makes it about £70/sqm, quite a bit more expensive than the composite panels at £15/sqm! Another cost to factor in would be how the internal finish would be done. Cut roof can have vapour barrier then just PB or even a thin layer of PIR taped to add more insulation and act as a barrier. The composite panels would need some internal timbers fixed to the purlins to board on to I guess. I know there will be lots of other costs I must be missing, but I just wanted to get something down in writing and out of my head. Hopefully then I'll be able to sleep tonight - and hopefully I can invite some comments/voices of reason to help me see a way through this. Can anyone give any advice re the above? Link to comment Share on other sites More sharing options...
JohnMo Posted January 11, 2022 Share Posted January 11, 2022 Another option would Posi rafters. These are pre made in a factory, which cuts site time down, ours are approx 9m long. Our flat roofs were ready for boarding over in two days. Used the gaps for running mvhr etc We then under draw with 50mm Short battens, then counter battened with a further 50mm, to give a 350mm cavity, this was then spray foamed. 2 Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 12, 2022 Share Posted January 12, 2022 i did mine with i joist, there are loads of different ways, maybe put some pictures up. What was the problem with the Nudura build. Link to comment Share on other sites More sharing options...
markc Posted January 12, 2022 Share Posted January 12, 2022 Roof construction depends on area, spans and pitch angle …. All work with (against) dead loads, snow and wind loads etc. 8x2 timbers is an old basic design and the methods mentioned previously perform much better. Link to comment Share on other sites More sharing options...
Iceverge Posted January 12, 2022 Share Posted January 12, 2022 Trussed roof an option? Link to comment Share on other sites More sharing options...
Tom Posted January 12, 2022 Author Share Posted January 12, 2022 Thanks all. Trussed roof not an option unfortunately. Hadn't thought of the I-joist/posi rafters. Essentially the costs would be similar to the standard cut roof as a whole I guess, except more for the joists themselves - online it seems like they are approx £10/m? Are they worth the extra? What makes them outperform traditional timber? We're not so bothered about routing cables etc as ductwork will be external. Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 12, 2022 Share Posted January 12, 2022 They come straight, lighter to use, longer lengths, mine were 10.9m with one central support. Do you have a design. Link to comment Share on other sites More sharing options...
Tom Posted January 13, 2022 Author Share Posted January 13, 2022 Thanks Russel. The roofs have a maximum span from wall to ridge of 5m, with angles of 14, 22 and 23 degrees for the three different roofs. Floor plans attached to show how they are arranged, the largest and smallest "barns" on the right and left respectively have their ridges running vertically up the page, the middle barn has it running horizontally - hipped where it meets the larger barn, and gable end and valley gutter where it meets the smaller barn. It's all single storey with vaulted ceilings. Link to comment Share on other sites More sharing options...
Tom Posted January 13, 2022 Author Share Posted January 13, 2022 Got some more info on the composite panels that I can ghet for £15/m: its Tata Trimapanel, which is a flat composite panel actually designed for walls it seems. As I'll be covering with another layer of wriggly tin I guess they would be usable on the roof. Anyway, SE looking at it now. Link to comment Share on other sites More sharing options...
Iceverge Posted January 13, 2022 Share Posted January 13, 2022 (edited) Go to a truss company. And ask for some of these. They'll even do the calcs for you too. Other wise a glulam ridge beam and I joists. In both cases. Batten + counter batten on top over roofing membrane. Airtight membrane below. Fill the void with pumped cellulose. Batten service cavity and plasterboard. Edited January 13, 2022 by Iceverge Link to comment Share on other sites More sharing options...
Simplysimon Posted January 13, 2022 Share Posted January 13, 2022 +1 to above re the i beams, just beat me to it Link to comment Share on other sites More sharing options...
Tom Posted January 18, 2022 Author Share Posted January 18, 2022 Thanks for your input both. I guess the issue with the I beams is I don't really need a big span (max 5m) - and they're going to come in more expensive than normal timber. If I go for the composite panels on purlins at the moment that seems cheaper than I-beam/timber rafters - and I've immediately already got a U value of about 0.17 with the 120mm PIR in them. Then I would fill the void created by the Z-purlins with say 150mm-200mm mineral wool and the U-value would be down close to 0.10. Tell me if I'm talking absolute bollocks! Link to comment Share on other sites More sharing options...
Tom Posted January 20, 2022 Author Share Posted January 20, 2022 Got a quote back to do the roof in SIPS (200mm deep): £72K + VAT ? Link to comment Share on other sites More sharing options...
Gus Potter Posted January 20, 2022 Share Posted January 20, 2022 On 13/01/2022 at 18:55, Tom said: ..Anyway, SE looking at it now. Good stuff. Sit down with your SE and look at your options now you are starting to get a feel for the cost. From time to time having to value Engineer can throw up other design options you may have not considered before. You may get a pleasant surpirse and save some money along the way. If you want post some cross sections so we can see the shape of the roof etc. Good luck. Link to comment Share on other sites More sharing options...
Simplysimon Posted January 20, 2022 Share Posted January 20, 2022 On 18/01/2022 at 12:18, Tom said: with the 120mm PIR in them. Then I would fill the void created by the Z-purlins with say 150mm-200mm mineral wool and the U-value would be down close to 0.10 have you priced this, blown cellulose in a 350mm i beam roof would cost 4-5k for that roof size, less than your 200mm quilt. Link to comment Share on other sites More sharing options...
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