Pappa

Good sound advice @Gus Potter and @Iceverge I don't have sectional drawings I'm afraid but I'll ask my architect to have a mess around with the stairs. My Structural Engineer is back from holiday next week so will speak to him and feed back here. At the start, you all advised me to get some joist companies to design something for me, I've had lots of quotes back at 220mm deep in I-joist and Web. Prices ranging from 1.7 to 2.7k including all fixtures. This is not far off the 1.6k material cost for 195mm deep C24 at 200 centers (which hasn't been validated by SE). So 220mm is the depth to beat as things stand. Thanks again for all your help.

Personally, I would have liked to go lower, but I'm going to hit the windows if I go lower than this, not only do I not want to build a ramp over the windows, but there is some very ornate Victorian architraves around the windows which I'm hoping to preserve. So I'm kind of stuck at 2760mm in the rooms below. Happy to compromise elsewhere eg. expensive/awkward attic floor installation. Thanks again P

Thanks for the relys. 2760mm I got the stair dimensions from stairbox and staircreator and verified with architect. Oh snap. I did not think of that. So I am putting in planning permission and these plans are likely to be accepted I'm told, but am I right in thinking building control do their own thing as per their interpretation of regs? So even though plans are approved, BCO could block me from proceeding with 1.8m head height?

Thanks for all the replys guys Its 2m, however they allow 1.9m if the stairs are walking in to the pitch of the roof at the middle and 1.8m at the end of the stairs. Please correct me if I'm wrong. 14 steps is the minimum to maintain a 42 degree angle as per regs. Agreed, however there are two drawbacks to that: 1) that side of the loft enjoys the best views out the window, we wouldn't be able to access those views from the master bedroom, instead we'd have to go in to the bathroom (frosted windows) and the spare bedroom. 2) That is the dormer side so the full head height ceilings would be wasted on bathroom and spare bedroom, instead the master bedroom would get the pitched roof. Thanks again.

Thanks again More drawings attached: As things are (195mm deep joists) I'll need to cut in to the existing ballustrade and move it along by 0.263m. this is despite compromises I'm making to make the winder as narrow as possible; and ceiling has been lowered as much as possible. Thanks again P

Excellent suggestion. I was looking in to just that and then I realised there is a purlin right there! Back to the drawing board.

Thanks for the replys So to be honest... I would rather go with 195 deep joist at 200 centers (Design 1 or 2) because splitting the span (Design 3) is going to cost me an extra couple hundred quid even before you mentioned the additional column etc. Furthermore, There will still be approx 90 joist hangers to install either way, so I can't see it making a difference in terms of labour... Obviously there is the installation of the steel beam which is more work. I'm glad you raised this, I've attached some more details below. The only thing its missing is MVHR ducting. I have no idea where the ducting is coming from however. I've not thought that far.

Thats great, thanks so much. Couple of questions: 1) I can see you've factored a 30mm gap, what are the pros and cons of 30 vs 50? Is lower possible? 2) What about Aerogel? Is the cost still 10x more than PIR? could it get the depth to within rafter depth? Is Aerogel still difficult to work with? 3) The required U Value... Is that not for the entire roof construction? Could I under insulate at the top of the stairs to meet head height requirements but then over insulate everywhere else; install triple glazed velux windows etc etc. 4) Is 9.5mm plasterboard adequate for a good skim finish on top? Thanks again P

Thanks This new design evades windows and doors downstairs. 400 centers. bottom right quadrant is made up of 72 x 170 timbers everywhere else 44 x 170 I'm not sure if the top connection of the RSJ is too close to the adjacent door. The RSJ is basically 5 x 10mm steel plates bolted together but I'm sure a SE will be able to come up with something more efficient. Thoughts please?

Thanks for the replys guys Sorry, I had my wires crossed, really, I was referring to blown cellulose/EPS beads. Rafter depth is 74mm. width is 46mm. 430mm centers. Thanks again P

Many thanks for your comprehensive thoughts. Yes, but none of this will be supported by the attic floor. We should be able to keep existing purlins to support the roof. I think we can design the attic floor in a vacuum without worrying about the roof. Good shout, never even considered this. So the RSJ we were considering as per @Iceverge plan above... The top masonry wall there is a door in that position at ground floor (which isn't a huge problem, we can brick that door). However, on the south side of the plan... that wall doesn't actually go down to the foundation. There is an opening underneath it for a bay window! So it's likely that wall is already resting on a steel beam (did they have steel beams 120 years ago?). We could potentially have the RSJ slightly to the left so the new spans would be 4m and 2m. Span table says we should be able to get a 75mm x 175mm joists across the 4m span. To keep the joist depth low. Whoops, The ceiling/timers have already been demolished. To be honest, that ceiling needed to be lowered either way otherwise there was no way we could do the attic conversion. Yes, I can! If I can use 170mm joists, I don't need to alter the existing ballustrades that cover the stairs from ground floor to 1st floor. If I have to go up to 195mm deep joists, I will have to make minor alterations to the ballustrades. Ideally, I don't want to touch the existing stairs because the Victorian carpentry is beautiful. Please check my understanding... If I'm keeping existing purlins, am I right in thinking I don't need to consider any of this this? The top half of the plan there will be a dormer which will be supported by existing ridge beam (which will probably need to be upgraded to RSJ subject to SE). But the floor shouldn't need to support the dormer roof. Is that right? Good shout. Deepseek/ChatGPT believes that if I bolt together 3 Steel plates, 10mm thick, 185mm deep. It could work. I prefer to stack modular plates sidewards as they're lightweight and can be manhandled without equipment/machinery that I'm not confident with. Another good idea, I'm happy to consider this but I would expect 175mm joist depth in return. We've still got the option of 195mm deep timbers at 200 centers to fall back on (unless someone can tell me why this is a bad idea). Ultimately, any intervention involving RSJ's must be able to make reduce the depth below 195mm. Thanks again for the great discussion everyone!

Thanks for the replys guys. Did you calculate this flitch beam or is it just a rough idea? If we can get a beam with that depth to work, I'm in! So Iceverge's Flitch Beam dimension prompted me to check the previous RSJ calculation performed by Deepseek/ChatGPT. There was a flaw in their assumption which led them to oversize the RSJ to ~350mm depth. They assumed the RSJ would carry the entire floor width however in reality it probably only carries half the floor (one masonry wall will carry a quarter of the load and the other masonry wall carrying the other quarter). Does this sound about right? As far as the foundation goes, both masonry walls go right down to the foundation. the foundation is clay but compacted like iron and more than a meter deep. If we go down the RSJ route, I'll have to get structural engineer to sign off on the foundation. Thanks again

Hi guys Can you help me understand what insulation I should go for on a pitched roof in an attic conversion. I'm hoping to have the insulation layer as thin as possible because I need to maintain 1.9m head height from middle of the stairs and 1.8m at the end of the stairs. I'm hearing good things about spray foam insulation. Closed cell or open? Do I still need to maintain a 50mm ventilation gap with spray foam? What about PIR? I understand that its hard to make a tight seal against the rafters. However, what if I fill the edges using cans of spray foam? I'm not keen on the gap tape, seems very laborious. How does Cellulose spray foam compare in terms of Cost and U-value versus PIR + spray foam around the edges? Ultimately, what will give me the thinnest insulation layer that is compliant with regs? Thanks in advance P

Thanks for all the replys Good shout, already reduce ceiling below as low as possible. I could go lower, but then I'm cutting in to the window and the beautiful ornate period window carpentry. I'm going to have to compromise somewhere and I would prefer to put extra time and money in to installing more joists at 200 centers and do some awkward drilling. This is great advice. Many thanks. Will more holes not reduce the joist strength? Is there an upper limit to how many holes? What if I pre-drill the minimum number of holes if I can proactively plan the plumbing/electrics? No doubt I'll need some last minute holes... Surely I could use a multi tool drill bit or something similar to drill the holes in the tight space? Thanks again P

Thanks. I've sent them an enquiry. The metal web companies i spoke to did use varying joist widths. However the depth was 220mm or greater. Yeah, I got the idea from someone above who suggested a system closer to a solid slab! I'll get structural engineer to review my calculations, but tell me from a practical standpoint, in terms of installation and routing services in a narrower joist space.... Will this unorthodox installation work? For example, I'll try to notch where possible, but there'll be reduced clearance for my drill if making holes in joists with 200 centers. So either I drill diagonally or buy a new compact tool to drill in tight spaces. Thanks again.

Thanks buddy I've spoken to two Posi joist companies and the thinnest they can go is 220mm. Do you know anybody who can make thinner joists? Good question, Yes! I've got two very good windows; some strong lads to pull the joists up; and plenty of clearance to pull them all the way in to the house. Just a thought about how I can keep 195mm x 45mm C24 Joists. According to my back-of-the-envelope calculations... If I go with 215mm centers or less... It is technically possible. (Lets round it down to 200mm centers for a nice even number when installing chipboard/plasterboard). I believe the extra materials/labour will still be cheaper than an RSJ. Is this practical however? It does mean I don't need to get someone in to install the RSJ (this is outside my competence/confidence). It also means I don't have an RSJ in the middle of the span which makes routing soil pipe and other services awkward. Thanks again

The problem I have is that I need to keep the joist size below 200mm otherwise I lose too much head height. I might be able to tolerate this, but the other problem is that I'm going to have to start shifting the staircase towards the top of the plan so I have the appropriate head height at the top of the stairs to satisfy building regs. If I move the stairs too far up, It means I have to start chopping in to the existing staircase coming up from the ground floor (not depicted on my plan). In a nutshell, I solve all of my problems if I can keep joist size below 200mm. Do you know if there are any ways this can be done without an RSJ? If we definitely need an RSJ. It's going to be quite big (according to Deepseek and ChatGPT) - 356 x 171. This means the RSJ will dip in to the 1st floor room(s) below. Do you know if an RSJ can be fastened on to a single skin party wall? Thanks again

Thanks for the reply. Yes, these are existing walls that run all the way down to the 120 year old clay foundation

Hi Guys I've spoken to some joist companies, and the smallest they'll make posi joists etc is 225mm. Is it worth splitting the span with an RSJ? that way, I should be able to design the floor in C24. Can an RSJ be fastened a single brick party wall? Here is the diagram again: I'll start a new thread when I'm ready to talk about insulation. Your concerns are noted in the meantime, Thanks again. P

Sorry, could you clarify what this means. Do you mean putting a steel beam in to split the span? That way the joists will be half the size?

Mid Terrace No other sketches I'm afraid. No cranes, everything will have to be placed by hand I'm afraid

Thanks for this. So I'm going to have a dormer at the rear of the house which is going to be warm roof construction. However, on the front side, its a pitched roof which is going to give me limited head height. I'm going to unfortunately have to stay with cold roof insulation on this side.

Thanks for the reply. Yes, this is for an attic. The space above the floor will be habitable space and therefore within the insulated and airtight envelope. I will be installing the appropriate vapour barriers and PIR thickness against the roof rafters.

Thanks for the comments guys. Any recommendations please? Great advice, I will change to 400 centres on the top half. Bottom half won't be possible for 400 centres, I'll have to stick with 300 centers as the span is too long. I've had a look at span tables and they don't give any advice when the span is this long. I'm having to manually calculate. I didn't even know they existed ! Can you send me some recommendations please? Thanks again P

Hi guys I'm attaching a sketch I've made of the structure of my first floor. I want to build an attic floor above this. Please can you advise which orientation of the floor joists is the most structurally sound? All of the brick walls in red are structural walls that go down to the foundation. I will refine and share my joist calculations once I commit to a design. Provisionally... I think 8 x 2 might work. Thanks again P