john0wingnut Posted January 30 Share Posted January 30 Morning Looking for some advice from the forum. I am doing a conversion of my conservatory into a proper room, so a rear extension. Existing foundation and dwarf walls are being retained, and the plan is for a timber frame to be put on top. SE have put forward their plans which shows the 150mm TF sitting on the 100mm block and 50mm overhanging the cavity unsupported, see below. I queried this BCO when drawings were being reviewed by them, and they responded, advising they would expect the soleplate to be over both internal and external leaf's. I subsequently raised this with the SE, who asks to speak with BCO directly, only to be told BCO don't get involved with design aspects of the building, so wouldn't comment. I feel like piggy in the middle now, and not quite sure what to do, and whether what the SE has put forward is ok? As an aside, I had planned to put a row of Marmox blocks on top of the exiting wall, however, their guidance states max 20mm overhang and TF central over the block. Ive also read online that if one has the rafters notched to the outside of the wall plate, this could cause an eccentric load on wall, given the 33% unsupported. I also had a read through the LABC manual which sites 20mm max for 140mm studs............. Keen to hear the views / thoughts / advice from others on hear. Link to comment Share on other sites More sharing options...
George Posted January 30 Share Posted January 30 140mm block inner leaf with sole plate sat squarely on top. It seems like they're letting the normal assumptions for a masonry wall drive the positioning - but this is primarily a timber structure. This means the cavity can be squeaked about - so long as there is a 50mm ventilated cavity behind the cladding, and the u-value of the wall is met, the masonry cavity can be adjusted to suit the timber frame. Link to comment Share on other sites More sharing options...
john0wingnut Posted January 30 Author Share Posted January 30 15 minutes ago, George said: 140mm block inner leaf with sole plate sat squarely on top. It seems like they're letting the normal assumptions for a masonry wall drive the positioning - but this is primarily a timber structure. This means the cavity can be squeaked about - so long as there is a 50mm ventilated cavity behind the cladding, and the u-value of the wall is met, the masonry cavity can be adjusted to suit the timber frame. The dwarf walls are already built and in place, as they were part of the conservatory, so there wont be a 140mm block, as its 100mm. The external face of the new TF will be vertical cladding, so masonry outer is staying as it is, with new TF dropped on top. Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 30 Share Posted January 30 You need more details on that drawing, a better understanding of how you are going to clad the outside would show what you need to do to get the two outer surfaces working together. regarding the sole plate, can you not use 200mm and span both leaves. Link to comment Share on other sites More sharing options...
ETC Posted January 30 Share Posted January 30 Fill the cavity with concrete. Link to comment Share on other sites More sharing options...
Iceverge Posted January 30 Share Posted January 30 You could use a 220mm stud wall and bridge the cavity. Cladding Battens Membrane OSB 220mm stud with mineral wool or cellulose insulation installed flush with the outer leaf of the cavity. A/T membrane 47mm insulated service cavity. Plasterboard Skim. Link to comment Share on other sites More sharing options...
john0wingnut Posted February 3 Author Share Posted February 3 Pretty much what I’m now going to do. 240mm I joist frame. Cladding Battens Tyvek UV Facade 40mm Steico Duodry 240mm I joist with full fill steicoflex 36 18mm OSB Majpel 5 membrane 40mm service void with steicoflex 36 PB and skim Link to comment Share on other sites More sharing options...
Iceverge Posted February 3 Share Posted February 3 (edited) Not the cheapest wall but it's an excellent spec. You could fill the cavity below the DPC with insulation if that's an option. Would help with thermal bridging. The majpell is an overkill in my opinion. 18mm OSB taped will be plenty airtight enough. Edited February 3 by Iceverge Link to comment Share on other sites More sharing options...
joe90 Posted February 3 Share Posted February 3 47 minutes ago, Iceverge said: You could fill the cavity below the DPC with insulation That’s what I did. (with a brick and block construction). 1 Link to comment Share on other sites More sharing options...
john0wingnut Posted February 3 Author Share Posted February 3 3 hours ago, Iceverge said: Not the cheapest wall but it's an excellent spec. You could fill the cavity below the DPC with insulation if that's an option. Would help with thermal bridging. The majpell is an overkill in my opinion. 18mm OSB taped will be plenty airtight enough. 3 hours ago, joe90 said: That’s what I did. (with a brick and block construction). Exactly my plan When the conservatory was built they have put some wool batts in the cavity, however, its not been done very well. I pulled out one section and can see to the bottom, where the banked concrete infill is. My plan was to remove the lot, then fill the entire thing with polybeads. Majpell could be removed as you say. It wasnt in my plan originally, as, like you say, I was going to tape the OSB joints, but when I spoke with the insultation supplier, they advised adding it. John Link to comment Share on other sites More sharing options...
Iceverge Posted February 3 Share Posted February 3 3 hours ago, john0wingnut said: was going to tape the OSB joints, but when I spoke with the insultation supplier, they advised adding it. Were they selling the Majpell!? It is the best of stuff mind you. For what it's worth all our window boxes from our house are 18mm OSB (40m2) untreated and we had a test of 0.31 ACH 50. In my view even good quality 11mm OSB, the stuff with a nice smooth consistent finish is plenty airtight. Link to comment Share on other sites More sharing options...
john0wingnut Posted February 9 Author Share Posted February 9 On 03/02/2024 at 23:20, Iceverge said: Were they selling the Majpell!? It is the best of stuff mind you. For what it's worth all our window boxes from our house are 18mm OSB (40m2) untreated and we had a test of 0.31 ACH 50. In my view even good quality 11mm OSB, the stuff with a nice smooth consistent finish is plenty airtight. They were quoting me for it yes. Something like £250, so in the grand scheme of things for my rear extension / porch its a small cost for the extra belt and braces, so I may still do it. Just in contact with We Build Eco now to see whether they will quote me for a shell kit, which I can then build myself. Link to comment Share on other sites More sharing options...
ProDave Posted February 9 Share Posted February 9 Why is this such a big issue? I wired an entire 2 storey timber frame house with this "problem" The foundations were built assuming a 100mm TF so 100mm block for the inner leaf, but then they decided to build the frame in 140mm. So the sole plate for the frame overhung the block by 40mm on the inside. BC never raised any issues with this and the house is still standing 15 years later. Link to comment Share on other sites More sharing options...
George Posted February 9 Share Posted February 9 It depends on the details. It might be OK, it might not be. Sitting a 140mm sole plate centrally on a 100mm blockwork should be OK. But the eccentricity of the 40mm overhang could cause problems, unless the 100mm block is held in place with concrete cavity fill and the floor. Link to comment Share on other sites More sharing options...
saveasteading Posted February 9 Share Posted February 9 4 hours ago, George said: the eccentricity of the 40mm overhang could cause problems, If you imagine the overhang being cut off at 45°, the forces from the studs will transfer to the narrower wall. In theory it's a bit higher loaded at the inner edge, but probably ok. Double up the sole plate and it will certainly spread the load. Link to comment Share on other sites More sharing options...
Mr Punter Posted February 9 Share Posted February 9 4 hours ago, George said: It depends on the details. It might be OK, it might not be. Sitting a 140mm sole plate centrally on a 100mm blockwork should be OK. But the eccentricity of the 40mm overhang could cause problems, unless the 100mm block is held in place with concrete cavity fill and the floor. 8 minutes ago, saveasteading said: If you imagine the overhang being cut off at 45°, the forces from the studs will transfer to the narrower wall. In theory it's a bit higher loaded at the inner edge, but probably ok. Double up the sole plate and it will certainly spread the load. The OP is now proposing a 240mm I beam type frame so I guess it will span the inner and outer masonry. Link to comment Share on other sites More sharing options...
Iceverge Posted February 9 Share Posted February 9 (edited) If you're going DIYing this @john0wingnut there's a lot to be said for sticking to off the shelf materials. Stuff that you can pick up easily at a local builders merchant. Dimensional timber, OSB etc. You could do a double stud wall or similar. Here's your proposed build up. Edited February 9 by Iceverge Link to comment Share on other sites More sharing options...
john0wingnut Posted February 11 Author Share Posted February 11 Hi all. It was actually the BCO who raised the issue with the overhang and asked me to discuss with the SE. I was also directed towards info from Trada / NHBC / LABC which advises of 20mm max overhang. The dwarf walls are 9 courses high, but do have a banked concrete infill which is just under the height of 1 internal block. SE came back with a new plan of using an IG9 lintel, but that just adds more complexity, so decided to go with a timber wide enough to span the cavity. I came across Steico joists in walls, which looked like a good option, which I am now exploring. Not exactly building merchant purchase, but there is a firm not too far from me that supplies them. Ive sent my frame plans to We Build Eco, who can then work those into a timber frame kit, which they cut in their factory then ship it up for building onsite. Also includes all sheathing, insulation, membranes, tapes and fixings etc. Will see what they come back to me with. I may well do it myself, or get a local chippy to build the frame, then I take it from there and finish it off. Link to comment Share on other sites More sharing options...
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