sean1933

Thanks everyone I'll speak with the building inspector tomorrow about site strip and if he wants me to take photos and mark GL before I strip back - SE suggested just using a peg at lowest point (site slopes a bit) marked for GL and take a few photos. Great point thanks both, I'll also try and get BC to visit on the morning of the dig - its a big house with B&B floor (so sleeper walls too) so want to avoid double digging if I can avoid it. One saving grace is the site is big and open (2.2acres) so no problem getting around it. I dug a 1.6m pit the other day to bury some rubble and the ground clay was absolutely solid at 500mm (as per our 6 bore hole soil test) so I'm hopeful the 1.2m our SE set as minimum dig depth may be okay (subject to a few old removed trees which may well have routes in situ so require a deeper stepped section. Yes our SE said we need clayboard for any trench fill foundation above the bottom 500mm. That being said he's confirmed its only required in virgin clay and not where there's any blockwork back filled foundation. Given we are building a 200mm cavity Brick/Block structure our trench fill is 750mm wide around the perimeter and 600mm on internal sleepers - annoyingly the SE has spec'd clayboard not only on the inside of the external walls but both sides of every internal sleeper wall so I'm tempted to just fill to 500mm and do away with the hassle/cost of claymaster and extra cost of trenchfill...but I need to do a quick cost/benefit analysis before hand. What I hate is the idea of either pre-buying a load of claymaster based on a 1.2m trenchfill only to be told by BC to dig deeper and needing to source more (it's notably cheaper direct from jablite), or overbuying from day 1 based on a 'worst case' depth. I'm still struggling with the concept of wasted materials as my profession is all about the detail, but I'll have to get over that or I'll drive myself nuts! Thanks Nod - looks very similar to what I envisage for our build. Am I right in thinking you had to put sump drains in or just a weak concrete top with a fall to deal with any potential under floor flooding... or did I make that up from a similar post? Reason I ask is our architect talked about putting in sump drains under the B&B but then helpfully put it in the spec to BC but left it as something to be discussed rather than pre-agreed at plan check stage...I'm not convinced its necessary as whilst site doesnt drain well due to heavy clay soil its not in a flood area and water table shouldnt be an issue. Also I was just going to put a DPM down with a bit of hardcore to stop any growth/smell but again architect has spec'd concrete (but without any real detail) which was a surprise to me when reading the approval! Another laborious expense I hadn't planned for!

Thanks for all the replies everyone i appreciate it. Can i just check the measure from depth as looking at Oz07 picture has confused me a little. I have stripped the topsoil off c.1m around where the foundations which because of the site (overgrown wildflower/deep weeds) has removed some 200mm (to be reinstated afterwards). Should i be measuring from this new 200mm depth or the original ground level - which will be used as reference for GL? I wouldnt normally be too fussed but its a big house with lots of sleeper walls and wide (600 & 750's) foundations so at £125m3 theres a lot of money that could be wasted here! Thanks in advance. As always the helpful responses are greatly appreciated.

Hi all, I've posted a couple of times here and keep meaning to put up an introduction post with plans/photos etc and haven't got around to it yet as so busy - will sort soon I promise! W'ere using one of the insulated B&B systems (probably jablite) as we're on a wealden clay site. I'm just starting to carry out the reduced dig and am trying to understand the best approach to the depth. Our SE plans (following site soil test) say a minimum depth of 1.2m from GL but to the bottom of B&B void is 450mm (best case as site is sloping), below that being trench fill. If I reduce dig on the entire site to bottom of the void then the foundation dig will only be c.750mm which for clay doesnt look that deep. My question is will BC look to measure from original GL (which will be reinstated externally) or the reduced dig level? Or would I be wiser to just scrap topsoil off, trench fill to the required level then dig out the void afterwards (which would no doubt be an arse and very messy). I looked at the NHBC tech document for guidance but its a bit flaky and suggests either could be an option depending on how long soil is left to dry out, but still unclear in my eyes. Appreciate a newbie question but I have heard stories of BC looking in the hole and telling people to go deeper despite firm grounding @ the base. Last thing I want is to have to dig another 450mm and trenchfill because I have got over zelous on the reduced dig! thanks in advance!

I just wanted to come back and say thanks for all the helpful advice - I moved house inbetween posting so was somewhat sidetracked, apologies for the rude silence! We're yet to make a final decision as I'm waiting on SE to confirm depth of rafters - expecting them to be pretty big at something like 250mm or 300mm so assuming thats right we'll probably go with something like 280mm FrameTherm(40) between + 50mm celotex underneath OR 230mm FrameTherm(40) between + 60mm celotex underneath which by my calc should be good for 0.10-0.11. Reason for the Frametherm40 rather than 35 is that it seems to be much much cheaper (140mm is c.£4m2 vs £9m2) and does the job assuming rafters are as deep as I assume. Subject to what SE says about I'm thinking: wet plastered plasterboard Battened service void VCL/Air tightness membrane Celotex rafters full filled with frametherm 40 (save for enough space for membrane drape) breathable membrane Tile battens slates Any further comments always welcome!

I know this is a relatively old thread but what did you go for in the end? I'm in exactly the same situation and struggling to decide/interpret what I can get away with from UKPN. 3phase system to be installed near the boundary which will be used as TBS then main supply once house and garage are going up. We much prefer the idea of building a brick hut (security and look) but the UKPN spec refers to a 100mm reinforced concrete roof slab which I'm struggling to work out how to build it (other than pour in situ), and it will look naff! It seems to be a requirement as the surveyor wrote on the papers to ignore spec written and build a hut thats at least 1.2H 1W 0.3D 'concrete roof'. I really like what you did worldwidewebs - I assume because your meter is in a cabinet that the roof didnt have to be concrete? I had considered doing the same and forking out for a surface mounted 3phase meter cabinet but they are something like £220, plus as we plan to use it as a TBS it still doesnt get us over the issue of space for sockets etc.. and I hate the idea of buying 2! Any further views greatly appreciated - its a relatively trivial issue but its causing me a slight headache regardless! Side point - where did you guys locate the TBS earthing pin, outside the kiosks or through the slab?

Thanks for the welcome! You make a good point regarding BBA approval and that would probably explain why I'm struggling to get clarity. I will take another look at all the options. I'm completely open to different options I just want to ensure the best bang for buck outcome (like I'm sure we all do) and take advantage of the deep rafter space. I have started with taking an above and between approach but from reading on here, despite it apparently being the best route it doesn't seem to be followed very often - many seem to go with full fill and insulated plasterboard. Do you perceive this to be to do with the extra external roof buildup/labour involved, or other reasons I'm missing!? Thanks Dave - a similar size to ours then. Are you rafters full filled I assume? What is your build up, namely do you have insulated PB? I'm approaching this build from a DIY everything possible (so most of it bar the brickwork/plastering) so a similar route to you. Only reason I was considering grey EPS beads is its going onto the walls and seems so cheap when there is a wide void compared to other materials.

Hi all I have been an avid reader of this website for the last 18 months and learnt a lot already. We have just had planning granted to build our 'dream' 5bed property and I'm now looking at the best ways to go about building. One of the questions I have surrounds insulation, namely the roof. What with the deterioration factors and ever increasing PIR costs we have already decided on using a blown bead (grey EPS) full fill in a 200mm cavity, married to a jablite (or similar) EPS between and over B&B floor. The roof build up remains undecided and thats where I would like some input. Our property is a large barn style property with a c.8m wide eave to eave roof - we waiting on SE advice but the rafters are therefore likely to be rather chunky (300mm at a guess) so I've been looking at alternative ways to insult, obviously with EPS being a hell of a lot cheaper than PIR. I am yet to run the numbers but I was wondering whether a warm roof with PIR over rafters and EPS fulfilled rafters would work. Does anyone have any experience of using EPS bead in the roof, or any comments - it would be greatly appreciated. Even if just to say dont bother! thanks in advance. Sean