Gus Potter

Yes it was a straight question. I was just asking as I design insulated rafts as part of my day job. Some of the stuff I do has a lot of my intellectual property built in. I'm happy to share some of my knowledge on BH, just don't want folk ripping it off whole sale for commercial advantage. As an offer if you want to have a chat on the phone then give me a call on 0771 308 1597, text me first so I know it's you, I'm also a bit deaf! We can chew the fat for a bit, happy to chat with you for up to an hour.. and you can pick my brains for free. If you want to take up my offer then send me some drawings and so on first. These of course will be kept confidential at my end. Great approach. I enjoy teaching and informing Clients. I find it a lot of fun actually. My starting point is to listen to you find out what you know and don't, then fill in the gaps in your knowledge and explain what often can be quite technical things in terms a lay person can understand. The objective is to give you enough information to allow you to make informed decisions. This sort of thing gives me a bit of a refreshing break, from drawing details / to structural calculations etc. once you have designed your first few hundred beams the novelty kind of wears off. You still have to concentrate though. In my day job I do this when pitching for business. If you inform a Client and explain the issues, how you might deal with them, highlight the things they may not have thought about, put forward other solutions, buildability cost and risk then it makes it hard for your competitors, you set the bench mark. If my competitors come in at lower fee rate, the Client is often now aware of what constitutes value for money. It works most of the time but some Clients are really stupid / just don't listen and go with the lowest price without valuing the service, ongoing support etc They will also then go with the lowest builders tender and end up nine times out of ten getting truly ripped off.

You have opened another can of worms here! At first glance the EA are saying to defer to the BS! The cost implications are potentially huge, the difference between a site being viable or not! This is a key point that contradict the English regs. I'll try and respond once had time to digest. Thanks David for joining the discussion.

Do you do this as a day job / business or are you a genuine self builder? The build light system shown is not compatible with a timber frame.

@David Cooke great post for spotting this. For the record @saveasteading CEng asked me for a second opinion on this as he had also spotted conflicts and wanted to sound me out. I can't take any credit for his astute discovery. @David Cooke both of us are interested in how we address this. I won't say any more just now as the credit has again to go to @saveasteading for spotting this anomaly. It's not just an anomaly, it can potentially lead to a significant increase in size of the drainage field. as you say David. . which is very expensive or if you don't have the space can be a major show stopper. I'll leave it at that for now and let @saveasteading to flesh this out for you when and if he has time.

You have not read and taken time to read and understand what I have written. Go back and read what I have written. Then take what I say and check with your concrete suppliers, seek advice. You may doubt what I say, but it would be a very good idea to check to see if I'm wrong. We are trying to help you here. It's not my money, I and others are giving you this advice for free! I do this stuff as a day job! I do the SE design, I kind of know how to do this. I've even written the structural calculations that go into some of the software packages that are used by the biggest supplier of cold formed steel buildings in the UK for their floor slabs. At what point are you going to wake up and think that maybe myself and others kind of know what we are talking about? If I was to bet on this I think SCC (self levelling concrete) is not your way to go as you are a novice. To hedge you bets and as you are not listening to the advice you are getting, what about just getting a part load and do one small slab to see how you get on. This way you get to practice. But I bet you have not taken any time to read the rest of my post about concrete strength ect. Look if you want to do this then it's you money. If you want to cut corners then fine, but it's false economy and your risk. I suspect you don't even understand the risk element and the risk / potential cost balance and I'm absolutely sure you have no idea how anything you change may impact on structural safety.. you should especially if you have kids.

It's gutting at times when you do a good job, no thanks, such is life.

How do you get into service that? We see it as now but once all the roof insulation goes in what then?

Ah.. I appreciate your dilemma, this stuff is not common bedtime reading! To help best, can you post a drawing of the wall construction details, with notes. Don't give us a "half drawing" I'm not interested in drawing your teeth or being a detective, give us it worts and all. I assume your cladding is vertical? If not it's a different animal, but still can be designed. There is actually a huge amount to consider here, but fundamentally, once you understand how each element of the cladding build up works and interacts it's remarkably simple and you'll end up thinking.. that is so easy! if it's well explained to you.. which some of us will endeavour to do for you. If you post a DETAILED drawing of the whole wall make up then myself and others will be able to explain what is relevant to you, how it works (have a quick check you are not doing something daft in terms of the whole wall) , the timber service classes you require, fixings, suggest best options, things to avoid, where you can maybe source what you need at sensible cost and arm you with the questions that you need to ask a supplier.

Great point Nick. Often we get what we call "water hammer in pipes" this is where there may be a bit of air, or occasionally pipe lengths that deflect. It sets up an oscillation in the "instant" water pressure ( potentially can exceed normal working pressures by a long way) that can destroy sensitive modern systems. over stress pipe connections. I would for all it costs.

Ok, my suggestion is that you don't rush this. It's a key step as if this does not turn out the way you hope you'll store up loads of problems that you'll have to deal with later. The great thing about BH is that what your are doing is quite common and folk will fesse up the mistakes they have made, so you don't repeat them. We just want to help you out! There are few if any on BH that will relish hearing that it has gone wrong for you when we could have helped. In that context. You say you have a ground bearing slab and that BC have signed off what you are going to lay your slab on. Thus it may be assumed that it is reasonably flat and level. But BC won't have checked this for you! In general terms level means that what you are laying on does not vary too much from one end of the building to the other. Flatness is roughly a measure of call it "undulation" between often 3.0m points. A slab can be level for corner extremities but go up and down like a roller coaster in between. I suspect you don't know what tolerances you should be working to. This is your SE's fault possibly, it may be buried somewhere in you specification? You say you have a 150mm thick slab. Roughly if what you are laying on is 15mm high in places then this can start to compromise the cover to the mesh. If the top side of your slab coincides with a high spot in the sub base it really eats into the performance of the slab as you are reducing what we call the effective structural depth. Ok your sub base is down now. We can have a look at how and where you finish the top of the slab. Now as you are doing this your self I can tell you that slabs often end up high. Contractors, in rush often leave slabs high also more often than not, bugger off and leave someone else to fix it. A over high slab impacts on everything that comes after. But lets turn back to structural strength first and a bit of rough techy stuff. You SE will have specified a basic structural strength of the concrete. If they are doing their job correctly they should specify a level tolerance both on what you are laying on and the top surface. I do this as a matter of course, it can go horribly wrong if you don't as you can end up with a slab that is only say 120mm thick in places.. and it's sods law that the thin bit will be under the point where there is a bit more load. Your basic might be something like a C28/35 concrete, there are lots of way we can specify concrete. It's then up to you / contractor to find a mix that can be work into place / p[properly compacted to meet the level tolerances and concrete strength. Basic concrete is made by mixing cement, concrete sand and water. When the water evaporates we have voids in the concrete, like the cell structure in wood. Thus we want to design a mix that is workable enough, but does not have too much water as this reduces the concrete strength. We call this the water / cement ratio. We measure how workable the concrete is by using often called the "slump" test. Google this and read around. It will help you lots! Now we can also add things to the concrete, other chemicals and powders which also can slow down how fast the concrete cures and remains workable. One easy way is to use what we call an air entraining agent, similar to Fairy liquid (never add Fairy liquid to any concrete of mortar mix, it's dangerous) , the bubbles of air make it more workable. Modern concrete mixes use other chemicals and agents that make the concrete more workable (basically lubricate the mix) but without reducing the matrix strength. You mention SCC (self compacting concrete) this is often also referred to as self levelling concrete. But the big problem with SCC or other self levelling mixes is that they need to be handled by experts that do it day in and day out. They can very quickly "turn on the novice" within a matter of minutes. If they turn on you then they are totally unforgiving! My advice is to look at a basic concrete mix with a bit more water, what we call a higher slump. Phone up a few suppliers, tell them what you are trying to do, your labour limitations, the weather you expect and the size and type of slab you are laying. If you make the right approach nine times out of ten you'll find folk itching to help you out! In my day job as an SE I often design quite "whacky stuff" I have a good basic knowledge but often find myself phoning the experts on a particular product to see where and when we can safely explore boundaries of design. I do check what they tell me, but I'm long past the point where I feel embarrassed to ask for expert advice, or just ask a daft question. If it's ok for me to do it then it's ok for you to call up a few concrete suppliers and ask for help? You have nothing to lose but a bit of time. You need to reflect on what you are doing. It's ok to take one step back to make two forward later.

Interesting stuff, early technology, not to be ruled out. As an SE I've always been interested in rock mechanics. It's a fascinating subject, not bedtime reading for all mind.

I agree. I've had a bit of a closer look. SE wise there is a sideways stability issue that may be questioned. This may be ok if the roof diaphragm is well braced and connected to the the main house wall. We can't yet see how you get into the extension from the house? Again this has SE implications. The drainage looks like it needs clarification and so on. Anyway, one should always not judge a job half done, thus I'm reticent to be too critical at this stage.

While getting rid of thermal bridges is admirable you need to remember that you SE needs to connect the structure together to stop it falling down, or deflecting, shrinking so much that you don't end up with a duff house. Now if you go over the score with your mitigation of thermal bridges, this can make the detailing so complex it becomes unbuildable, I'll have to introduce work SE arounds for example, the build cost and detailing time starts to rocket exponentially. Your builder will charge you for what's on the drawings.. and if it looks hard, add a premium, but often never actually deliver what you are paying for. Keep the design simple. To do this you have to embrace holistic design, from the foundations / ground investigation up. This makes it hard for self builders who want to split the design into different stages which is common on Build Hub. The trick many miss is not to get an experience designer on board early, even if just to keep a watching brief on what you're up to pretty much from day one. I do this from time to time even though my real work may only start later in the project. it also gives you time to find out if you like me, and I you! it often gives us an easy exit route if we don't gel, little money / programme time, if any, is lost. It make a lot of sense at my designer end as if I've kept any eye on the earlier process I can often avoid having to sort out a mess before I start on my own part of the design.

Try and post your drawings, you'll get loads of targeted advice if you do. How refreshing to hear, well done!

For me this is a interesting challenge to design a solution. I appreciate that you find it far from interesting and it's a worry. Stage 1: One option I would be inclined to look at first is to install another beam or beams below the ones you have if it turns out they are so heavily corroded they are well below capacity. Remember we would look initially to design these for the latest loadings prescribed in the design codes. The following is based on using a trick we deploy when we are needing to support say a large opening in a historic building or repair steels, more applicable to you. The existing beams have a point of support on say the main walls. Thus in the round we know these are the strong and proven points. It's a reasonable starting assumption the walls locally will have pretty much finished settling in and around the existing supports, thus we want to put our new point loads at these positions. Let's say, as we don't have much information that the beams are supporting the ground floor flat but also an internal spine wall that runs up the building and it turns out there is a fair bit of load. It make the challenge more interesting. This is in a bit of reverse order. How do we calculate the loads and do some other thinking. Well the loads are easily calculated once we know the building layout, floor and roof span directions. From that we can calculate the forces on the existing beam/s. We would want to split these between permanent loads ( dead loads from the materials) and imposed ( live loads) from people, furniture, snow roof load etc. Next we would want to have a look at the existing corroded beams, try and identify their section properties when new and as they are now, corroded. We can then make a reasoned guess as to what their original design strength was and what it is now. The extra thinking comes for example as one obvious question is why has it not fallen down to date? Have the beams deflected a bit too much and shed load elsewhere. Remember a building does not often fall down until it has exhausted every alternative load path. This is what SE's often call redundancy, which helps stop what we also cause disproportionate collapse, like the domino effect. We would want to get a handle on this before we access any confined space. Any Engineer is going to want to do a pretty comprehensive dilapidations survey. This heads off any "adventure’s seeking to take advantage, but it also protects the honest residents if some further cracking occurs during the works. In summary stage one is doing some fag packet calculations, a survey and from that we can get a feel for how the building is "working". Stage 2: Again not quite in the right order but it easier to hold the narrative. Lets say we can install beams under the existing, then do some bolting etc to support the loads, maybe by stiffening the webs of the existing beams or by using other simple plates / angle sections. This means the new beam/s load the supporting walls in the same place, proven to have worked so far. The big thing is that there is a good chance you won't be able to get the new beans into the basement in one complete length. Thus splices will be required, which tend to flex a bit. Also critically the new beams won't take up the load unless we pre deflect them. For them to carry load the building would need to settle onto them.. high probability of cracking as we move up the building. There is a technique we use on historic buildings where we get the new beams in with a bit of clearance and then jack between the old beams and say the top of a new beam to pre bend the new beam so it takes up the load. We have calculated the loads so have a bench mark as to how much we need to jack / pre bend. We are only talking mm here so we would use say dial gauges. In practice what we do is jack it a bit then run up stairs and see if cracking is starting or if existing cracks are opening up, doors starting to jamb an so on. This takes time, it's not a one day job! Stage 3: As above and in reverse order. The new beams often need to rest on a new padstone below the existing one spreading the load from the existing corroded beams or we need a spreading arrangement. This is partly a detailing and practical thing. But once access is gained with a fair wind usually there is a way of applying the load to the supporting walls in the same place as the existing beams are. Again simplistically we just trick the building, it's sub structure into "thinking" nothing has changed above. But this is not an exact science and 99% of the time some minor movement will occur and that has to be communicated to all parties. Comment: Infilling the basement or sifting the loads to a different place invites lots of potential settlement problems and so on. You would need to investigate the basement floor and the soil under which is going to be less consolidated than the soil under the main load bearing wall for example. Construction safety is going to be the driver here in terms of a structural solution. I would look to have at least head height clearance for folk to work. Two means of escape from the basement. Make sure we have a good ventilation strategy during and after the works, avoid any welding, use of volatile paints etc and make sure there is compliance with all the regs relating to working in confined spaces. In summary it may turn out that the SE solution is not complex, it will likely need a diligent / experienced and well supervised Contractor. The hard part is herding the rest of the cats, project administration and agreement of where the liability lies etc. This goes along way towards explaining why many designers don't want the hassle of a project like this, the design fee has to be attractive not least.