Gus Potter

It's a fascinating subject. For all on BH there are some general recommendations in the design codes, say for movement joints in brickwork / concrete blocks but mostly it's left up to engineering judgement and that is based on looking at the whole design holistically. Movement in buildings is one of the dark arts.. it's taken me 40 years to grasp the basics and I still learn from day to day. A green oak frame is just a hard wood timber structure and we have a good idea about how this will move about and how to detail the other bits of the building that are connected to it and allow for the diffferential movement between the materials. All houses move about, bend in the wind and settle for example. Take a passive / semi passive raft foundation. It sits on a thick layer of insulation which is a bit "squashy" and here we that into account. When designing I look at the materials, what needs to be supported, say walls and glass and how they behave and interact, then say OK.. how can I force the movement (which I know is going to happen) to a place where I can control it better and how do I design the movement joints to cope with this particular structure. A simple example may be where you know you have tiled floors transitioning to say engineered wood floors. The common sense thing here is to force the design so the movement takes place under the door threshhold where it can be hidden and not crack the tiles. Probably because the folk had loads of experience and understood the materials they were using. Sadly in the modern world this is often no longer the case. Do you know if these buildings have fallen down? Mind you they may have masive cracks in them?

I assume you have gas hobs etc and showers running off a gas combie boiler and not on electric. Potentially do you have 4 ovens running on xmas day.. make it make sense please or is BS 7176 not all DEI?

I have, good solution! Eating some humble pie at my end. Hopefully still pals?

I'm trying to get my head around how this is an economic build. Yes go for the wow factor.. I love oak frames. They live, breath and move about, but ICF walls are stiff for example. Then you need to sit them on some kind of foundation, could be a raft and all the time you add unwanted weight. For me a small oak frame building should almost "float" on the ground and move with the ground and it's nuances. But if you take this design approach and then go putting in sensitive finishes such as large format tiles on the floor then you need to either spend more money in the ground or re detail your finishes. There is often no free lunch. But with a bit of eccletic thinking you can often have your cake and eat it.. if you put in the work to understand the building, the ground and put in plenty movement joints. Incedentally when designing raft foundations we might choose to make them very stiff with lots of rebar.. say if there is a mining risk. Here we want the raft foundation to "stay together" so the whole house tilts a bit without breaking it's back. But we also may design them to be quite flexible as the ground may settle differentially (a different depth of good load bearing clay from one end of the building to the other|) In some ways an an oak frame is suitable for a flexible raft (often cheeper) as we just take the view.. everything is moving about so let's live with that. So long as it does not fall down then it can be ok to have a house that cracks a bit and lives! Maybe that is an eco friendly approach to design? Well a plus + for that getting an SE in early. Good designs are about taking a view on things, exploring options, ruling out others, review, develop and review again. You have done the right thing gettting an SE on board early. This will allow you to look at the design holistically from the load bearing ground up. There are great savings to be made by adopting this approach. Mnay Build Hubbers don't do this. Cracking observation. That is giving me food for thought! Why.. because @saveasteading has tackled a really complex roof on a steading conversion. They got a competant local joiner to template the really difficult joints, some of the hip angles and joints were hugely challenging and then they executed the work effectively. Ok that was done in soft wood but you could easily do the same in green oak. The connection design is similar.. dowels or bolts. Oak is stronger than soft wood thus the connections (which often govern the design) are easier to make work. My gut feeling is that a local joiner may not have the heavy tools / fabrication facility to cut complex rebates say in green oak.. but for a simple oak frame it think it may be doable on site.. after all a simple oak frame is just like a traditional cut / purlin type roof.. with 1/3 checks in the timber for the ceiling ties, half checks at the ridge and so on.

Why?

No get this check by an SE. @IanR and all. Please don't dish out this advice / suggestions without knowing about the design philosophy. I respectfully suggest that you need to know not least about rebar anchorage, bond length and how that changes depending on the embeded depth in the concrete even if giving general advice on rebar. For all and at say @IanR there is some great general advice here on BH but a lot is contradictory when talking about raft / ground bearing slabs. The main reason is that the ground varies from site to site so needs a bespoke solution. It's not just the ground but often the slope of the ground and the water table. Funnily for me as a designer it's not the concrete slab or the insulation it sits (Kore etc) on that is the hard part to design. The really hard part / that risks my PI insurance (and your house) is often how much you need to dig out below the slab or raft! This is where the main risk lies.. in the ground under the insulation.

Nothing "thick" about your query. Good spot! Check with your SE as they should have provided this dimension. It's important for both top and bottom bar. Often on a raft found the top of the ring beam is in tension (hence the top bar) when we design for say soft spots or want to shed load (called partial fixity) from column point loadings for example. The detail you see with the top bar is probably the SE making sure the corner of the building doesn't sink (critical desing case) with the weigh of the external leaf if there is a soft spot at the corner.

Will be up for a bit, I'm researching some stuff I need to dig out for tomorrow, I'll drop you a quick note.

That's a good starting point recognising that some folk can shine a bit. There are good an bad eggs in life. Ah.. dealing with the public! or is it worse being a sub contractor for say McAlpine or a smaller firm that use you as a subbie. Seriously Nick we probably have a bit in common. If you fancy chewing the fat then PM me.

Good point Nick. To add a bit. If you nip down to you local B&Q and look at the floor slab you see what we call tied (call these sawn joints) joints at 4 - 6 m spacing. About every 18m you'll see a movement joint, tends to have metal armour each side with a big bead of mastic. B & Q slabs are mostly ground bearing slabs, no rafts. As an aside when I'm designing pallat racking systems we try as best we can not to sit the legs straddling the primary movement joints. Ok so 140 m sq. Say it was a rectangle 9.5 x 15 m ~ 142m sq. You could probably do this as a ground bearing slab with a few sawn joints or non at all if you up the anticrack mesh density. If well laid and cured.. looked after then with a screed on top then it can work as you point out. I'm cautious as while it worked for you on that site it won't apply to all slabs. You make a good point about intermediate and ring beams. These if not detailed and understood properly can tend to lock the slab edges and say the middle in position. This locking effect increases the shrinkage stress in the slab that cause trouble cracking wise. It's a great job as self building and renovations for example test you skills, for me the more I learn the more I realise how much I don't know! Indeed. We may have a fair bit in common. I was a local building Contractor for 20 years before trucking off to Uni to become a Structural Engineer and Designer. It's almost a natural progression for those of us who enjoy their job. You build stuff following other folks designs, realise you can do better at times, learn, improve and implement.

To add a bit to my last post. Structural raft foundations and ground bearing floor slabs shrink (mostly) and move about. Sometimes we need to split these up if you have a big house. It's quite common to various kinds of joints when designing industrial building slabs as they tend to be big. Optimised design is also looking about how the slab will move and shrink in terms of what we are putting on top of it. For example, we need to think about resting glazing on the slab or near to, positions of walls and so on. No point in having a massive slab movement joint right under your lovely large format kitchen tiles or sensitive glazing. Bigger slabs can move 15 - 20mm, no tile decoupling matt is going to cope with that. True optomisation results from holistic design. But it's hard (impossible?) to plan down to the last detail on a self build. The secret is to build in flexibility / options and that way you generally make overall savings.

Progress! Well done. Here are a few nuances about raft design which I hope helps. When designing these for say a less experienced Client; self builder or someone (say a cabinet maker or mechanic) that want's a small industrial unit I'm always keen to understand and make the Client aware of the potential for selection of builders and how their skill set is reflected in the prices you get back. . I also want to know how experienced the Client is, have you done this before? how much do you want to get involved in the technical design and decisions that underlie the final drawings and specification that I produce. This can have a big impact on the actual cost. Ideally you want to make the job easy enough so more builders can price.. get a bit of competition going. To try and explain briefly. For a reinforced concrete slab to act properly as a raft we need to for example control the thickness... ideally it should be of even thickness...but definitley not too thin in places. The slab can end up being too thin if we don't specify a level tolerance for what is is sitting on. Now if I say on my drawings that the EPS or insulation under the slab needs to be between +0.0 mm and minus 5.0mm then most local builders would think I'm nuts and add on a cost, tell you that your SE is an idiot or worse just not do what you are paying them to do. Let's select a general tolerance of 0.0mm to -15mm for how level the EPS needs to be. This is still quite a big ask when the winter rain and snow is running down your neck! That said we don't want massive low spots as it stops the raft from sliding as the concrete shrinks. If you make it harder for the slab to slide during curing and drying out a bit then this can make the cracks worse. We may want to put a screed on top of the slab with underfloor heating pipes. Here we need to control the how level and flat the top of the slab is. We may say that from a datum no point of the slab is to be more than 5.0mm higher and no more than 10- 15mm lower. If the slab is too high then you run into trouble with the cover on your UF heating pipes, particularly if in the heat of battle on site you need one pipe to cross over the other. Now there are some folk on BH like @nod and @saveasteading that are highly experienced at this and can work to tighter tolerances... but they know their stuff! If you are a novice at this then tighter tolerances are not for you! On paper I can make a raft pretty structurally efficient (I've a tool box of tricks and different design methods) but your chances of building it for reasonable cost as per the specification are probably pretty low unless you have a lot of experience and can adequately supervise (needs experience and time) and monitor the delivery. A properly optimised raft is one that performs structurally, is designed to suit you skill set in terms of access to builders, amount of suprevision, location, finance, surface finishes and so on. The motto is .. keep it simple!

Yes as the plastic introduces a slip plane which destroys the composite action between the masonry above and the lintel. If you bend a beam (also a composite beam.. composite lintel) downwards then there is compression in the top, tension in the bottom. When you balance all the forces within the beam you'll find not least that there are axial shear forces along the length.. the top of the beam is in compression so it shortens.. the bottom is stretching getting longer.. so there must be some axial shear forces in there. If you put a cavity tray in then the longitudinal shear force is not transferred between the brick courses above and the concrete lintel.. add a bit of water to that.. as that is what cavity trays are for and you have a good slip plane. Hope that makes sense? In this case you need a non composite lintel. This is roughly how a composite lintel works.. but you often need sufficient masonry each side to act as bit of a buttress to the imaginary arch to be on the safe side. If you look at the manufacturer data for composite lintels they give you this information on the number of bonded courses of brick you need over the composite lintel to make it work. Many manufactures data tables use a safe working load (SWL) which has the factor of safety (FOS) built in. This is fine for normal day to day stuff but watch out when you go for the longer spans as the deflections could cause other issues with say bifold doors below or sensitive renders. Always check to see if the lintel manufacture shows their loads as the total load or a UDL load (load per metre run of lintel) In the round I often use non composite lintels as the extra cost outweighs the on site risks of bad workmanship, folk making late changes (an enthusiastic heating Enginner coring / slapping holes in the theoretical arch etc) to the design or propping an faffing about.

Good point. I get a few folk a year and look at stuff ourselves where these cracking sites come up. Then you lift the bonnet and find all sorts. Most of the issues with these dilapidated things on small sites lie around servicing it and associated legal burdens. My mind set is like you perhaps.. ok lets go back to the beginning and see where the problems really lie as above. The seller want to sell.. you want to buy! Now here you need a bit of good faith but at the same time you don't want to incurr full on professional fees each time you look at a distressed property. You can get around this by either doing a lot of learning.. and by that I mean a lot or keep someone like me on a retainer, basically phone a friend. I have a couple of Clients who work this way. One is based in Glasgow who does both commercial and residential property development.. and we often chew the fat.. probably 1 in 5 to 10 projects end up getting taken forward to the bidding / financial negotiation stage. Remember one of the many common laws of business.. turnover is vanity.. profit is sanity! What you say is right.. it's about managing the seller's expectations... but not giving away just how you are going to make the project financially viable as some of these folk will just pinch your idea and advertise again using your hard work! It's a mucky business! For all though. Sometimes you see a property with a nice clean title and a friendly seller that is just stuck. Case in point. I worked on one years back where the public road was subsiding into and towards the house that was built lower down the slope. The highways were claiming that the house owner was liable... which basically made the place unsellable. The solution was in effect to use the first floor of the house as part of the stability system to retain the road retaining wall.. kept the highways department happy and that made the project fly. One trick here was to dig out quite a lot of soil and replace it with EPS as this reduced some of the soil loads behind the retaining wall . The off street parking was then built on top of the EPS which also served as great insulation to the semi basement wall. I'll do a sketch sometime for all. To conclude.. usuallly it's a service thing.. but sometimes the project can fly with innovative Enginnering.. which floats my boat! @Jimbobjones Good luck and hope you can do a deal!

Thanks for all the useful comments folks.. will mull it all over... lots to learn!

To add a bit. When we want to build on rock to get going we need to get a feel for how the rock is laid down and it's properties. We start with the compressive strength. That would be if we found a homogenous piece with no cracks, no weathering etc and crushed it in a test rig. Usually that gives a big number. But we need to know this number if we were to bolt a steel column with a thick steel base plate directly to the rock with a bit of grout under it. Here we have a higher localised compressive stress, like under the heel of you shoe. But often rocks have cracks in them. It's not solid rock. The cracks can be filled with other material. What we also need to know is how the cracks are spaced and their approximate width. We also need to know the direction of the cracks and plane. Rock (depending on type) can crack in 3 dimensions so it takes a bit of working out. Think of say leggo blocks. If all the leggo block interfaces are horizontal, vertical, interlocked and nice cubes.. they will carry more weigth. But if the leggo is slanted then they can slide over one another thus will be less able to bear load. If we have wide cracks in the rock filled with say clay/ silt / organic soil then the water gets in and this all acts as a lubricant which lets the blocks of stone slide more easily. When resting a building on sloping rock we need to work out if the rock will peel away / slide towards the open face or not and to make a judgement we need to look at the above not least. If it were to do so it would pull your supporting columns with it. Generally most rocks can take lots of load. Even if they have cracks they are fine so long as we have some confinement. If the rock is reasonably confined against sideways movement it can't go anywhere!

Love this! Watched the video, brilliant, love how direct glazing is explained and the cost element for example. This is the sort of stuff I enjoy, fraught with potential structural problems, reg compliance and legal burdens. In Scotland, not least the Church of Scotland, WRI etc have been selling things like this off and they can go really cheep. Sometimes you can pick them up when folk have bought them off the church say, lost their shirt and have to sell on! If you want to buy something like this you need not just to get all your ducks in a row (lots of research and learning) but also spot the elegant and cost effective design solution. Up front this is often getting the site services and foundations sorted out. There are bargins to be had but research and engineering solutions are the key. To be honest you often need to have an SE on board that also knows about drainage and all the rest that has lots of experience before you put in a bid for these things. You then need a good solicitor to wrap up the deal legally! In terms of cost anything that cantilevers often attracts extra cost. Building straight onto rock.. often a joy. I've done the odd build on rock in Scotland. Often it's not the rock (even some slightly weathered rock.. will often happen with Tuff) but BC and the NHBC that cause an issue as they bang on about frost protection. Often they just need a bit of education about frost heave, how ice crystals grow, their water demand and grouth rate. Don't forget it's just a house you are building which is pretty light weight. Funnily there are lots of houses in the UK built on rock.. extending them can be easy at times once you get your head around the common constraints and convincing BC that you don't often need to dig out the rock to put concrete back in the hole!

Hi all. I've had a bit of a rake about on BH and read around some of the great stuff you have been posting about AI and how that may fit into the self build market, conversion of farm buildings to domestic and building extensions. For me I started out some 40 years ago in construction before we had computers and the internet. Quotations were typed out and sent by post! To provide a bit of context I now work for myself as an SE and Architectural Designer. Prior to that I worked and trained mostly with a cold formed shed steel company that sold a lot of buildings in Australia and the UK. They were the biggest supplier (of CF sheds, some we turned into houses etc) by a long way in the UK. My role there was to develop some the engineering calcs, do a bit of R & D and design raft slabs etc. But the funny thing was that it was almost a software company that sold steel sheds via franchises and this is done via the internet.. a web site. To summarise.. AI I think is just a natural progression that needs to be used as a tool like any other. But I'm struggling to work out where to go next! At the moment my bread and butter income comes from designing extensions, loft conversions, knocking a lot of holes in walls to form open plan spaces. The other part of my portfolio is what I call "whacky and challenging stuff" for want of a better word. This could be anything from an interesting self build, farm building conversion to the odd bit of dispute work say against the NHBC. Now technology we know moves fast and I've been trying to get my head around whether AI is a threat to my income stream say over the next ten to fifteen years.. and then I'll probably hang up my boots. Here is a link to the RIBA where they talk about this: https://www.architecture.com/knowledge-and-resources/knowledge-landing-page/no-turning-back-41-of-architects-now-using-ai?srsltid=AfmBOoqSGRmmzU3V9DcDtLc4hEeimXg3_kxSkilpP1ux1xJ6xEoRjH8O To save anyone but the very keen reading . In summary there are a lot of mixed opinions and some Architects are dabbling with AI. There is some interesting points on design responsiblity, copywrite, who is training the model and providing the data that AI learns from? There is an element of justifiable nerviousness. Another rough quote I saw elsewhere was, AI is not a direct threat to say the Architect / SE in design experience terms but it could make you much less competetive if you don't use it.. like we use excel or word.. which is a strategic business threat. If you go bust then.. Personally I know I have to get my head around this in some form or another if I want to still be in business in say 5 or 10 years time... but where do I focus my learning effort. At the moment I see this panning out as having access to better (cheeper) SE visualisation and Architectural rendering of finishes. This could maybe extended to show how say a traditional cut timber roof works to act as an aid to the contractor when pricing. Funnily I use a pretty high end structural steel and timber detailing package that does all the 3D stuff and produces the fabrication drawings anyway. But it is an SE package and does not do Architectural renders for example. Often I'll add some screenshots to a complicated 2D SE design drawing so folk can see how it fits together. But could AI know just how to pitch and angle a screenshot to capture all the tricky bits that a builder needs to know? I just can't see AI at the moment helping me design a padstone at the end of a beam on an 120 year old stone structure, with 5.0 metre bifolds under with some lateral stability problems. Then you have the temporary works.. propping and so on.. all to be cost effective. But where is can help a lot is in basic visualisation if it is cheep! As a one man band my unique selling point is my experience and the fact that I wear two hats, SE and Architectural designer. This make me competetive financially (one stop shop) as I am able to "see the whole design picture", and this is attractive to some Clients.. but not all! Now scale that up a bit to say a small Architectural practice. Again they have a USP but I think it will be the big outfits that may, as always wil use this first.. which at the moment puts it out of reach of the self builder. I'm tying my ownself in knots here as am just learning! I think AI will be at first low cost but then the software fees will rack up.. and this has to get passed on in some form or another. The question is.. Will it make self building / extending more affordable? Personally I hope that there will always be a need for a designer like me, small and in a niece market. But one hard law of business.. how much do you cost! In ten years time someone may call me an "Artisan Designer".. I hope! Maybe a way of explaining my raw thinking is to refer to Adam Smith, The Wealth of Nations; circa 1776 where he explains the division of labour. AI is in some ways doing this.

Stunning! Your hard work has paid off.

I could have worded that post better. Still pals I hope?

Yes thanks for the offer. xxx

To question (a) correct, (b) first part .. yes they can make the thermal detailing easier depending on what level of insulation you want to achieve. Sometimes it's cheeper and most effective to go for a simple strip found that a local builder can do in their sleep and add a bit of extra insulation elsewhere in the easy places.. this can be called the compensatory design approach to insulation for example. Design is often about making educated compromises on some details so you end up with a better job performing and buildable job at the end of the day. It's surprising what you can get a raft to sit on in terms of ground bearing capacity. I've done some whacky ones with an allowable soil bearing capacity of 30 kPa in soft SAND near the coast... a nice value is normally around 100kPa about ten tonnes a square metre. But that is not all of the story. A low bearing capacity soil can also be a CLAY soil where is swells and shrinks. This makes things more tricky as while the soil may carry the load ok it keeps wanting to move up and down over the seasons. You can do rafts down to quite a depth at times. Down to 1.0m is perfectly doable at times but beyond a 600mm depth of hardcore the NHBC want the fill to be specified and designed by an SE say. Sometimes (you need to be brave and get your sums right) we dig out the heavy soil and replace with insulation and a less dense material so the soil at the base of the excavation does not "know" we have popped a house on top. Here is an opportunity to use say Foamglass Geocell as it is light weight, good bearing capacity and has a quantifiable U value.

Yes. A reason is that on self build you have much more control. Mistakes will get made, people won't turn up on site when the should and so on. The Kore stuff may fit on the draiwng you give them but you'll need to pay for a good site survey. Also say you start digging and you hit a soft spot in the ground.. what do you do.. phone Kore or a friend? Hey, if English is not your first language your get a prize for putting me on the spot! (calling me out) You starting point is to get some base prices from say Kore, and so on. Then run it by your SE and see how you value engineer not just the foundation but also the superstructure. For all we know if you get prices from different folk their interest is not yours! They just want to sell you stuff! A lot of people on BH are their own worst enemies. They want the cheapest price for everything, but in the heat of battle they end up loosing money... as they don't know when to pay for some professional advice. It can be done and you can drive down the build cost a if you take time to identify what you want and then negotiate with builder.

My objective is to save you money. It's not often I start a new topic but here are some of my thoughts. Please excuse the spelling and grammer as I'm a bit dyslexic and off duty from my day job. Bit of narative first to let you know I know my stuff! Over say the last 10 years I've seen a lot of enthusiasm in the self build market for insulated rafts in one form or another. I got into this as an SE and developed the design calculations for the biggest supplier in the UK of cold formed steel industrial buildings, about 750 to 800 descent sized buildings a year. These are lightweight buildings with low floor loadings, like a house.. that sit on a big thick layer of squashy insulation. an insulated raft. I had a loads of fun at Capital Steel Buildings (CSB my employer), Gary Watson, their CEO, his directors in Australia not least put their money (a lot of 500k plus) and faith in me. Gary Watson the CEO who lives in Dalry, Scotland! So this is not some abstract concept for me! He lives just down the road pretty much, stuck his own money behind me and we sold a lot of buildings in the UK. Many thousands! I know a bit about this insulated raft malarky in terms of structural and geotech design! That said if anyone wants to engage with me on the theory then I would be delighted. It is a bit of a dark art, coupled with plastic and yield line theory design. I really push the boat on it but am always safe. Serviceablity needs discussed but I know the maths! For all the corners of a slab tend to be the bits that are hardest to get to work. For any curious SE's lurcking in the background. I took TR34 as a basis and then developed from first principles.. there is no "funny FE " software used! It's all verify able by excel! For all in some ways the design of insulated domestic rafts is very elegant and often cost efective. But get it slightly wrong and you are in trouble! But make no mistake we were doing this stuff in a hard and competetive commercial market probably before Kore rafts etc and Tanners design became more main stream in the self build market. For me I'm delighted that self builders are getting on board with insulated rafts and the whole passive house thing. Aside I wish I knew about this stuff when I did my self build, with hind sight I made some mistakes. Ok here is the rub. There are folk now jumping on the badwagon that I and others developed 15 years ago but they don't actually know what they are doing! Manufacturer's are creating insulated formers that come at a premium cost and claim they are at the cutting edge.. they are not often in terms of buildability! I know this as I as above developed lean desings. These folk are there to sell you insulation and their building system. *Kore etc are there to sell you insulation! *MBC et al are there to sell you timber frame! *You as self builders want to get an SE or Architect at the same price as a washing machine engineer. My advice is to look at simple stupid alterantives that you can get off the shelf! that can be a bit of PIR insulation cut up to avoid waste and perfom the same function. Example: someone pitched up the other day wanting to use a space frame. Someone said oh Kore do a thingy that can support the outer leaf. I think why as an SE can you not just specify a couple of braces in the frame so you don't need that complex outer "thingy" detail? Ask me if interested. Now folks I hope you have found my rant informative or just funny / daft. Design should be fun. The design process.. well if you don't like what I've written then you can rule it out.. Design is often about ruling stuff out and that lets you focus on the things that matter. To finish. As an SE I love, and it is a challenge to explain to a lay person how stuctures work. I have Man bag, in that is ruler. I pull that out to explain how buckling of a beam works. There is a soft rubber and my hard plastic measuring tape. I use these to show how the different layers of an insulated raft work.. the rubber is less stiff than the tape when you put one on top of the other..this lets folk see that the insultation is the critical design layer. The maths are complex but the Client immediately can grasp what we need to do, why and the raw cost implication.

Good thread. But you do know this varies over the day? Stop being wanky and go back to basics! Your taps and flow valves will wear over time.. I piss myself laughing at you all talking about 0.1 bar pressure, static or dynamic..oh and a couple of rough bends in the pipe and it bollocks! I can tell you that your qualitative values are complete pish in terms of long term performance. In a sealed system 0.1 bar can be lost by a mugfull of water weaping from a joint! Yes I'm the philistine.. We have been using direct cylinders for at least 30 years, well I remember it was about that long ago that I fitted one, it is not rocket science.. just needs common sense. My prediction is that your plumber will part company with you. To avoid this identify what you can live with.. start by asking the other members of you family... as good design is for all. I'll say this again. There are folk on BH that are trying to make UFH and a hot water supply into a complex subject. Yes in a big hotel say it matters a bit but for a self build.. get a grip! You are off your heads! I have been into UFH and the like long before most of you were born! @nod are we of the same age? and sometimes the elegance lies in the simplicity, low maintenance and reliability costs! I leart my skills and design skills from the Scandinavians and industrial sector, the UK self build market were late to the UFH game, ICF and insulated rafts. Funnily I was doing a lot of this stuff before the self build market got a foothold. Why! 1/ In one or two years time the valves will get sticky / pumps will wear, the boiler will wear and things will start playing up. You just can't seem to see this folks! 2/ Your average plumber will have no clue how to service all the shite you have installed. 3/ If you sell the house how on earth are you going to run this by a surveyor. I could go on. Keep it simple folks and it will save you money! Maybe test you flow rate on the incoming service pipe. Open the pipe, get a bucket and let it flow free. Time it to get the delivery of a quantity of water. Do this in the middle of the night at 4.0 am, at 7.00 am and at 4.30 pm when the kids come home from school. Do this over a couple of months or so.. the water board get leaks and muck about with the water pressure! Check between winter and summer flow delivery. Then you may realise how your calcs are a not applicable! Is that it? You probaly need to fit another £ 5.00 automatic air vent from screwfix!