Gus Potter

Hello TANK. A few thoughts, some explanation which may give you some pointers and another few for all who may be considering knocking down a wall say between their kitchen and dining room in a modern house to form an open plan space. "I'm wanting to install wall piers on my outbuilding as the length is 6m. Asked about this in my original thread but had no response. I wouldn't ask without first doing my own research, but I can't seem to find a definitive answer (maybe 'coz there isn't one!) either by Google or with the likes of you tube. You probably won't as there is a fair bit to it.. "My footings lie around 750mm below where the screed flooring will be. Do the piers always rise with the rest of the wall at the footings, or can they start from the screed (which will be 50mm on top of 50mm insulation atop hardcore.)? The first 5 courses are eng brick which then gives way to thermalite." Normally they rise from the footings. "Im hoping it's the latter as I have overlooked them and will need to do a little remedial work if I'm to go back to the footings... " Yes in the spirit of BH there is always hope and mostly a solution..only available on BH of course.. this can take a leap of faith but here goes with some technical stuff first then a possible solution TANK depending on what other factors come into play on your particular job. The first thing to recognise is that often a wall needs to resist both vertical (from say the weight of a roof or floor) and lateral wind loading.. even internal walls from time to time as you get different wind pressures occurring inside a building. A big set of bifolds accidently left open say on a windy day. Yes folks, we need to "have a look" at this possibility "just in case" to make sure the design is safe. In general terms if you take a brick wall (no big windows!) of say 3.0m long with a good return corner of masonry at each end, built off something solid on the bottom, say a foundation and held in place by a stiff roof at the top then the masonry will "span" in two directions.. a bit like a concrete floor slab that is supported on all four sides, you often see these descibed as a "two way spanning concrete slab". However, partly due to the way the bricks / blocks are shaped/ laid a standard UK masonry wall is stronger in the horizontal spanning direction than the vertical direction. Now at 6.0m horizontally that is a long way for a thin wall to span horizontally so it can resist particularly the wind loading.. you don't get much if any help here in terms of horizontal spanning resistance when you come to check the wall is ok. Also, you need to have a look at how the roof is supported on the wall head as sometimes this can cause an unwanted bending effect just to add to your problems. Very simplistically and just to make a corollary. Think of the wall as a series of two beams.. one goes horizontally, one vertically and they both interact and share some of the load. We "know" that a deep steel beam is often "stronger" than a shallow beam, for example a deep floor joist will often carry more load than a shallow one of the same thickness. The same applies to a masonry wall, the thicker it is then generally the more load it can carry. TANK. At the moment you have a wall with no piers so it's effective thickness is the thickness of the wall assuming it is a single skin. At 6.0m long it's strength is going to come mostly due to it's capacity to span vertically. I think you have recognised that it is probably not quite going to "cut the mustard" especially if you live in a windy area? As a starting point we often say lets put in some piers at 8ft centres ~ 2.4m and see if we can get this to fly. I use imperial units as some of the modern codes still use empirical design to some extent... we know it works! When you put in a pier it often has the effect of increasing the effective thickness of the wall..so in other words you are making the "wall beams" a bit deeper. Now it will carry more load, and this is how you get it all to work as we have made the wall " effectively thicker" by introducing piers. That's the end of the general theory bit! Turning back to TANK's issue. It look like there is no enthusiasm for digging up the floor and building up a pier off the founds. Anyway for a retro fitted pier to properly work it needs to be fully bonded into the wall. Ideally, fully bonded also means that both the pier and the wall should be build at the same time. This is because when you lay mortar you need to "tamp it down" so that the bed is evenly compressed for example and bonded to the masonry above and below the bonded in brick/ block. The bricks, blocks of the pier need really to physically interlock. Some may say.. well use a wall starter kit. Fix it to the wall that is built already and build the pier. This does not work as many starter kits have small coach screws with plugs. The ties also can slip vertically so they do not properly bond a remedial pier to the existing wall.. you may as well use wall paper paste. The reason they don't work is that you get an effect called "complimentary shear" this is a vertical force that occurs between the remedial pier and the existing wall. Wall starter kits typically can't resist this force as the ties move up and down within the profile of the wall starter kit. TANK. What about this? Rather than piers go for some wind posts. These could maybe be timbers (or steels) running vertically from the "structural slab" up to the wall head. What you are doing here is splitting the wall up into small panels.. the lateral wind loading is resisted by the timbers and the timbers also stiffen the wall so it can carry more vertical load. Sounds bizarre but.. timbers are cheep. They will stick out from the wall a bit more than steel but you can use them to support the ends of small shelves? Your floor buildup just says "50mm on top of 50mm insulation atop hardcore.)?" I'll assume you have say a 100 to 150 concrete slab under.. if not then.. ? you may have some serious issues with your floor cracking up and nothing to fix the base of the wind posts to. Assuming you do have a solid structural floor slab to connect into. Here is the lastish bits of the puzzle. The floor (structural slab) will go up and down as the moisture content of the ground changes and the loads on the floor vary (call this the "dumpling under the slab") , the walls hopefully will move not so much as the founds will be deeper. How do you connect a wind post to the slab / dumpling that could move up and down? you use a flexible bracket that is "bendy" https://www.strongtie.co.uk/products/detail/large-reinforced-angle-brackets/265 You can't use these brackets above as the the slotted hole (fixing) is too close to the bottom of the wind post if you go for timber, steel usually ok depending on what the profile is but I posted the link to give you an idea as to what might be a "bendy" type bracket! In the world of design it's ok to sometimes let things bend and recover as steel is elastic..your toilet bowl / floor tiles less so!. So long as you don't go over the score! Now that is a potential solution that will avoid you having to dig a few big holes in the floor to retrofit the piers all the way to the founds. Even if you do you can see that you will have to find a way of properly bonding the pier to the Thermalite. This Thermalite is last elephant in the room.. What you need to do is to find a fixing that will transfer the wind lateral load in the wall to the wind posts. You can often fix brackets to the sides of the timber wind post with square twist nails. You fix the bracket to the Thermalite with an under reamed resin anchor. Attached is the Fischer test report which gives you some pointers as to what fixings can be used in Thermalite blocks, the under reaming drill bits and so on. You can find the drill bits and installation instructions on the web. TANK.. hope this helps. You will probably need an SE to prove this but hopefully it can be a simple process, not too costly compared with BC knocking you back and / or digging up a big bit of the floor in two or three places then making good the DPM and so on. Even if you do this you'll struggle to bond the new pier in effectively and make a good looking job of it. Even if you don't need this retro work for BC then please be safe and get it (the wall) checked out. For all. The above issues often crop up when you want to knock down a wall along say the back of your house to form an open plan room right along the back of the house. Often you end up with a pier / column of masonry sitting in the middle of the external elevation wall . In the old day windows and doors were made of good solid wood / or metal framed and they stabalised the masonry. Now windows are plastic / aluminium framed with "rubbish" brackets that just don't stabalise the masonry up the sides of the window / door opening. Thus often you need some kind of wind post. Normally this is ok and easyish to do but when you encounter Aerated blocks (Thermalite say) or bricks with holes in them the fixings become an issue. That said often there is a solution. All the best TANK. Fischer Test Report for Thermalite_Report(1).pdf

Hello Babak. Please be very careful here. Unfortunately I have seen cases where folk have been a bit too hasty with the demolition and one case where the planners prevented them from rebuilding. There was some other history but it can be that serious! It looks like you have a fair idea of what you want and have a "design" in mind. Also, it looks like you have already put a fair amount of thought into build method, servicing it and so on. What about contacting some SE's? As you are proposing ICF for part of it there are a good few that have a grasp of the fundamentals of this and from their point of view it's often more interesting than just doing the day to day stuff, so they may think.. yes that looks like a project I would like to do. I'll also do the "Architectural side2 and building approval side" as the Client already has a good grasp of what they want and, as I'm drawing it up I'll do the structural calcs as I go along.. it's a very efficient way of doing it! In summary there are a few SE's who also do the whole package, calcs, drawings, a bit of the Architectural design and building approval so don't rule this option out. Even if you don't go this full route there are SE's who will give you a good few pointers and offer their contact base to you. There is a perception that for design you have to first get an Architect.. then an SE.. then a contractor. But all these folk often work and collaborate, learn from each other anyway. So don't rule out approaching an SE, explain what you need. You may get a pleasant surprise as they may say.. well I can do most of it but I know an Architect that can say deal with the parts that are outwith my expertise.

Jilly, thanks for the kind words. much appreciated. The main thing is that it seems like you are marching on now and hey, you probably got a load of stuff done before the material prices went daft due to covid.. so extra bonus to offset the stress. If you look at the cost of doing it now in this climate.. well you may well be in profit already! Jilly makes a good point here for all. If your designer turns round and says this is going to break the budget ask them why. You need to cut them a bit of slack but they should be able to explain to some extent and identify what parts of the job are causing the budget problem. If you just want a house too big for your budget then they should just tell you.. take the rose tinted glasses off folks and here is why.. On refurbs / conversions it's a different ball game and this is where a good designer should be able to identify the areas that are contributing to breaking the budget. Once you know this you can then look at finding a solution. Sometimes it's just not possible to convert say with the money you have, but at least you know why and be in a position to judge if the compromises you need to make may outweigh the benefits.

Hi MdeB I think you're taking a risk. As new build the substrate, founds, what is under the slab ect will still be settling in, adjusting to the ambient moisture regime and possible curing if you have masonry. There will be quite a lot going on under the screed and the supporting structure that you can't predict. The aspect ratio of the screed looks borderline. For a simple concrete ground bearing slab in an extension we consider amongst other things the aspect ratio. Here you have 5.8 / 2.8 = 2.07.. for unreinforced concrete slabs in say an extension once the aspect ratio goes over 2 then it is a flag. For a thin screed.. potential trouble. for slabs we often locate saw joints to control the location of the cracking. I would screed it, saw cut it into three sections 5.8 /3 = 1.93m x 2.8m as soon as it has hardened enough (24 hours) and leave it for as long as you can. Keep the screed covered and wet for at least 7days (preferrably a month!) so it cures, then slowly dry it out. It's a conservatory.. it could be pretty hot.. misstreat the scread and it will return the favour later! Even at these saw cut panel sizes you may still get some hairline cracking unless you are using a screed with polymer / steel fibres in it. Be safe and use a decoupling mat. You don't have to saw it all the way through. If the screed is 60mm thick then saw it to a depth of 40mm. You promote the cracking here as you have weakend the screed. What your are doing is allowing it to crack where you want it to and trying as best you can to limit the size and orientation of each crack. It's important to put a sheet of plastic under the screed so it can slip over the substructure easily as it cures. Get the screed right and the laying of the tiles should be the worry free part. All the best.

Good point here from the_r_sole. It's hard to compare designers prices. You can get a set of BR regs set of drawings and calcs cheeply, but that is all you will get. Just enough to get your approval. You can get them really cheep if you are happy with saying say.. ,x ,y and z will comply with clause x, y and z in the regs.. very few drawings or supporting info required. You'll have your approval but the info you have will be very limited..trouble ahead if you don't have significant experience of building and dealing with individual contractors. It will then be up to you to develop and communicate the detailed design information to the contractors ect who are pricing unless you are really doing it all yourself. The less info you give them generally the more heavy they will be with the pricing pencil or build in caveats to allow them to charge you for "extra work". It will also fall to you to take on the liability / responsibility if you get it wrong, BC come to inspect and say. NO! Put it this way. A good skilled trades person who will also keep you right on a DIY build (maybe by saying.. are you sure you want to do this?) may cost £250 - 300 / day as an average UK price. £31.5 - 37.50 / hour. Now chuck in some plant / van tool costs and so on. Just say 40 quid per hour = £ 1600 per week. That is just for one person. For two.. £ 3200 per week. Now an experienced Architect or designer can easily save you this and more. Go cheep and you risk getting bogged down trying to resolve buildability issues that crop up because you are just relying on the drawings you have use for regs approval, trying to source different materials that will still comply with say the U value requirements and so on. The test when comparing fees is to ask this of the designers. 1/ Your fee is this. It is higher than the others so can you convince me that this extra fee will result in an overall saving..mitigate the fee? Tell me how you are going to do this. Explain to me how say you have thought through the buildability aspects so it's easy for local trades persons to build and thus they will be less heavy with the pricing. Are you sure that the materials can be easily sourced and on time to avoid delay on site? Where is the risk in this project to me, can you tell me where the risk lies so I can make an informed descision? A good designer will think about all these things but that "thinking time" and experience comes at an upfront cost. 2/ Your fee is this. It is the lowest! Can you convince me that I'm still going to get the same service and effort of design thought on my build/ project? Can you assure me that you are giving me enough information so that a contractor is not going to get free access to my bank account while at the same time my job is not going to go off progamme, I'm not going to get into trouble by inadvertantly by doing something that may breach the regs.. go for the completion certificate and get knocked back.. ? If your designers don't like these types of questions then.. keep looking. As a last thought.. a washing machine repair "Engineer" may cost £ 60 per hour.. this is your home. While the professional fees may seem a bit steep initially, once you look at this in the round and if you don't have significant experience then a responsive and supportive designer can be a good route to follow.

Ah! Preparation is the key in my view. Clean off all loose debris, loose mortar at the wall heads and so on, even give it a hoover. This way you'll be less tempted to over spray. You save on fluid and this will maybe tempt you to spend more on the chemical spray. Long lances? well you'll get the easy to reach bits that are drafty but the worms etc will seek out the parts your "Heinekin" delivered "blind" from below has not reached. If you want to do a good job and are just not looking for the paperwork then set up some proper access and do it thoroughly. Not sure on costs but you could sequence the works so that you can use the access platform for something else at the same time, say if you need to cut out and splice the badly wrotten rafters?

Yes, you're right James. Some deflection takes place as you add say masonry which is a constant load, call this a dead load. The rest comes from say floor / roof loading which is variable load, call this a live load. Thus you have the "intial" deflection from the dead load. Then a variable deflection from the live load. Generally we assume for most domestic applications say using steel lintels that the deflection response is linear up to a point. Thus your total deflection is the sum of the dead + live load deflections. Precambering is an option but for the self builder just needing the odd beam it can be a bit costly. Best if you can to just make the beam/ lintel say a bit bigger. As Peter says (I think) if you have too much deflection, intial or otherwise you can just create something that does not look good. Too much and even the best door mastic operative will not be able to hide a significant downwards bow! Spot on Mike..go for bottom supported as the doors (glass mainly) are heavy and introduce a fair bit of variable load to a lintel as they open and shut.

Good point Peter. SillyBilly looks like there is more to this than meets the eye. If the wall is going to be an internal wall at the end of the day then why lightweight insulating blocks? Is there an issue with the founds say and you are trying to reduce the loading? The wall looks like it is 6.3m long , usually after about 6.0m you start to worry about significant shrinkage cracking in cement based blocks (as opposed to clay), assuming the materials have been looked after and the mortar mix is not too strong. Even with walls shorter than this you get some shrinkage cracks around the opening depending on the surrounding wall geometry. Much also depends on the wall height, geometry and how the SE say has designed it. Vertically spanning, two way spanning and so on. To give you relevant pointers then a drawing or two would help. If you are just worried about the wind loading in the temporary condition then your starting point is this. If you look at the probability of the wall being loaded by the wind over a two year period then the wind loading is reduced (by some 35 to 40 %) as the normally calculated wind loading is usually based on a 50 year return period. But the wall may have been designed with a certain amount of compression, say from the upper floors / roof. Often if you have a good weight on a wall then compression is beneficial as it counteracts the tension stress developed on one face of the wall by the sideways wind loading. On the practical side you could consider introducing a couple of temporary wind posts on the outside of the wall, you may have the space to do this. We know that aerated type blocks are hard to get a good fixing to so when you are building the wall introduce some bent leg tie straps with a twist. The bent leg would turn down the inside face of the wall and where the strap comes out the outside face you have the twist. Now screw fix the strap to the side of your wind post, go for timber posts. Fix the posts top and bottom. When ready remove the temporary timber wind posts and recycle. The bed reinforcement looks great on paper but for a brickie it's another hassle and makes it harder to keep the coursing level. If you really need it SE wise then fine but the brickie will charge you more to bed it and for the extra time they will have to take to get everything level. You also have the cost of the bed reinforcement and time to procure. If you're laying the blocks your self then make sure you have a practice with bed reinforcement as although the flattened type S/steal stuff looks great it still makes it harder to maintain level coursing if you are working on a 10.0 mm mortar bed and standard units sizes. If you deviate from this then you'll often find that you can't marry up the wall coursing where it ties into other walls. Hope this gives you a few ideas.

Hello Moggaman. Working on the basis that your purlins are supported somewhere along the span? maybe a splice? below is part of a steel spec. "All steelwork is to be fabricated by a CE approved fabricator and comply with the BS EN 1090 series requirements. Site welding is not permitted. It is the Contractor's responsibility to check all steelwork dimensions prior to manufacture." "Shot blast to grade SA2.5 and painted with a zinc rich primer. Nominal DFT 80 microns." Shot blasting comes in varying degrees of "cleaning", SA2.5 is a common one for domestic applications. The primer spec gives your local fabricator a bit of play as to what they can use.. the tins of paint are actually drums of paint so they can "use up a drum" on your job. DFT means nominal dry film thickness.. in other words it looks like paint not just a sniff. It's worth doing as a matter of good practice and can help protect vulnerable parts like the welds and bolts / fixings. In summary, if your steel is in an indoor, heated and fully protected and non aggresive environment then strictly speaking no need for painting / treatments. If it is running over a bathroom and you may have moisture? Would I leave the steel raw on my own house? No. The primer can get rubbed a bit as it's quite soft so don't panic if you see a few bits scuffed off. Last thought, it often comes in two colours, red or grey if that matters?

Hello flanagai. It may be that an SE say or an Architect that has a good SE background is best placed to help here. At some point some calculations may be required for BC purposes if you encounter the unexpected. If you think about it this way. You'll be doing the rest from underbuilding up. The groundworks could be considered as a stand alone package. You are already "project managing" it. You can do a lot to help yourself and keep the cost down. Let's say you engage a local contractor to lay the founds and take the underbuilding up to wall plate level. To price this they will take your drawings, work out the amount of dig and muck away, the volume of concrete, amount of say blocks, wall ties and the amount of say lean mix cavity fill you need. Somewhere they often say.. "based on the drawings" Now for the self builder one problem that arises is if you need to make the founds deeper, say you hit a soft spot. In some case the founds need to be shallow if you encounter rock. Let's take the deeper as more common. Here you have many options; use say trench fill, just make the founds thicker using higher grade concrete if not that much, lay the foundations deeper and make the under building higher. However, there comes a point where your masonry wall thickness needs to be increased if it gets too deep, this is partly to do with the soil pressure on the side of the walls under the ground. All of a sudden things can grow arms and legs. Here an SE type person is often best placed to advise on the most economic option. What you can do is ask the contractor to give you a price based on the drawings but also give you rates for say extra dig, cost per square metre of each masonry leaf. Break this down into a cost say for up to 900mm below working site ground level and up to 1500mm below ground level. Also ask for rates to support the ground if you need to go deeper. Basically ask for rates for extra work. Now, if it turns out once you have excavated the ground that you need to do more work then you have a set of agreed rates. This can transfer more risk to you but you have more control. On the other hand you can just say to the contractor.. you take all the risk.. but you will pay for this.. very rarely will the contractor be the looser here. This is where an SE can really help. Get them in early and they will help you identify what rates you need to get in terms of extra work. Once you have set this up you should often be able to measure up your self and agree the payments. Now let's say you do need to do extra work. The SE will have a discussion with you and the builder and say here is the most economic way of overcoming the unforseen. In summary doing it this way will result in you paying for what you get.. often no more and if you all get on ok and you pay your bills on time you may get a bit extra for free as you'll know. For all, many contractors like working for good reasonable and fair clients so will occasionally go a bit less heavy on the invoicing.

Hi Jack. If fixing to a slab then "underhanging" removes issues of the reduced edge fixing distance, that being the distance from the fixing into the concrete to the edge of the concrete. If over hanging then this problem arises. At 10mm variation between the high point and low point you are in the ball park regarding mortar bedding thickness. If you have 5.0mm mortar bed at the high point then you'll end up with 15mm thickness of bed at the low point.. all within acceptable limits. The following is based on the assumption that you have not cast the slab to high! If you have then that is for another day. Enough of the theory! ..some practical stuff. Here is a method of bedding a wall plate to get a quality job. The first thing is that timber is not straight when you lay it down on site.. it may have been when it left the merchants! Lay your wall plate down on plan and it will often have a bow, look at it sideways on elevation and it will be bowed that way too. And just to add to the dilemma it will also often be twisted. Usually you have a DPC between the timber and the slab. Staple the DPC to the bottom of the soleplate. Next get some wet timber, say 100 x 22 mm soft wood. Rip this down lengthways so you have little strips of different thicknesses, say 4 x 22 , 6 x 22 , 8 x 22 and so on. Cut these into 150mm lengths.. like kindeling for the fire. Put the sole plate on the slab, pilot drill it so the screw can slip freely and part fix it down to the slab with say 120 mm x 5mm screws and plugs. As you do this get the sole plate as straight as you can on plan. Often you only need screws at 900 to 1200 mm centres so it's not to onerous to do. Also, you can use the screws to take the twist out as you can either put them close to the edge of the slab or more inboard. Fine to do as they are just temporary fixings and non structural. Now work your way along it putting in the timber packers to get the top level. Try as best you can to keep the packers away from the stud positions. You'll not always get this bang on so don't worry. Just don't put them where you have a cluster of studs say taking high loads from lintels etc above or too close to the corners. You need to slacken and retighten the screws as you go to get the packers in. You'll not get it perfect but do the best you can. Once you have got it all as straight as you can unscrew the sole plate, put down the mortar bed and reposition the screws. Slowly tighten the screws while giving the top of the timber a tap down with a hammer. Leave it all to set. After a few days of dry weather the packers will shrink and you can wiggle them out easily. Point up the small holes they leave. Don't use dry timber for the packers as it will swell and lift the sole plate.. and you won't get them out later. Leave the screws in place as this helps keep the sole plate in place while you are putting up the kit. Once the kit is up follow the SE's instructions for permanant holding down fixings and so on. For all. The above is a bit lengthy but if you are self building and ordering a kit (TF) the manufacturer will often have a clause in their spec about how level the sole plate has to be. Deviate from this and it can let them shift the blame to you if something is not quite right with the kit. If you are going traditional masonry construction then you may have a trussed rafter roof. Again, the wall plate tolerances are covered in the manufacture's clauses. Traditionally, less so now we used to make the bedding of the wall plate part of the brickies work package, particularly in Scotland when doing TF. This gives a clear line of demarcation. Brickies are not so keen on this now as.. All the best Jack and hope this helps, or you can adapt the above principles to suit your slab.

Hi Jack1962. As a general rule of thumb. Mortar bedding should be between 5 and 20mm thick. Less than 5mm and the bed is too thin, more than 20mm and the mortar will start to crack, shrink, loose strength as it's too thick. Sole plates need to be evenly bedded so you can see that at less than 5mm thick they will also be hard to gently tamp down to level (like bedding a brick), more than 20mm and the mortar will start to just spill out of the bed. The mortar will tend to be less compacted at the edges and more in the middle so the sole plate will want to twist (rock) one way or the other. Also, once you exceed 20mm of mortar you can impact on the performance of any fixings holding down the sole plate. The other thing to look out for is how much the sole plate is overhanging say masonry below. Generally for a 90 -100mm wide sole plate you want it over / underhanging by no more than 12mm. For a 140 - 150mm sole plate no more than 20mm. If you exceed this then a number of issues arise. The first is that the sole plate can load the supporting structure below (say masonry) eccentrically (usually not about it's centre of gravity) and this can introduce unwanted bending type forces, local over stressing in the masonry for example. Again you can have an issue with the fixings as if the timber is too far overhanging the fixings can end up too close to the edge of the timber / masonry / concrete and thus become less effective. If your masonry is well off being flat (level along the top) then if you get really stuck you can use an extra sole plate that is fixed to the masonry on some bedding to partly sweeten it out. Then put another sole plate on top which is packed with structural packers. Each solution needs a bit of thought as one size does not fit all build methods. Just be aware that timber shrinks quite a lot perpendicular to the grain so the more horizontal timbers you introduce the more vertical shrinkage you'll get. this may or may or be an issue. If your walls are just not straight on plan then? each case needs assessed. If you have a variation of 5mm over 5.0m then it looks like the brickie / person preparing the support for the sole plate has done a pretty good job!

Hello Tank. To get the best out of Build Hub you can try doing a quick sketch on a bit of A4 showing the ground level, your foundation etc. Don't worry about the quality.. no one will slag you off for a bad drawing..you need to see some of mine! Take a photo of your page and post it. Sometimes it's easier to make a drawing than to try and explain it in writing. The main thing is to have a bit of fun posting on BH and don't worry about using "technical" speak..just spill it out! I'm guessing but have you a found and you are trying to work out how you lay the masonry so that when you get out the ground the brick / block courses are level? If this is the case.. (most founds are not that flat and level) then for example you can lay concrete blocks "on the flat" which is the wide side down and adjust the mortar bed thickness..aiming to keep the mortar thickness to less than 20mm.. does not always work in practice for DIY but that is the aim. If you have not done it before it takes a bit of thinking out.. but great when you get it right!

Belt and braces..to add Squirt some expanding foam into the joints between the gaps, tape it up (plenty info on BH about cheep effective tapes) as the foam is not that vapour tight but really helps keep the insulation in place. Sleep tight.. and.. in the winter; don't put on 5 gallon pots of stock on the stove steaming away all night, avoid drying vast amounts of washing without opening the windows. It's you own home so you know to let the "steam out"..just be sensible. Keep the heating on at low so you get some residual heat permeating though to the outside walls and into the roof space, you move the dew point outwards and you'll be fine. In terms of heating cost, it's a few quid extra a year?, no point in analysing this to death as you have a refurb /conversion. Just do your reasonable best and march on! Start thinking about the kitchen units and the enjoyable stuff! In other words don't go away for example, let the place freeze, come back and make lots of water vapour. It's like an old car with leather seats, they crack if you keep them in an old damp garage, take it out and crank up the heater.

Yes putting the insulation under the slab is fine. What you are doing is bringing the slab into the inside of the insulation envelope, just like if you have some masonry walls inside. In some ways you have a big storage radiator (hate to say it but often called thermal mass) which can help stabalise the temperature in the room. It's important that you carry the insulation below the slab up the sides so you "wrap" it fully in the insulation blanket. There are practical benefits in that you often have something more solid to lay your floor on and build non load bearing partition walls off.