Gus Potter

Hello @iMCaan Concrete founds are fine under water and as others say seepage at this level is quite common. If you are wanting to drain the site a bit I would avoid if possible placing the drains below the level of the top of the concrete found. What can happen is that folk go mad and dig deep French drains near their founds. If in clay soil this can dry it out in the summer and your building could settle. If in sand the fine grains could get flushed out from under the founds.. and they settle. I've seen this on old stone built houses / steadings / shallow founded corbelled brick founds etc where folk put in a new drain deeper than the existing founds surrounded by pea gravel and a few years later cracks start appearing in the walls as they have dried out the ground under the founds that have happily sat there for decades. Most of the time movement is caused by blocked drains and tree roots and that is what you read about on the internet. Thus when new building we don't think "the other way round"

Me too.. to add some of my thoughts to yours. Before you go down the piling route I would make sure that you rule out the other options first and can justify to yourself that you have given it your best shot. @Zak S "To me rebuild could only be justified if I build a standard 2 storey plus a loft kind of house. Given the foot print it could easily reach 600sqm but I onky need 450-500 sqm. But want to have decent size ground floor. To follow up on Joe's point I would also ask myself.. can I design something that uses part of the existing foot print only and retain the existing founds? Then add some new founds similar to the existing to "fill in" the gaps. You have gathered some info by way of your GI report but you also have an existing building sitting there. I would want to have a look at the existing building, determine it's age and also when parts were added and altered. You see this with old steadings.. as say a farm grows they add bits, change it to accommodate larger farm machinery and so on. Different loads are added to the ground at different times. Now this can be a mine of information as it lets you observe how the existing building has performed on the ground over many decades. You can see cracks, where it settles / heaves and so on. You can also calculate the load the existing building is putting on the soil under it's foundations. Already you have some hard evidence on what the ground can carry. This also gives you clues as to where you need to target any further investigation. Could be trial pits dug with a JCB. It requires some experience to interpret what do see, if a lay person don't be put off using your common sense. It's your house / project so have fun with it and explore what you see. The ask say the Geotech / SE folk to explain your observations. Also have a look at what is next door as the ground conditions can change a lot over a 100 metres, but right next door can provide the best clues as to what you may have just over the boundary and thus what you may have a few metres away. I have had a quick look at the GI report (only in terms of the house though) and while underpinning the existing founds is always an option it's labour intensive and if you have high ground water.. a horrible job and difficult to get right. So this means cost £. You have what looks like two water tables on the bore hole logs BH1 & 2. This is not uncommon. Very roughly the rain falls / your drains leak / you have old soak aways and want to put in new ones too. This water soaks into the ground and can get trapped on top of a less permeable layer, like CLAY. But below this layer of CLAY can be fractured rock or gravel overlying fractured rock, like sand stone and at this level lies the main aquifer. Often you find gravels / sands / silts before hitting more competant rock. This is due to weathering of the rock during ice ages and so on. This goes some way towards explaining the refusal term? The lower water strike can be a primary aquifer that the water board use to extract drinking water from so watch out if you want to start messing with it! Always check with the water board early just in case. The upper water level could be attributed to what is sometimes called a perched water table. The water gets stuck by a layer of less permeable material and can't drain straight down to the lower water table.. thus it is "perched". You see I have used capital letters above in places. You can also see this in the bore hole logs. Take borehole 1 log. At a depth of about 1.6m you find "..slightly silty sandy CLAY with fine roots" The capital letters indicate that the dominant portion of the sample is clay, hence in capital letters. Clay is less permeable than sand so it is preventing the water from draining down to the lower level. Now above 1.6 you have ground with sand and sands can transmit a lot of water. Yes there is a bit of made ground but this may not be extensive. Farmers were / are not daft and don't tend to build steadings over made ground. But they do often have a midden outside (MADE GOUND) and this gets picked up on a GI! Also if you pump like mad to keep your excavation dry you can suck out the fine portion of the sand, this weakens it and can cause a lot of trouble. Ideally you want to design a domestic found that is situated above the water level and avoid these issues if you can when dealing with sands. There is much discussion on BH about clays swelling and shrinking but if you can be sure they are deep enough and always saturated this won't happen. I won't go into great detail but you can also find fine roots at depth. Yes they will rot but settlements are negligable and can be accounted for. On the bore hole logs and in the "site work" section you can see they have carried out two types of penetration test. The SPT and dynamic probing. The dynamic probing is complex so I'll leave that for now. But the STP number provides the Engineer with a rough (well very rough) feel for what might be worth a closer look. A rough rule of thumb is to take the STP number and multiply by ten to give you an indicative allowable bearing capacity. The allowable bearing capacity is a measure of what load you can put on the ground (with factors of safety) without the ground failing or you house settling too much. Take BH 1 at 1.6 m SPT = 8 and BH 2 SPT = 9 at say 2.0m. Take the lower value 8 x 10 = 80 kN/m^2 ( 80 Kilo Newtons per square meter) which is about 8.0 tonnes per square metre.. quite a lot! But.. above this where want to put a found (could be a raft) there is water shown in you logs. Sands get their strength (bearing capacity) from partly inter granular friction in laymans terms. For this to work each grain bears on the ones around it and each particle has a density of say 20 kN/m^3. But when the water rises the density is halved (see Archimedes or the nautical members of BH) so you have half the friction. Thus when doing conceptual stuff we divide the 80 kN/m^2 by two when in SAND type soils to give an allowable bearing capacity (ABC) of 40 kN/m^2. Now you can often on a low rise building you can get a simple ground bearing raft with insulation under to work (with a fair wind) using an ABC of 40 kN/m^2. Make the insulation thicker and you are in passive house terratory. You may ask.. what about the roots rotting? Well we just design the raft to account for the ground settling a bit and the odd soft spot. You keep the raft as high as you can and away from the ground water. I would kick the tyres on this first before committing to piling, even if to rule the idea out and free up brain space. I hope this gives you a bit of further insight and is of use.

Yes, it's not ideal but the masonry bonding futher down is more appealing. If that joint is falling right under a truss suppot point then you may want to see if you can take off a bit of the wall plate and form the joints else where. Remember that you also probably need to fix a truss clip there. Roofs are quite forgiving under certain circumstances and up to a point.. look at many old roofs and how they sag. But also remember that you don't want a dip in your new ceiling.. this may irritate.

The GI report make for interesting reading. What are you proposing to build, how many storeys and do you have a rough floor plan?

In principle the half lap joint looks neetish.. what is less encouraging is the masonry below and it's bonding. Wall plates do an number of different things. One is to spread the load, another is to often act as a binder which ties the wall head over discontinuities in the masonry below. It may be ok but hard to say more without more detail / information.

All the best, make a list and pick their brains, offer to pay a bit more for the advise.. is goes down well. From an SE point of view it's often easier to spend half an hour or so chewing the fat with a Client than going through the process of invoicing, also a satisfied Client may just recommend you to their friends!

Hi Foggy. This is not uncommon, owning a house is a journey. Often it's beneficial when you get stuck to go back to basics. Start by going round the inside of the house and look at the openings.. doors and windows, wait for a windyish day and go round with the back or your hand feeling for drafts. If you have older double glazing you'll find a few. Often this is caused by the opening part of the window loosing it's shape as the glass holds the opening part square. What then happens is that the window does not shut right and make a tight seal on the gasket. Also the gaskets loose their flexibility so the locking mechanism has to work much harder to make a seal all round. Pick a window and watch some videos on utube about how you adjust the window / door mechanisms. Have a go at it so the mechanism is working freely. Next look at the gaskets. There are load of different types but you can get sample kits to help you identify what gaskets you need.. see link below for idea. https://www.handlesandhinges.co.uk/double-glazing-seal-gasket-sample-pack/ Now have a go at replacing the gaskets if need be, you are already on your way for little expense. Next run you hand along the bottom of the skirting and find the drafts, note the position. At the same time see if there is mould at this level and note the position. Look up the walls in each room and note the position of any damp / mould. Next go into the attic and see how the insulation is laid, check to see it is not touching the underside of the roof / tiles and that the eaves are clear and the insulation is not clogging the eaves ventilation. Also look to see if you have downlighters in the ceiling that might be letting steam from say the shower into the attic. The key in the attic is to look for places where you have moisture getting in and hitting a cold surface with no air flow. This applies elsewhere too. Now go outside and look at where the outside walls are most exposed, check you gutters, down pipes and that the rainwater drains are running clear. Look under the floor and see if it is ventilated and that the air space is not full of rubbish. See how it all works. Once you have got a handle on this then turn to your cavity wall insulation. Cavity walls in the 1920's were fairly new and the cavities we often smaller than they are today. The cavity wall insulation may be missing in places so look for odd mouldy spots in funny places, see if they marry up with the cold / exposed areas on the outside and ground levels that are say maybe higher and cooling the wall / stopping it breathing. You need to be a bit of a detective and read/ learn a bit but once you get a feel for things you'll have a much better understanding on how your house is working fabric wise. Next look at how you are living and using the house and identify the sources of moisture. Compare with your observations. Once you have done this you'll be much more informed and thus able to target your spend most effectively and or make a small change to your living regime. It may be that by introducing ventilation in the right place, cutting out the drafts in the wrong place and adding the odd bit of insulation delivers the best return. Then if money to spare you can introduce more technology. All the best.

Please be safe. You are building two storeys so odd / unexpected / bad things can start to happen and you have higher loads than say single storey. If you need to ask about the block strength then you don't have sufficient experience in structural design to make sure what you are doing is safe.. and be able to prove it now / later if you need to sell. Sorry to be blunt. What about running this by say an SE, they may be able to say.. hey you can save money here as you are over designing, but here you need to up your game. Ideally an SE / experienced designer should be able to save you more than their fee in the round?

To provide you with another level of security confirm in writing whether the quotation includes complying with all the current building regulations and recommendations ( Building Standards) in Scotland and the tender drawings. You'll be surprised how encompassing this can be.

Hi eandg. I'm guessing here as you have posted no plans or other info. Being higher if a different animal. I'm guessing but if you have 1.2m from the boundary then it's good to have a levelish portion for you to clean the gutters, walk round etc. have a look on the net at what is a called a reinforced masonry retaining wall. There are lot's of different options other than a gravity retaining wall. On the soft side this type of reinforced wall can be faced with facing brick on you neighbours side.. yes I know it may cost a bit more but in the long run you may want to be friends with your neighbours or find you need their good will if you need to later access their land for some reason. When / if you come to sell you maybe want to be able to say.. we have great friendly neighbours. I've do the odd job where you actually dig out a lot of the soil and replace this with ICF type blocks. What you do here is to reduce the sideways load on the wall (lateral earth pressure) as the ICF blocks are stable and much less dense than the clay.. Sounds bizarre but this can be a cost effective solution when looking at things in the round. You do have to watch the water table as ICF blocks float so you need to make sure you put enough ballast load over the top to avoid .. embarrasement. Post some sketches.. plan view and cross sections if you can / have the will, sit back and let BH members take the strain.

Hello eandg. Generally a 1600m retaining wall offers up a lot of options, provided the wall is not "structural"... say holding back soil that is offering restraint to your neighbours house foundations. I would start with the fun part! Could we use gabion baskets say and grow things out of them. Could we use gabions with cheep infill and face it up with some dry stone dyke with inset bench seating to compliment any outside seating area. What you do here is to create butresses and put seating in between. What about a timber crib wall, you need a bit of space behind them to install but a well constructed crib wall will last for many years and is accepted by say the NHBC as a solution. Again you can grow things out of them and almost create a water fall effect of foliage that gives colour all year round. There are many different types of retaining wall. For a lowish wall such as this a gravity wall is often a good option if you have some space. In this case a starting point for the base width would be about ~1.2m if you have a reasonably stiff clay and good drainage. The footings are taken down to the level where the soil does not contain any matter that will decompose, below the top soil. You'll still get some minor settlement but these walls cope fine with these conditions. What about posting more info on your site and the type of ground you have.. you'll get loads of ideas here on BH.

Hello iMCann. Unfortunately there are at least a couple of other things to consider. Generally a wall of 7.0N (Newtons / mm^2 compressive unit strength i.e each individual block) can carry a surprising amount of vertical load if the wall is loaded about it's centreline and not subject to other sideways forces. But beam and block flooring can introduce quite high local stresses where the beams rest on the blocks, an over stress can lead to local crushing of the block. The walls may also be designed to resist horizontal wind loads which cause bending (flexure) in the wall. Even internal walls are subject to wind load! Imagine you have a big set of sliding doors open and a gust of wind blows in.. it can load the internal walls sideways. It's usually not a problem on domestic houses but on large industrial buildings or houses with two storeys and a vaulted ceiling this "internal wind loading" needs checked. The flexural stength of the wall is partly dictated by the unit strength of the blocks. The higher the block stength generally the stronger (up to a point) the wall in flexure. I would err on the safe side and check first with your SE. Seems odd though that an SE would spec 10N blocks when 7.3 would have done. At the same time ask your SE how to phrase the question (so they don't give you the run around) to the supplier so you can ascertain the strength of the block if 10N are indeed required with some evidence to back up any claim they may make, even though the blocks are stamped 7.3N.

To put a different slant on this, but hopefully an encouraging one for BH members. After the Grenfell tragety the insurers that were providing PI cover to designers (also to the folk that manufacture cladding systems and anyone else involved) were lost and backed off, many were trying to figure out what would follow, what it all meant. The mortgage providers, who partly rely on designers PI cover and input were stuck as they are there to write insurance business..not to study the regs, fire design and so on. However. Things seem to be settling a bit for the domestic market as many houses don't exceed say 18m in height. The building regs Uk wide (fire) are actually not that bad for low rise structures so it's a bit of a less risk area. Unfortunately in the panic even self builds / extensions got caught in the net to some extent and lenders backed off. I have seen a big surge in the amount of info you need to provide when renewing your PI cover in terms of fire risk. It's gone from about one page to six or seven and you also need to provide a statement about.. almost how you see the risk..the insurers are really trying to cover all their bases. Fundamentally though, if you are self building / extending you can still have a timber cladding and a timber frame. If you are less than 1.0m from the boundary then it's an issue but that has always been the case pre Grenfell or not. Often there are other factors that make the fire design a mute point if close to a boundary.. privacy.. over shaddowing, massing and so on in terms of planning anyway. I'm mulling this over.. have spoken to a few folk that make a living as underwriters, brokers and lenders. What I did was when filling in my PI renewal form was to embrace the questions and lay out where the risk is. My premium went up by about 7% so good result as I was expecting it to double at least. Given my experience it may be worth while discussing with your designers what they would charge you to provide a lay persons summary about your design and the fire risk. The aim is to reassure and educate potential insurers. Always bear in mind that insurance companies need to write policies.. and they need you!

+ one. Add up the labout time to fix it.. you'll need to find someone with the artisan type skills to do it. Compare with a new covering with maybe a guarentee?

@MJNewton Thank you MJ for your initial post and the follow up. It is a great informative thread. A great demonstration of how a pragmatic approach, working together with suppliers / contractors / the technical advisors can lead to a good outcome without having to resort to stripping things out and all the grief that can bring. Thanks again. To add a bit of food for thought... Often you hear bad reports about flat roofs, like all things in life bad news travels faster than good in general.The contractor /suppliers / installers / roofers are the first to get it in the neck when you start to see ponding / puddles on the roof. If you are thinking about a flat roof then you need to take quite a few steps back to prepare the structure in advance.. this needs forward planning! To explain I'll use the following example. You build an extension out the back of your house. Much of the below also applies to new build. The roof timbers / joists.. could be metal and often span from the new front wall of the extension back to the line of your existing rear elevation. You often want to open up the rear wall of the house to give a more of an open plan feel. You need say a steel beam / big concrete lintel to make a wider opening to access the new extension. This beam often rests on the old walls each side and adds point loads where before the load was spread more evenly. These point loads travel down the walls and often adds a bit more load locally to the founds that have sat there quite happily without moving. This extra load often causes some extra local settlement unless say the house is on solid rock. Your new beam / lintel bends a bit in the middle. Near the ends of the extension the rafters are supported on a good length of wall, maybe the side /gable walls of the extension.. but half way along the existing rear elevation you have a found that is settling and a have beam that is bending. Thus you already have a dip that causes water to pond. Steel beams bend under their initial load say from brickwork / floors / roofs above.. then they bend a bit more if you have say snow on the roof. But often they go back up a bit once the snow melts. But timber is a bit different. Timber (concrete to a lesser extent) suffers from what is called creep, it's a natural material. You can see good example of this if you go into an old pub with exposed beams.. observe the sag / creep. BS 5268 for example recommends a deflection limit of 0.003 x the roof joist span for some applications. So a 4000mm span can deflect 0.003 x 4000 = 12mm! But with modern flat roof membranes you can already see trouble ahead. What happens is that the poor builder puts everthing in flat and level and a few weeks / months later it is no longer flat, the steel has deflected, the timber is creeping and the founds are settling locally. You then see ponding on the roof and start asking questions. If you look at each element in isolation it is often not much in terms of settlement / deflection.. but when you add all the movement together you can soon see how the flat roof now has low spots in it. In summary make sure your SE or structural designer knows that you are going for a modern " true flat roof" early on and this can avoid later issues.

There are often two mind sets.. 1/ To look at the wild life as a problem that interferes with your financial gain 2/ To accept that you are interfering with the habitat and make a serious effort to provide an alternative (during the build) and enhanced habitat once you have had your way.. development wise. If you are inclined towards point two then you can look at providing a "temporary habitat".. enhanced feeding, stacking cut foliage, even pallets to encourage insects, to provide shelter, natural food for young birds and a safe space for birds and so on to dive into if a predator is about etc. A building site does not have to be barren. Calvin you seem a point 2 type.. you have time now to act on the alternaitve habitat, supplementary feeding and so on. Take this approach and you will enhance the wildlife and not have to worry about the chimney. Then when you finish your project you can enjoy not just your new house but all the wildlife that has been turning up during the build.. what more could you ask for? Have to say ten out of ten to you for posting about this.

Keep it simple stupid. Have designed some houses with the plant rooms in the attic and it works fine. I would recommend that for future proofing you design the plant room in the attic so it can carry some good point loads from say a couple of really heavy cylinders.. say 500 litres each plus the self weight of the tanks. That is about 1.3 -1.5 tonnes working load. A lot of the stuff you see on BH is actually cutting edge stuff.. ASHP etc, the theory is explained in depth by the some members.. To do this easily (furure proofing) you can use steel beams or just position your load bearing walls.. easier to do. Also work your way down and put the manifolds in say the towel cupboard, laundry room if a big house.. you have good crisp bed sheets and so on, even put the manifolds in the bathrooms.. makes them super cosy! It's a luxery but let's just enjoy a bit of that! To afford you future flexibility separate the floors into zones. Put a manifold on each floor supplied by pipework of a decent bore. It won't cost you that much extra but you may well be glad later if this is your forever home. Yes, we have a wide range of heating controls available at the moment, smart controls and so on but who knows what will be available in the future. Best to run some good large bore pipes from the attic to the floors to give a plenty flow at a low pump head. In the future you can restrict the flow if need be but much more difficult to increase if the bore of the pipes is too small layout is restrictive. The laws of hydraulics won't change. I

Radian. To get the best response can you do a sketch, take a photo and upload? Give as much info as you can.. if in doubt about how much info to provide just chuck it all in!

Yes it's a dilemma as to a fair charge, Architects / SE's and so on get a bit of a slagging on BH but it's social media! I've been a member of BH for a couple of years (I think) maybe a bit less so very much a newcomer when compared with the mods. Also, there are some past icons such as Jeremy Harris, still plenty folk that really know their stuff contributing and most importantly new members that are coming on board with new ideas as building technology is developing.. The new members, for me, keeps the site refreshed and the "old folk" on their toes! Another attraction for me is the humour that members come up with.. it's a great site for sharing ideas having fun with like minded folk and exploring ideas. I post stuff about the SE side and the odd bit about the gorund but I learn a lot here and enjoy reading about what folk are doing and so on. It's a good friendly site. Ok punloos.. your into electronics and systems.. but you have a brain and common sense. That is the key. Just ask your questions on BH, don't worry about asking a daft one, BH is not a knowledge test! Often I think that folk worry about getting trolled, using their own name. Some folk crop up here on BH that clearly are professional developers asking for free advice, some are trying to play the planning system.. but on the whole I think many are just honest in their intention.. yes if you are in detailed negotiations to buy a plot then remain anonymous but after that.. ? In summary just keep asking the questions, best to ask rather than build it and wish later you had not asked the question! Oh and have you had a Greggs Steak bake yet! I know the guy who was part of the development team on the steak bake..So THERE! my claim to fame

@LSB It's good to ask questions, that is what BH is about.. sharing views and knowledge. I learn loads here and enjoy reading the posts on ASHP etc and stuff that is just interesting. From time to time I may be wrong..but better to ask questions now rather than after something is built. LSB. An experienced SE, Building Contractor, serial BH self builder will often just look at beam/ rafter sizes and form of structure and go..that looks a "bit tight" lets have a closer look at that. I can see that your 150 x 47 rafters are tightly sized.. based on the one page of calcs you have posted. Getting "it tight is ok".. that is economic design. I use my own name here on BH. I'm not criticizing your SE but I am highlighting that the rafter choice should be justified in your calcs, particularly in terms of your deflection limits and whether the rest of the roof covering and internal linings are compatible with those limits. It's important to recognise that if you have say a steel cladding over a timber joisted roof then while the timbers may be strong enough to carry the loads they may bend (deflect) so much that it over stresses the fixings on the cladding. The cladding supplier will specify what the maximum deflection of the underlying roof is to be. Exceed this and you void your cladding warranty. Sometimes on an lean to extension at a 4.0 m span rafter it's easier to specify a 195 x 45 rafter.. this gives you plenty depth to insulate the roof, run the services and ventilate either by an air gap of by way of a membrane, sarking and almost full depth insulation between the rafters.. much food for thought!

Welcome @un355 You'll find a lot of great info here. If you have never built a house for yourself before then I used to say pre covid that with an Architect, some support, a good builder who will hold you hand a bit and a fair wind you could save some 10% - maybe 15% tops if you do a lot of the admin type of stuff yourself, research deeply and not pay too much for the plot. Admin would include things like; sorting out the services, making the phone calls etc. Now, with the current fluctuations I would reduce that to say 5 - 10% to be on the safe side. That is based on building a like for like house you can buy off the shelf. However, it depends on how you measure affordability. With a self build, even on a limited budget you can get a house that is "bespoke" just for you. Also, building your own home is something special.. it's your own castle and relatively few folk do this in the UK. You get that quite satisfaction, sense of achievement from self building and that has value.

@jfb I have a similar issue that I'll need to face up to this year when I come to do the roof. It may be worth looking on the net as there are some reclamaition yards that provide a reclaimed matching service.

@puntloos"Can you rephrase that last sentence?" Hi puntloos... of course! On reflection I should have chosen a different form of wording, but enthusiasm took over! BH has a great range of members, from folk that are just starting out to serial self builders, professional Contractors, Architects, SE's and other real specialists that really know their subject inside and out. One common trait many have on BH (whether just starting out on you first DIY project or not) is that we spend a lot of time just thinking. I had a recent discussion with a highly experienced Architect (40+ years experience) where we mulled over how much time you "charge for" and how much "brain time" time you really spend on a project. Some folk on BH will for example be sitting having say lunch, a Greggs bakers experience, putting the bins out.. just day to day stuff and presto.. into your head crops up an idea about your build / DIY project, how you solve a problem. This applies to almost everyone including those who make a living from design and construction. This thinking time often does not appear on any invoice / balance / spread sheet. Things can become counter productive when there is a lack of clarity, understanding between the design team and / or the contractor. Most commonly this is precipitated by a lack of effective communication. If you are say a novice self builder then you learn as you go. At some point you may become frustrated that things are not to your absolute satisfaction as you have been learning plenty..and this can start to eat away at you.. thus counter productive or words to that effect?

EW is ΣW (Sigma W) which is the sum of the all the factored load (W is the loads from the ULS column) on the beam. Thus ΣW = sum of total ULS load =0.46 + 0.45 = 0.91 kN/m (kilo Newtons per metre run) * 4.0m (beam length) = 3.64 kN (kilo Newtons) = the factored design load applied to each beam. Zyy Req... Z is called the section modulus and the yy indicates about which axis of the beam the value of Z applies to. The British Standards and Eurocodes use a different axis of notation. Your SE is using the Eurocode here. The letters "req" are short for required. The SE is working out what value of Z (a geometric property) the beam needs to have as a minimum to carry the load. To calculate the Zyy required we use the formula Z req = maximum bending moment / maximum design stress allowed in the timber. The bending moment is shown as m = 1.8 kNm and the design stress is 14.8 N/mm^ (Newtons per mm squared). We need to make the units compatible so to convert kNm (Kilo Newton metres) to Nmm (Newton millimtres) we multiply by 10^6 as below. Thus Zyy req = 1.8 * 10^6 / 14.8 = 121621 mm^3 = 121.6 x 10^3 mm^3 this is the minimum value of Zyy required to carry the load. Next the SE is checking to see if a 150 x 47 timber has a Zyy value greater than the required minimum. Zyy for the proposed timber is calculated by the formula b*d^2 / 6. Beam width = b = 47mm, Beam depth = d =150mm thus Zyy = 47 *150 *150/6 = 176250 mm^3 = ~ 176.3 x 10^3 mm^3 (millimetres cubed) > 121.6 x 10^3 required thus OK. The Iyy is called the second moment of area (units mm^4 ) and you'll probably see this on the next page when the SE checks the deflection.

Lots of great views expressed here. For me the test to apply to your design team is this... Ask them where they see the risk to you in the design. That could be on the Architectural side.. say bespoke materials price fluctuations.., the SE.. where is the risk.. is it in the ground or some where else, the folk that are maybe providing services to the plot. Where they are confident that say the doors, windows, roof will perform in the long term and where they are less sure about long term performance. It might sound a bit odd but a good professional will explain to their Client where the risks lie, inform them and let them make their decision. They will also explain / outline how they are going to save you a fair bit of money that will mitigate their fee maybe make it cost neutral!.. but they won't tell you how until you sign on the line. It 's a case of seeing if they are open on honest about the whole design. You too need to accept that if you want something special you'll probably need to take more risk if you want to keep the cost down. In summary just have an open and honest conversation. If you don't hit it off then find someone else, even if their fee is a little higher, in the grand scheme of things a good designer can save you thousands sometime tens of thousands on a self build. Also remember that designers love working with engaged Clients who communicate with them too! often you'll find that they don't bill you for every hour they work and that when you look at it in the round your shopping about has been fine up till now to enable you to get a feel for the cost but has now become counter productive in the relationship?