Gus Potter

You may ask what is @George on about.. ? to the lay person it may seem odd that when you remove load from a brick wall it can make it unstable. How does that happen? Imagine you have a brick wall 2.4m~ (8 feet high) That's about in old money 32 courses of brick. The mortar is old so not "sticky" thus it can only carry a downwards load. Envisage 32 bricks stacked up on old crappy mortar.. if you can. Now the wall is held in place by the floor and the ceiling at the top and above that by the rafters say. . Imagine if you gave it a sideways push.. ? For it to topple (basically) you would just need to over come the self weight of the bricks that are left for the wall to bend beyond it's "sideways tipping point" . But the chimney stack adds more weight which makes it harder to topple. You can try this at home with say Jenga blocks. Add weight to the top of a stack of blocks and they are harder to topple when you push sideways. Also if you remove one side of a chimney and use gallow brackets you can suddenly make things a lot worse as the gallow brackets cause a "toppling force" that is unexpected.. that is why for one reason BC have clamped down on this.

Hi Ian and all. I can see where you're coming from Ian but there are a couple (well a lot of steps actually) of steps missing. Generally Ian your right about typical slab edge TF line loading only being about a tenth of the load that a 300kPa insulation can carry at 10% compression. Say Ian's load is 10%.. that results in a settlement of 3.0mm at the edge of the slab say. Now it gets a bit more complicated as that amount of movement at the slab edge is on the boundary of crack / over stress the slab if it is not reinforced. For all G and J sum it up the concept.. sometimes we need to go back to basics! Simplistically we make a concrete slab that generally spreads the load over a large area and that all works fine. Take a 300 kPa insulation at 10% compression at 300mm thick. That's 30 tonnes per square metre on the face of it but it will; compress like fury, crack your walls, make your floors off level, burst cladding fixings, maybe cause the roof to leak and stop your doors/ windows from opening and shutting.. ! When we design foundations and raft slabs we as an absolute limit the settlement to 25mm for a domestic dwelling over a 50 year life span. Now it's not just the compressibility of the insulation it's all the stuff under that... the soil and so on which adds to the amount of movement / settlement. Take the outside walls for example.. these load the slab edge and thus at the slab edges we get an overstress / too much compression of the insulation. Simplistically we calculate what the insulation can carry at the slab edge. The bit it can't carry we throw back into the slab by reinforcing the slab with steel bars until we get it all to work. The technical term for this is what we call a semi flexible edge thickened raft.. The semi flexible bit is important as what we do is balance the movement / flexibility of the layers with the reinforcement and usually the insulation behavoir so the slab does not fail and crack too much. Now that all sounds complicated but all we are doing is to look at how "squashy" the layers of stuff are under the concrete and design the concrete raft to cope with that. In summary when it comes to EPS or anything like that think about how much it needs to compres by before it achieves it's declared strength.

I've used this in one of my designs..The Client was keen on it and wanted to make it work .. good results so far!

At the end of the day my gut feeling if you have trees is to go for a strip found if you can. You compensate by using more insulation elsewhere. The trees are a big thing. Can't say much more with the limited info you provide. The raft designs I do are nearly always goverened by the compressibilty of the insulation.. and I've been doing that for decades like Olaf. Tanners (TSD Ireland) also know their stuff, working with them just now. Look folks this is nothing new. Like Olaf and the Canadians putting insulation under concrete is not that hard. In Sweden and Canada the frost heave is a big thing.. in the UK we assume the frost only goes 450mm down tops. For insulated rafts we need to know how much each layer compresses by. If you take an insulation that states it has a 150-300 (tops) kPa compression strength.. that on the face of it looks like much more than a soil with a kPa of 75 -100.. but the fine print says that it will achieve 300 kPa at 10% compression. Now say the insulation is 300mm thick. You load that up and it needs to compress by 30mm to achieve its design strength. Now 30mm compression will play havoc with the concrete slab.! It won't work! When desgning these things I look at the soil first to get a handle on that, often I can put that to bed and just look at the insulation, it's U value and compressive strength at 10% compression.. Then the loads and flexibility of the slab.. see how much it can bend by without cracking and not needing daft amounts of rebar. I do a bit of juggling to balance the loads and the job is nearly done in terms of checking the structural strength. Unless there are point loads! Now the nightmare starts. You look at buildability, how much you can pour in a day, where you need joints for shrinkage etc. These are actually the hard parts! This idea that folk have.. you put insulation under a slab (to make it a passive slab) and now you need to be a specialist is a bit off the mark.. the insulation is just another layer above the soil with a different elasticity. Semi flexible passive rafts (a nuance but designed slightly diferently) tend to work down to soils with a bearing capacity of 40 kPa if the ground water is well down. The insulated raft is not often technically a challenge. The trees are important as these are the things that can add value, ammenity and don't forget.. things like shading! it looks like you are going for the PH concept.. so you have maybe lots of glass.. try if you can to look after the trees and not consider them as an obstical and take advantage of them. Can they provide shading when considering over heating in the summer. As an SE/ deisgner I look at the site.. what makes it attractive.. say the trees and then try an find a solution that preserves the character, ammenity and then see if we can say use the shading effect and greening/ colour contrast to enhance the surroundings.

Ok to disagree. My own feeling is that it's worth while paying the SE for a visit. This achieves a number of things. 1/ It lets the builder know that you the Client are not alone. 2/ I can say that I can't remember the time when I went to inspect a job and found nothing wrong. Builders often swap materials, hangers, connections, nail types and don't follow the nailing schedule on TF. Then you have fire stopping, vapour barriers etc... long list 3/ When I go to site I also look at fit up and if everything looks ok for the next stages say.. the insulation, how are the drains looking and so on. Now is the time to nip things in the bud. As a project goes on builders tend to come under more financial pressure as they like to get as much profit out the job early on. If they feel they have a weak Client it's human nature that they tend to let things slip as the project goes on. Ok @joe90 I agree with you that you should tell the builder to get it right.. but sometimes the presence of an SE, Architect or QS say can concentrate the mind..avoid later serious disputes.. especially if any visit is at short notice or unannounced.

Gav.. You have a couple of choices.. Take the risk that you have built something that is not safe.. bad move. OR Pay your SE to come to site and sort it out. Then try and claw back the SE fee money from your builders. My advice.. pay your SE and do things right and remediate if need be. If BC spot anything you'll have to pay anyway to get it fixed.

How much water do you want? Say average usage of 150- 200 litres per person per day. With a bit of storage you have more than enough? That makes me think.. how is the aluminum registering? You say very high but just how high? make sure it's just natural rather than an indicator of ground contamination. AL is often present in ground water as a trace element anyway.

WTF.. I suppose I must be far right now!

If you really want the house you need to find a good friendly SE that knows about this stuff. Expect to pay about £700 to 1200 for this service. I do this for some of my Clients for these types of properties. I identify the issues and we go back to the vendor and say.. this is what we think it is worth but importantly we identify the issues. This often informs the vendor and we justify our offer. Remember that a sale has to be equitable! Some Vendors stick to their guns and hope someone daft will come along.. Trust your gut feeling as I think you are right to do so.

Well done! Who told you a proper core would take 10 min? This is a proper core sample not making a hole for a duct say where you can afford to be rough. Coring a slab for techinical investigative info takes time. If you thrash into it you won't get a proper slab depth. Every 10mm matters when you are coring a slab for strength analysis / verification. From memory the going rate is £100 - 150 a core on a 150mm anticipated slab depth with a recovered sample (in good shape) for the operative time. If you have managed 4 in a day then that is not bad. Each one needs to be logged, photgraphed etc and they need thinking time too!

Big thanks from me Dave for sorting that out.

No your not Dave! That is an error, 2 + 5 = 7 inches ~ 170mm deep. Can you edit at your end?

To add a bit. I wrote this in response to a previous post by @Jawbkk who is thinking about fixing some floor joists that have had notches and holes drilled in them. One solution is to plate the joists up with ply. Simplistically the loads need to get into the ply.. round the notches and holes drilled for the electric cables and back into the solid timber. But if you use screws / nails etc then you need many and you can't get enough adequate spacing and edge distance for the nails etc. Also the more nails / scews you use the more risk you run in terms of damaging the already weakened timber. By using a structural glue you transfer the loads over a much wider area. You often just need a few screws to clamp the timber while the glue takes up. To check this type of timber joint if the right glue is used the SE now knows that the glue is stronger than the native timber. Then the SE checks not least what is called the rolling shear in the ply wood. This is the check to make sure that the diffferent layers of ply don't separate. The ply layers are bonded with a structural glue so why use a potentially inferior glue between the ply and the native timber? With a fair wind it then all falls into place and you have a sound solution.

You are getting there. Have you worked out what it has cost you to date?

Hello all. I know that a lot of folk talk about using D4 glue but this can lead to trouble. The designation D4 relates to the durability of the glue..durability is related to for simplicity the weather exposure.. you can buy D4 from say B & Q, Tool Station, Screwfix and loads of other places. Now I'm fine if you want to use "D4" that is bandied about on BH for sticking some non structural floor boards together. I'm not OK about you using this sole designation D4 (durability) in a stuctural application. A Tesco / Asda etc plastic bag is durable (probably deserves a D4 rating) for lying for years at the side of the road but it's not structurally strong! Glulam beams for example are bonded together with a structural glue that conforms to for example BS EN 301 which deals with glues that have structural stength AND the durability rating which is D1, 2, 3 & 4. Why would you not want to use a glue with both a structural and durability rating that the Glulam folk use? A structural glue requires both stength and durability. Structural glues tend to be resin based.. like old fashioned Araldite that your Mum and Dad used for fixing their glasses... Cascamite structural glue does the job and has both a structural and durability rating. Please folks can we stop recomending D4 when we are discussing bonding structural components.

There are two aspects to this. Generally if it is more than 600mm above the ground then it becomes a "structure" and thus it needs to be "structurally" designed. One reason for this is that beyond this height it can also sway sideways. In my day job as an SE I often design decks that are above 600mm in height. To roughly size the timbers I use an old rule of thumb for joists at 600 mm centres which is: Joist span 3000 mm. Divide by 25mm to convert to inches.. 3000 / 25 = 120 inches. Divide this by 12 to convert to feet = 120 /12 = 10 feet. Half of 10 feet = 5.. if it is a domestic application and I want to be safe.. and make sure I can later justify the fixings for the handrails etc I'll add 2 to that making 7 inches so I need a 170 x 50 joist. This is a thing that folk often miss.. make you floor deck too thin and you'll have problems later with the hand rails and other stuff. Skimping on the decking joist depth can be false economy.

Fair enough. For all on BH when we are dealing with old structures you get a lot of lintels that were known to work compositely with the brickwork above. The lintels can also be timber with some gently arched brickwork... these old builders were clever folk! Over the years you get folk changing windows say and hammering things about, installing ducts for extract fans.. which breaks the composite action.. hence the failures.. An easy way of quantifying this is to go to the load tables of modern concrete lintel suppliers. Some lintels are called composite lintels.. which interact with the brick above.. don't go putting in DPC's or cavity trays with these as the plastic make a slip plane which make them ineffective. Some are called non composite which hold everything up above on their own.

Quick reality check folks. Make things complex on site and you'll often pay for something that never gets delivered by the contractor. Wrapping the bottom of internal columns in Aroegell is daft.. because you need a small quantity (costs a lot) and someone has to go in the van to go and get it! That excercise just to procure the stuff could cost you £150 -300 or more which could pay for a lot of thicker insulation elsewhere at no risk. You may need fire protection so a bit of extra glass wool solves the problem and get you below the 0.7 U back stop value to stop regular condensation. The key thing that a lot of folk seem to be missing here on BH is that you can do a compensatory U value calculation. If you have a steel column poking out inside the house you need to make sure it does not drip with condensation. Again a back stop U value of 0.7 is about right with a vapour barrier to be on the safe side. There is a massive diference between an internal column and a steel column on the external wall in terms of cold bridging. An internal column is founded on the heated dumpling of soil so the heat loss is much less that the basic software is often telling you. You then compensate for the extra heat loss by beafing up other parts of the insulation envelope. But for such a small extra heat loss from an internal column.. it's next to nothing. This is a common sense approach that a lot of folk on BH are missing.

Oh and if you do this you will find that you are falling under the CDM regulations not least. Please spend some time investigating what you're getting yourself into.. hence the percentage fee rates I mention. How well are you insured to do this? It's not just to protect you it's also for your Client in case you get run over by a bus. You have a duty of care and while it sounds great to help folk out when you do so things have to be set up correctly.

To put a bit of an impartial slant on this.. @nod is highly experience, has time and also has a day job and contacts in the construction industry? Now I'm not going to cast stones as I use to be a Contractor and also had plenty of employees and subbies that I could chuck at my own stuff and make the money work. Nods figures are probably achieveable if you are set up this way.. but this is a self build forum and I can tell you (I do this as a day job) that Nods rates are "optomistic". But also Nod works like a fiend seven days a week! Nod has some serious commitement and deserves the reward. But often folk on BH have young kids so you can't do what Nod does. My hat goes off to Nod for commitment but most of my Clients just don't fit into Nod's mould and we need to reach a compromise and the build cost goes upas a consequence.

Given the size of this project and complexity my advice is to diplomatically extracate yourself from this. If you are intent on pursuing then you need to be commanding a fee of somewhere between 8- 12% of the build cost. Set out the deliverables, get paid in stages and get a good QS in right now.

The following I hope gives you a flavour of the design but also the liability that lies with the designer and that, I hope, will help folk on BH get their head around some of the SE type fee costs. Well done @Alan Ambrose for digging out this article. The author takes one of the CPD courses on Eurocode concrete design I have been on and interrogates it in a bit more detail.. and makes a good job of it too! For me this is part of my day job so am familiar with the terms / design calcs etc and how the Euro codes go into more detail and the theory behind it.. such as restraint conditions, ageing, restraint at slab ends.. a long list. But for all on build hub for the critical thing to take away from this is the bit at the end of the article copied below. "Firstly, the design need to be realistic. Blindly throwing reinforcement at it can do more harm than good. Good detailing is particularly important for complex geometries. What’s more important is to try and have simple geometries in the first place". In other words you can have a fab house but keep the underlying structure as simple / stupid and buildable as you can. That drives down cost, reduces risk and lets you spend your cash on the thing you get to see and enjoy. Hiya @SteamyTea If I could I would give you a double cup award.. I've learnt loads from you.. For all there are some folk like Steamy and to do a name check @MikeSharp01 who are fantastic educators. I'll give it a go and try and cover some of the concepts for folk on BH. but excuse the spelling and grammer please! For all the below applies to lots of other things you might do not just basements. Ok take a simple concrete reinforced beam spanning between two walls loaded from above. The bottom of the beam is in tension and the top in compression. Concrete is quite "stiff" which means that under compression is does not compress much. The bottom of the beam is in tension. Concrete is not so good at resisting tension so we introduce steel rebar in the bottom of the beam which has lots of tensile strength and now we have a reinforced concrete beam. But steel is quite stretchy compared with concrete. For it to work the steel rebar needs to be bonded to the concrete at the bottom of the beam. For the beam not to fall down the tensile forces in the top of the beam need to balance the tensile force in the bottom steel rebar. But to achieve the balance the steel needs to stretch first thus we get cracks in the bottom of the beam. The same principles applies to basement walls. Call this behavoir primary cracking. To limit cracking we add more rebar area so the steel does not stretch as much. In other words when we design reinforced concrete we make sure we have enough rebar so it does not fall down.. then check that we are limiting the amount the steel stretches by so we limit cracking. Now it gets complicated. In a reinforced concrete wall we have a large area of concrete that as @Alan Ambrose article points out undergoes a number of "experiences". Concrete goes through a number of phases when you pour it in to say a basement wall. Broadly speaking cement (part of the component in concrete) needs to start getting it's act together within 4 -8 hours ( temperature dependeant) and this is covered in the BS standard for example. This means that the chemical transition gets underway. This causes a lot of heat which makes the concrete expand. After say 24 -48 hours the concrete starts to "bind".. the aggregates, cement any additives and we get what is called plastic shrinckage.. which can be quite a lot.. in other words the concrete is still in quite an excited state for a better word. After a bit of time the concrete starts to dry out and we get what we call drying shinkage. On a raft slab.. say on an EPS raft slab we also get funny behavoir at the slab corners call "curling".. which is basically all of the above having a laugh with us. @SteamyTea The secondary reinforcement is primarily intended to deal with the behavoir of the concrete during the curing and drying out process. It is a bit of a guess but if you look after your concrete for say a month after it's poured you can do a lot to help yourself in terms of getting what you have paid for. Quality of workmanship is key. I've copied below a bit out of one of my specifications that I wrote for a self builder for a floor slab. The idea of this is to provide simple instructions for what you can do on site to address what is a very complex problem. Designer liability: From the above you can see that this is fraught with liability if you are say an SE / designer like me. My PI insurer goes nuts and wants to know exactly how much liability I'm taking on. @Alan Ambrose article deals with Eurocode design which if applied well results in lean and cost effective design.. but it is predicated on all the Contractors etc doing their bit very well.. the construction industry in the Uk is not often up to this standard. So when it goes wrong I'm for example the first easy target. Now folks if want to pay me for taking on that liability then I'm fine with that.. but the only way I can do that is if I come to site often to make sure that the contractor is doing "EXACTLY WHAT I REQUIRE" no if's or buts! However while that sounds great you will need to pay for an experienced contractor who is used to having someone like me holding thier feet to the fire. Self building is about finding the right compromise.

It's this thing where folk think PH and you need to eliminate every cold bridge.. even columns that are internal.. it looks great on paper but the cost outweighs the benefit.. your door handles are a cold bridge for example. Building houses requires a common sense and practical approach. Yes you do to you achieve a back stop U value. For a column like this that is founded say 450mm below finished floor level then it may achieve the back stop U value anyway with the fire protection and associated air gap. Yes it usually is enough.

Yes.. some of that mesh looks really conjested. I would send some photos to your SE before pouring and make sure they are happy with the layout. You'll need to be right on the ball with the concrete compaction. Scottish BC may ask for photos of the rebar.. if they don't match up with the SE's design you may have a problem! Be safe and get approval for this now from the SE and then you can sleep soundly!

I like this approach.. it's like passive stack ventilation simple and effective. No maintenance. In principle I'm all for PH type houses.. what I'm not in favour of is designing in elements that will require costly maintenance in the future and possible early replacement.. When I see folk doing this I think vanity has taken over from sanity. Off now to support England at the footy.