Gus Potter

Ok it's not worked out that well. If you are embarking on a big TF stick build project then you need a saw bench made from timber with a good top quality chop saw. You fix solidly the saw bench to any floor and at night you take the chop saw home so it does not get nicked. Expect to pay around £700 - £900 for a good saw and blade. On a big kit you need a saw with a 300mm blade that does compound cuts. Don't mince about. This way you can fabricate a TF frame to the same tolerances as a TF fabricator. OK so you are not 21 years old any more. Making Tf panels if working on you own.. on site and lifting them up. You square the panels with the odd OSB board. Then lift them and sheet after. To stop the sheets dropping you use a temporary ledger at the bottom and then tilt them into place. Now I'm an old codger I've figured out how to stick build a TF for the folk that are 60 plus.. and have done it myself.

Yes that sums it up but also the ground swells up.. some grund can swell / shring a lot.. several inches / cm! Things like trees and the desication of the ground are essential to know about. Nick makes a good point here. If your motivation is to achieve something close to passive then a raft is a good simple way of doing it. Yes the raft may cost a little more than a strip found but it can be much easier to build in some circumstances.. it's simple if you take care and lay the insulation and rebar correctly. All these things come with a lot of complexity. Say you are in a Radon area then a raft makes the Radon Barrier easier to detail out / buildability for example. Trees for example in clay soils can make things much more complex. But just say you have good ground and just want a raft as Nick favours and because YOU CAN and WANT IT.. no harm in that.. it's your house and your design decision. If you have good ground then this can be easy to achieve without chucking loads of rebar at the slab and making it massivly thick. You need some kind of floor anyway! It usually needs a bit of edge thickening in some form or another as folk want to put in big glass doors these days which cause point loads at the slab edge. One biggy that BC / NHBC ask about is cover for frost and height to DPC. Generally height to DPC from ground level for sensitive wall cladding is 150mm which is about 50mm less than the thickness of an ideal raft slab on good ground. Now add 300 mm of EPS to that takes you down to 350mm below finished ground level. Add say 150mm of type one and now all our materials (which are not suceptible to frost) extend to the min of 450mm required for frost cover. Box ticked. For all.. Raft slabs.. and a bit of info that may help. There are argueably three at least kinds of generic rafts. Within each type there are permutations. There are others but let's run with this for now. The main types are: 1/ A rigid raft. This tends to be a bit of a beast. We may use this in a domestic context where we have past mining that can cause the ground to move / crack horizontally at the surface. This used to be associated with long wall mining where you get a rolling wave of horizontal movement in the ground. We don't often design this way now in a domestic setting.. as they shut all the mines long ago. 2/ A semi flexible raft. Here we thicken the edge a bit but if you have heavy walls the EPS say at the edge compresses too much which causes the edge of the raft to rotate. To stop the rotation we reinforce the slab so it carries some of the vertical loads and stops the rotation of the edge. 3/ An edge thickened slab. Here the thickening at the slab edge deals with the frost cover and the differing line and point loads around the slab edge. The slab it's self is just designed so it does not crack thus has a light reinforcing mesh. Now within all these generic types there are permutations and the design is often driven by how good and consistent the soil is under the whole thing. If we have ground that has local soft spots then the edge of the slab and internally often needs to be thicker so it can span over the soft spots and thus needs more reinforcement. In summary it's a pretty complex undertaking in term of the structural / soil things you need to know about . but the solution is often simple once you draw it out. The main thing is to spend a bit of time and money understanding and investigating your ground as this reduces your risk and helps you design the right way. With a fair wind a raft slab does not always cost a lot more when you take everything in context.

Ask what would you home insurance say! Do you think they would pay out if your only arguement was.. well other folk have done it? Builders often tell you what you want to hear not what you need to know.

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?