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Week 7 - It's a steel!


Red Kite

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The video shows the black plastic membrane going down this week, followed by an enormous quantity of steel. What you dont see is that with all the rain the pump has been running continuously to try and keep the rainwater at bay - getting lots of water on the slab at this point is a bad thing as the membrane floats up which is a 'bad thing', its a bit better now because of the tons of steel on it!

 

The black membrane goes down in three layer - first the sealed flexible which is heat sealed and goes up over the shuttering and is sealed around the pipes. Then a rigid layer in sheets on top to protect it, and then another flexible plastic layer. The aim here is that the whole slab is waterproof and no water comes up through it. We will also have additives in the concrete itself which makes it 'waterproof concrete',  and talking to the guys  this pretty much makes the membrane redundant - but B Regs requires two 'just in case' methods of waterproofing.

 

Then in goes a ridiculous quantity of steel mesh (rusty brown in the video) in two layers standing off the slab on blocks of concrete referred to as 'Mars bars' and separated by 'chairs' (if I have the terminology right). Its a really good question why there is so much steel? We think we could build a skyscraper off the slab and it seems overkill for a lightweight timber frame house. However the Structural Engineer designed it and since he is a Professional (with a capital P) it seems he can add contingency and safety and 'just in case' (same as the waterproofing) as he sees fit and we get no choice at all. Its all backed up with pages of abstruse calculations on the bending strength of steel and concrete - the joke about SE's is that 'they have their moments'! The only comfort is that it should withstand any earthquakes we might get in Wiltshire ,and its all backed up by his Indemnity Insurance so, apart from the cost. its all wonderful. Also interestingly the contractors said that 10 - 15 years ago slabs like this only has a tiny amount of 'anti crack' steel mesh that worked just fine, so it seems that those earthquakes must be getting more common (or more likely SE's more cautious). Though in an age of trying to be Eco and 'save the planet' you might have thought there would be a move to design to 'good enough' and not to seemingly overdo the huge amounts of steel and concrete (really not a green material - especially with waterproof chemicals) used in construction. But having just watched 'The Accident' (a C4 drama where a building collapses and kills lots of kids and  they are all questioning the construction methodology and start apportioning blame) on catch-up then perhaps they have a point.

 

After the mesh has gone down they started on the L Bars that tie into the slab mesh and stand up into the vertical walls so they hold back the ground around them.  They should finish these and the 'Kicker' next week. The Kicker is a small upstand around the outside of  the slab that gets poured at the same time as the slab and then the walls are shuttered up from this. They estimate that the slab will get poured next Monday - and they plan to pour it in a single exciting day!!!

 

The other progress is that the Planning Officer signed off our Planning Conditions. This was after a bit of to and fro on the materials - we showed her samples of the render (cream K-Rend silicon), roof membrane (slate grey IKO) and the grey cladding panels. Seems like she didn't quite understand these - they are Rockpanel which is made from Basalt stone (pretty Eco) and finished in a textured grey - apart from the snazzy finish it looks a bit like render board.  After a couple of rounds we got there, and now we can technically start the house construction. Up to now all we have been doing is 'foundations' to keep within the letter of the conditions which preclude 'house construction' until the Planners have approved the drainage, materials and landscaping! Luckily we had no 'pre-commencement' conditions so could get on with work while the Planners faffed around!

 

See https://www.dropbox.com/sh/th9f6e3cel5dm1q/AAAfsWdAH184J75bCNUUtzVra?dl=0  for the weekly videos

 

 

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Yes this is a fixed price Groundworks plus drainage contract with a specialist contractor. We have been talking to the contractor for over 5 years, at that point we were going to go with Weber Haus (the Euro put paid to that idea!) and he does a lot of the GW for all the German OSM's. Weber Haus have used him for 10 years plus and would not use anyone else. He is happy and able to work to the tolerance the Germans ask for (about 5mm!) and comes highly recommended. This for him is not a big job - hate to see bigger. So yes we are paying top dollar, and I am sure we could have gone cheaper (or DIY!!!! ) but the quality  and knowledge is absolutely top notch. I heard some advice at one of the shows that was 'work with the best possible people you can afford' - we can just about afford this and so far he is proving to be the best. As a fixed price contract, as long as you dont have hard deadlines, it doesn't matter if there are weather delays, but as you can see they are cracking on.

 

Also worth considering that we would  likely have been on really deep trench fill foundations (or piles and ringbeam) for a more traditional foundation design; expensive  and a pile of unknown risk. Raft foundations btw dont work well on sloped sites! (for some reason MBC declined to offer their brilliant raft foundation solution). Building the way we have adds some cost but takes a lot of the risk out (if you dig that deep!!!) and gives us two houses on the plot and lots of additional basement space. The rule of thumb is that the basement floor space will cost you per m2 'about the same' as the upper floors - but you get the foundations as a bi product. So then its down to how much space you want and can afford. Given we had a height restriction of no more than the existing bungalow we couldn't go upwards, which left outwards or downwards. Outwards wouldn't have allowed two houses on the plot easily due to planning and site restrictions (we tried!). So we went downwards, which works well with the sloped site. Certainly not the route for everyone, or for a cost sensitive DIY build (I couldn't imagine doing this as a DIY or even PM + Subcontract project like so many brave souls on this board seem to do), but as with all self-build you get to choose exactly what route you want to go down (or 'choose your brand of insanity'). Though were that route takes you is a different story........As they say 'its a journey'.

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Great to read this - I'm just planning the GW & foundations for our project at the moment so it is super helpful! Will look at movie as soon as I get to a bigger screen than my mobile! Mine is a replacement for a 1930s house also built on a slope. Current house has no damp (good free-draining compacted sand/sandstone), so like you, feels like the new ways of working are adding in a lot of cost for marginal benefit - all helps the economy go round I suppose!

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