MortarThePoint

Well the demolition guys did a pretty good job of lifting all that 100mm Celotex. Most of the foil has come off and many are a bit curvy. I think its GA4100. It's going a bit brown in the sun and rain, but I guess that's only on the surface. The garage plan uses 100mm mineral wool insulation in a 100mm cavity. I am wondering about the merit of ripping this down to 450mm wide slabs and widening the cavity to 150mm to include this. It would save about £1000 and make use of about half of this pile. I think if I had to add the foil back on it would end up being more hassle than it's worth.

We're going for hot water sink and a loo. I don't fancy being repeatedly told off for traipsing filth in to the new house ? A shallow pit is a good call. Combined with a low lift and you've got most of the win of full lift. There might be a call to the Structural Engineer coming up. That said, I think I remember reading somewhere that pits aren't allowed any more due to fume buildup. Is that beyond a certain depth or something?

We're going to be pouring the raft for our garage/workshop in a couple of weeks. It's a real treat for me to have this space and part of the reason why we are doing the whole selfbuild thing. It's my last chance to think of some of the details to add and I thought I would ask people to fantasise a bit. They can be simple details (e.g. I am thinking of asking them to cast in anchors near the door pillars for pulley winches) to that 'if only I could' thing (e.g. I'd love a car lift but that's not on the list unfortunately).

I'm a fan tolerances like +something/-0 (e.g. +50/-0 mm) as it conveys which way pain lies.

Don't trades worry what the next one in line is going to think of their work? I couldn't work in a way that has contempt for the next guy when it's relatively easy to take some pride.

Lead Acid batteries are about to be in massive demand as people wander out to their cars and find the batteries are stuffed. RAC has had a special offer of free replacements which I think they will regret. A friend got a text message from their car dealer to say their car needed charging. Makes you wonder how much they know these days.

Thanks, I'm sure you're right. One of those things you churn on in the middle of the night having been woken up by a 4yo.

No holidays for a while at this rate. (more thinking Covid-19)

Well I had lots of time to think of some ideas overnight: 1) It can be off foundation centre and that's fine to a limit which I'll find out from the Structural Engineer. 2) The exterior wall at the rear (top of frame) could be pushed back a bit. Currently the wall to foundation is centred on the cavity. Below DPC, the inner leaf is 190mm blockwork and the outer is 100mm blockwork. If that wall had its overall footprint centred then that is a 45mm shift. Due to the walls higher up being of thinner blockwork, the cavity side of the 190mm blockwork will be more highly loaded, but I'd hope that over the two courses below DPC that effect would disappear(?) 3) The inner leaf of a cavity wall carries most of the load. I know that the wider you go with the cavity the less structural the outer leaf becomes, but I don't have any load sharing proportions to back that up. My cavities are to be 100mm. *IF* the outer leaf is only supporting its own weight and the inner leaf is supporting its own weight plus all the floors and roof then you could arguably centre the load on that basis and it yields 95mm. If the outer leaf takes 1/3 and the inner leaf 2/3 then its about 45mm. Arguably double those shifts would yield the same moment on the foundations as was originally designed in, just in the outward rather than inward direction. 4) Use thinner stronger blocks. The spine wall running the mid-length of the house is designed as 215mm/10.4N blockwork below DPC and 190mm/10.4N above DPC (a). If that 190mm/10.4N was thinned to 100mm/21N, but still centred on the lower blockwork, it would yield half the change in block width, so 45mm (b). They would be special order blocks and the wall would be more vulnerable to lateral loads, but there may be something there. Even 140mm/15N blocks offcentre from the lower blockwork could give 40mm or more. Thinner blockwork doesn't help the first floor as that's already 100mm, but could help on the ground floor. Could go as far as 100mm/21N shifted over 100mm (c) but that feels iffy. 5) Get the groundworks guys to dig and fill next to the misplaced section of foundation, or at least some piers at right angles. I can only imagine their reaction but it is their mistake. I'll see how they navigate it when they measure up for the below DPC blockwork as they are doing that too. I haven't discussed the misplaced foundation section with them yet and want them to kind of work it out for themselves rather than have me point it out. Hopefully some mixture of the first 4 will get things back on track.

That's worth knowing, thanks.

Thanks, I'm a bit of a nerd. That and buying QCAD which was the best £30 I've ever spent.

I think I have probably been too trusting. They used an EDM to mark out and I checked the mark out as best I could, but more experience would probably have made me do some profiles or outside foundation marks for checking as well as a bit of double checking with tape measure or EDM.

I'm hearing good vibes which is encouraging ?

They clearly didn't dig centred on the line. They used a 600mm bucket and I guess they may have widened only on one side. That's the only way I can guess they have done it. No profiles unfortunately.

Thanks, this is good to hear. I worry about it being off centre putting a moment on that part of the foundation that effectively increases the pressure below one side of the foundation section. I expect I'll need to check with the Structural Engineer.

Separate to a discussion on tolerances, I wanted to start a new thread focused on a possible issue with how my foundations have been positioned. I used a tape measure to measure lines the groundworks team had sprayed out lines that represented middle of the trenches and they agreed with the drawings to within what I thought was an acceptable amount typically less 30mm difference. Since the trenches have been dug and poured it is hard to measure dimensions, but I am sure that one of the easiest to measure dimensions is out by 200mm. The wife's face was a picture when I explained to her that the kitchen had just shrunk by 200mm though it's actually the whole of one side of the house. Worse, we are preordering precast flooring that needs to fit. This is a bit of a worry. I measured centre of trench to centre of trench using a tape measure in the three positions (along A, B and C) and all three were under by ~200mm. I sent the drone up and I think this image captures the problem. The orange lines superimposed are the walls construction drawing. I laid some timber in the trenches to have reference dimensions. Now I'm hoping the bottom left hand corner is due to lens distortion so let's not go there yet. I don't have an EDM or anything to do better 2D measurements, but those three 1D measurements have me worried enough. With the rear wall (top of frame) centred on its trench the wall going left to right along the centre line is off centre of its trench. It just seems like that whole trench is out of place. That trench is specified to be 850mm wide and have a very structural wall on it which is 215mm wide. If the blockwork was laid out ignoring the trench misplacement it would look like the image below, 200mm off centre (blockwork in blue on top of foundation concrete in grey). That level of off centre looks alarming to me. What do people think? Am I rightly worried or does everyone have walls well of foundation centre? Some comfort and cold beer required.

I measured how they had sprayed the foundations out, but can only imagine what they would have made of it if I had started measuring the trenches as dug before the pour. Not particularly practical with a tape measure as well due to tench side collapse potential. Well since it has been poured and gone off I have been out with a tape measure ? I think I'd better start a new thread for thoughts on that.

Just to be clear the "That's nuts!" comment was about the swimming pool ? My groundworks guys dug to sprayed lines and the only measuring going on was for level/depth. BCO didn't go down and measure any stiffness, but gave things a good eyeball.

Is it normal to survey between dig and pour of the foundations to check that they are correctly dug? That's nuts!

I've seen there is quite a big gap between theory and practice so it is very useful to have real world numbers. My "brick or block level" was thinking above DPC, but I would hope the plumbness applies below DPC too. I don't find it stressful dealing with large tolerances as long as I have a feel for what they should be. Measuring something say 30mm out and having no idea what is reasonable is where the stress comes in. I'm cool if a room ends up 30mm smaller, but not if a doorway does. Some things are largely aesthetic (brick level and perp alignment every other course), some things functional (having a crapper in the corner of the living room rather as intended next door in the toilet) and some structural (plumbness).

I'd prefer to readjust. It's always been a pet peeve of mine when people have not thought about tolerances and default set very tight. I saved a previous company I worked for >£30k on a 10,000 fps high speed video camera that was proposed to confirm that an event happened within less than 100ms. I was able to show it using my phone at 120fps and we went with a rig using a 1200 fps point and shoot camera with an LED attached to a calibrated signal generator setting a reference timing. The event was always faster than 50ms. Would have been cool to play with the 10,000 fps camera though.

The groundworks crew used something like that to spray out the foundations. I measured it with a tape measure over the following weekend and was really impressed. It was about +/- 30mm typically and I was using a tape measure over rough ground so some of that will be on my measurement. The sprayed line has a width anyway. I did notice that they had misread a dimension though and lined up two walls that weren't supposed to be.

I've found some NHBC guidance for some of these: http://www.nhbc.co.uk/NHBCpublications/LiteratureLibrary/Technical/filedownload,15912,en.pdf but it's to do with finishes really. Brick or block level: +/-8mm over 5m, +/-12mm over further Brick or block plumbness: 8mm over 5m, 12m for higher but 8mm per storey

I want to be sure I have realistic expectations as it saves grief and means that I can tweak to accommodate the appropriate tolerance in advance.

That's some serious bedtime reading!