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Hi all, pretty crappy discovery this week of a very small crack running directly through a concrete padstone under a new steel beam picking up the one side of our new extension, running through mortar joints and blocks down to the slab (or as far as I can see). It's a concrete block sized padstone built into the inner skin of the wall built from dense concrete blocks. There's no crack evident on the outer skin and the foundation is probably 800mm thick there due to having to excavate all the soggy clay, so I don't think there's any issue with the foundation at this point. See link below for a video - apologies if it doesn't show it very well. The roof has been built to SE spec, and has been designed to potentially hold a 2.5t hot-tub on top. The bizarre thing is there's barely any load on there at the moment - just the steel beams themselves plus joists and plywood. Any ideas why this could have happened and likely remedial works required? I'm hopeful it will be just supporting the beam and chopping out and replacing the padstone.

You also need to remember that there are different types of lime mortar. I'm not sure how familiar you are but broadly you have two types Natural Hydraulic Lime (NHL, available in 2, 3.5 and 5 in increasing strength and reducing free lime content) Non-hydraulic lime (also technically 'NHL' by name but no-one refers to it as that) also called air-lime or you might see 'lime putty' This can be in the form of quicklime which needs to be slaked, or as hydrated lime ('bagged lime' at the builders merchants) which has already been partially hydrated I don't actually know how low carbon the different lime mortars are, but my understanding is that they all require burning either pure limestone or some mix of limestone and other stones in a kiln at very high temperatures, and that energy has to come from somewhere, which could be burning coal I guess. I just asked ChatGPT about this and got the below summary. The issue here is that non-hydraulic lime sets very slowly and isn't very strong. It's the best thing for old buildings with solid walls and no DPC, so while there's carbon to be saved, it might not be something you can get sign off from a structural perspective.

It aounsa like you've been having some similar experiences to me! Where did you land with window detailing in the end? Is there anything you can share?

Thanks for this, although it looks like your roof is a cold/hybrid given ventilation above it? I assume that doesn't matter in terms of the VCL, and that it probably needs to be chased in somehow.

My VCL is going in on top of my plywood sub-deck, which butts up to the existing stone-faced external wall of the house. Where does the VCL go at the house side? The stone is uneven, so I don't think I'd get a good bond if I tried to adhere it to the external face of the house.

If you search for a 'Tony tray' you'll see that a lot of people have wrapped the ends of the joists in 600mm+ DPC, which can be taped to OSB and into the parge coat on the walls.

We are putting a frameless glass balustrade around our roof terrace for our extension, but I'm also struggling on speccing what it should fix into. I spoke to Q railing who manufacture the profiles that the glass fits into, and they said that I need to speak to an SE to make sure whatever it fixes into can take a 0.74kN/m load at 1100mm, but wouldn't help beyond that. I spoke to the SE who has been designing the steelwork for our renovation/extension and he didn't want to touch it, saying that I need to go back to the manufacturer. I feel like the simplest answer is a reinforced concrete ringbeam sat on top of the outer blockwork, but I'm at a bit of a loss as to who can tell me whether or not this is compliant, or whether BCO will know that it's overspecified and sign it off. Maybe I need to find a new SE for this bit in particular? Any suggestions?

You're specifying a warm roof, so no ventilated void, therefore the quoted post doesn't apply (unless I've missed something).

The other photo

Builder needs to repair this hole which he still has the stones from (pic below). He wants to use a 'standard' mortar mix and is concerned mainly with it being structurally sound. It's worth noting we'll be taking some stone out below and adjacent to this to create a doorway, and that there'll be a block wall from the new extension butting up against this corner. He knows I would prefer lime where the old walls are concerned but that's more relating to pointing and plaster than the repair work. Any thoughts?

Just wanted to send a message to say you seem surprisingly chipper given the absolute sh*t show in evidence here. So many people would just accept this as 'what it is', so kudos to you to get sorting it! I hope you retained a significant chunk of the ££ against the builder.

That sounds interesting but I'm also not that keen on the idea of having to replace something after a few years to keep it watertight. I've come to the conclusion that the easiest option might be to cast a 200x100 ring beam on top of the outer skin as a mini parapet, which means I can fix posts or the balustrade channel (if we go frameless) into the concrete. I would cast drainage channels through the ringbeam at intervals and have single ply membrane run right through these into collector boxes, which avoids having the posts/channel in the drainage pathway. I did a sketch somewhere that I can dig out if you're interested.

I'm trying to reduce cold bridging where there is a change of floor level of 370mm (two steps) in our extension. The builder is constructing a dividing wall with blocks laid flat because it is buttressing a 200+ year old wall of the existing building. I want to put in a layer of thermoblocks into the dividing wall because this will be a break in the insulation under the slab. This is what he wants to do: And this is what I'm proposing, but I'm not sure where to lap the DPM from the two slabs. I'm using a 35mm concrete paver cut down to make up the 65mm marmox to a full 100mm course. Which of these is right in terms of where the DPM should lap? I should probably also say that the drawing is directly underneath an opening between the two rooms hence why I've drawn the floor covering, but either side of the opening the blocks will continue up, hence why I need Marmox and can't continue the PIR from the top slab over onto the blockwork. Any other comments on this plan?

I've just been through this process and decided that instead of going with an aerated block, I'm going with plain and simple dense 7N concrete blockwork for both skins, with a 150mm full fill cavity with Dritherm 37 and an insulated 50mm service cavity on the inside with mineral wool batts, finished with plasterboard and skim. This gets me under 0.18 U value requirement. I plan to parge the internal face of the inner blockwork to make this my airtight layer, meaning that I don't need to worry as much about taping the back of socket boxes etc, and makes running services super simple. I've read (on here and elsewhere) that thermalite blocks are prone to cracking, and I assume that would compromise my airtight layer if the blocks crack as the parge coat is likely to crack as well.

I'm glad you asked (and I should have come and updated this). Don't bother with the Flir - really poor thermal resolution (60x80) and lacking things like set high and low points. I returned mine and bought the Topdon TC001 for about £200 and I'm much happier with it. The app seems better and more fully featured and the image quality is a lot better. It will also track the hottest and coolest items in the frame as well as the centre spot.