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Ok, I appreciate the advice. I won't give up just yet but will bear what you said in mind. Got a few weeks before I have to get off the fence.

Perhaps my diagram isn't clear. The part marked as 'additional flashing' is intended to be the side flashing, hooked over the Roof Maker flashing apron. That appears to be similar to the GSE adaptor noted on the Velux ODL kit and pictured below? I am working on a solution to the SVP so that might go away but I'd like the option to use Roof Maker's Luxlite (fixed or opening). @JohnMo, in your opinion is it Velux or nothing with the GSE kit then?

Can’t. It’s the highest point in the system.

Thanks. Unfortunately I can’t use Velux as they don’t do a fixed pitched window. My picture shows only the vertical join between the GSE tray and the window flashing apron. What would you add here to address your side-flashing point? I thought I’d covered that?

My architect helpfully placed the SVP too close to the roof light for my ensuite so it now has to be non-opening. After a bit of searching I've ended up with the Roof Maker Luxlite LP. If anyone knows of a more economical solution for a 35º pitched roof fixed triple glazed window please let me know! This roof light has to fit between 2 PV panels mounted in-roof on GSE trays. Am I correct to assume that the roof light unit butts up to the GSE tray and flashing is used to maintain the water proofing? If that detail is correct then for a 1134 mm wide PV panel, assuming 5mm clearance each side, my roof light has to fit into a 1144mm gap. Deducting the 194mm from each side for the rooflight flashing apron then my window structural opening is restricted to 756mm. What do you think? Thanks!

Sorry, autocorrect interfered: “Onshape works in most browsers”

Insulator works on any device with browser. I use Home Design 3D on my iPad, iPhone and MacBook.

I’m using Onshape for my detail work. It’s a fully fitted CAD that runs entirely in the browser, and is free if you don’t mind all your drawings being public. I also use HomeDesign3D for 3D visualisation.

Right in the middle? Upward and downward combined vent? Neat.

I've put the questions posed here to him. His responses have been confident and reflect the debate here so I don't believe he's out of his depth. I hear your points about the formers. If Building Control do not accept the SE's design then he doesn't get paid.

That’s the position of my SE too. He would not recommend type 3 fill in my case (structural raft) as it has a lower bearing pressure capacity than type one. He says the free draining nature of type 3 will allow water to percolate beneath the raft. Also the nature of type 3 could damage the EPS. Finally type 3 will be more expensive than type 1. However, I’m really grateful for this debate as I’m learning a lot!

Thanks Nick. Great feedback which I will discuss with the SE

Ok, thanks Russell. To be fair there's a lot of other detail that I haven't shared. I decided to go local for my SE so I hope I don't regret it.

Not especially, only that the SE had experience of insulated slabs. Why?

@Russell griffiths I have just asked them exactly that question!

My SE has just completed the first draft of the foundation design for my twin wall timber frame house. He's proposing a hefty structural raft to counter the clay soil and trees. Note the internal wall of the timber frame is on the slab ring beam but the outer is on the insulated upstand. He's specified Jackodur but could be another brand.

Unfortunately I have to be in London now on Thursday so won't be joining you all. Hopefully there will be a next time!

@saveasteading In my case the slab is roughly 15x9m, passive design so 400mm EPS on layer of sand, then on ground, 175mm structural raft on top with a ring beam around the perimeter. Clay with trees ground condition so heave has to be mitigated.

Update after chat with SE. The slab is a 'raft' not ground bearing, so it will be 175mm thick, with additional edge thickness for the walls. He's planning on two meshes set 40mm from the top and bottom. The top mesh will be sectioned around the joint lines. 50mm formers will be used to make the joints in the slab - no sawn cuts. Each joint line will have an intermediate mesh at 58mm (1/3rd depth) from the top to maintain the structural strength. We've agreed to extend this intermediate mesh so that I can use it to lay the UFH pipes. He stressed again that fixing a burst pipe will be more complicated than just digging out the concrete and backfilling with a bit of mesh, but admits he knows nothing about UFH pipe strength/resilience. Any views on this?

I believe the SE has designed a direct ground bearing slab on top of a crushable layer (to handle ground heave) hence he wants to include movement joints cut into the slab.

@Nick Laslett me too, so not giving up yet! However, I am finding it hard to source any ‘professional guide’/standards for UFH in-slab to pass to my SE. All the suppliers seem keen to push their proprietary UFH tracks and screed solutions, perhaps because they can sell more ‘stuff’?

That’s sounds like one solution though might be hard to execute. No polished concrete. I’m assuming the joints to stop cracking aren’t full depth as he mentioned cutting them in after the pour.

MBC’s quote was high, and assumed benign ground conditions. They also said I’d probably get it done cheaper locally. I’ve got clay and trees on the site. I do agree though that if my SE can’t get his head around the UFH in-slab then I’ll go elsewhere. Re the flow temp, I’ve used LoopCad to estimate it based on a 3kw peak heat output over 100m2 ground floor and a 300mm spacing. If I go to 200mm then I’d probably be where you are at.

Conceptual, yes. I’m using the MBC passive frame which is a twin wall structure with cellulose insulation in between. The internal frame is load bearing on the slab. External frame is on the EPS. Then there’s another small concrete base for the low brick wall ‘skirt’ (timber clad above).