G and J

I haven’t thought that far ahead, but I note that EPDM appears to be the standard approach at the mo. And I appreciate the help. I suspect PIR and Superfoil is a good option but my habit is to try and get my head round all sensible options before going for what seems best.

Fab. Thank you.

PS. Designing out the flat roof is not an option for us, we did think long and hard about that at an early stage.

OK, I can understand all that. Is there something I can do to my flat roof though? Or is the simple fact that it is a flat roof such that I just have to accept the short lifespans and risk of water ingress?

Drainage plane! Isn’t that the EPDM?

So build up from top would be…. EPDM Ply PIR (tapered to save the thickness of furring as it is only a small area. VCL - i.e. airtight layer Ply Posi-joists Mineral wool stuffed in everywhere I can get it inc. around MVHR ducts and plenums. And the theory is that as long as I get enough PIR on top without too much cold bridging then the VCL will stay warm enough to avoid any form of condensation. Am I close?

Hmmmmm. If the joists are only 202mm and I fill the void with mineral wool I’ll not get a good u value at all. The context is that we also have a large set of patio doors in that area, so I’m having a fret that that area will be the coldest bit of the house.

It's got to be your call....... In an ideal world we all find a plot in a location we want to live AND can build the house of " our dreams " whatever that means to us at the time In reality, unless money/location has no limits, we may have to compromise on something. We have a project in our "dream" location (i.e. the one that will suit us going forward, leaving our "dream" location where we built 30 odd years ago), but in both cases some compromise was necessary. If the external look of the property is one of your key drivers, you've got to go for that? If it's the location..... As an aside we're in the same LPA as Alan, we may well have lucked out, but our pre app was spot on.....application granted in full.

So is this an example of the one third/two thirds rule? In which case I need a goodly bit of PIR to keep the ply warm. Which superfoil variant did you use?

That sounds good. I’m not sure how thick the top chord of our posijoists will be, so that’s another bit of info on my info shopping list. I think Superfoil only needs 20mm unventilated airgap to achieve max. R value so that should be ok. We’re lucky it’s only a tiny area - 3m x 5m with about a third of that taken up by a skylight, so I won’t even need that many staples.

Ah I see, thank you. I can understand the ply to distribute the weight (of, in our case, a few pigeons and a very occasional window cleaner’s feet), but doesn’t that mean that the ply layer cannot breath at all? Does that matter? Also, are those clips standard things? I’ll be onto Superfoil on Monday (lucky Superfoil lol).

In either case you will be living with your new neighbours, so ideally you want to take them with you......our experience is as others have suggested (we bought a bungalow on a very narrow plot ...its' pair one side, 2 storey house the other side....we wanted to demolish and build 2 storey house using footprint). Before we bought people in bungalow said they would object/ people in house said " no problem". In end all neighbours objected (including those opposite), although those in the bungalows pair only objected on basis of how work would be done. None of the objections had any relevance in terms of planning policy. Permission was granted in full. Job of work now to take new neighbours with us, including listening to their worries and undertaking party wall awards in a non combatative way (however we feel!)...so which house do you prefer?

Like this I mean….

I have looked at Superfoil. I’m guessing you mean the Superfoil laid directly onto the top of the joists, taped to be your airtight layer, then PIR on top, then ply, then furring strips, then ply, then EPDM?

I can move the skylight up to preserve the 150mm upstand around the skylight if I thicken the flat roof, what I can’t do is raise the parapet as that risks us contravening our planning permission. So I guess knowing how big the upstand (watertight layer to parapet) has to be helps me understand what I’ve got to work with.

Just for you, don’t show anyone else…. I think the joists either side of the roof light are 202mm to give some scale.

Not now. We don’t want to go back through planning. There be dragons!

So, we are now looking at our flat roof. Architect has suggested cold roof inside parapet so cross flow is not as easy as it could be. Not got the details yet but drawing suggests insulation is within the joists so presumably the assumption is mineral wool stuffed in and around the 202mm posi joists. I think this means at best a u value of 0.17 ignoring the joists, so in reality equivalent to circa 100mm of PIR on top. If we convert to a warm roof do we still need a 150mm upstand to the inside of the parapet? If not then I’ve room for a layer of PIR above the joists.

I throw a rope over the ridge, secured to something reliable, thence secured to my ancient and rusty scaffold tower. I’ve got climbing kit but that’s not what I mean. A cheap poly rope with loop knots every few feet help enormously and prevent the tower teetering if it’s prone to that.

Just thinking through all this, and I can’t work out how the membranes would be done in practice. Is there a way of sealing the lapped wall membrane to the roof membrane? It could be just laid underneath it but I can’t see a way of that join being taped up.

Bear in mind that trusses at 400mm centres would mean I’d have to attend slimming world like forever! :-0

Goodness, I’m so confused. What are the things you wouldn’t omit? And by airflow do you mean in the loft?

Plasterboard ceilings need to be supported on 400mm centres? Good heavens, I’m so out of date. I’m standing in my kitchen and both upstairs and downstairs ceilings are 12.5mm plasterboard straight onto 600 centres joists or trusses. But then, this house was built a third of a century ago. How times change.

PS we’ve a 40degree roof. And yes, we know that we could achieve the same headroom improvement much cheaper and easier by going to 39degrees…

I was braced for the grimace re PIR in the frame! I do note your concerns…. and I will price up blown cellulose for the loft….. however as for the ceilings….. I think I’ve understood, so I’ve drawn up what will be our version. I know we could easily raise the ceiling more, but I hate chamfered rooms. I did briefly consider raising the tie enough to accommodate everything, wires and ducts. But it feels like more work and materials thus more time and money than plasterboarding straight onto the bottom chord of the trusses.