Posted By: Victorianecorequires a re-roof anyway

Ideal - this is your chance to do the best kind of roof insulation, which is EWI-alike on the roof as well as the walls. Tiles and battens stripped, 11mm OSB3 sheathing laid across the rafter tops (if you gapfilling glue and screw it, all edges supported, it'll be airtight too). Glue 150 EPS or woodfibre on top of that. Then breather felt, long-screwed downslope battens, slating battens, slates, Glidevale 250 or equiv continuous ventilation of the downslope batten space, in one eave, uninterrupted over the ridge, out the other eave. 100 insulation fitted between the rafters, from below. 250mm total, optimum before diminishing returns set in, crazy to do less, while you're at it. Roof 'EWI' joins continuous with wall EWI, none of the usual eaves thermal bridge. None of the usual fiddly interruption by partitions, timber members etc that's usual with doing all the insulation from below. The roofspace is fully insulated, part of the internal environment.

If your're already taking the tiles off, it's a lot more straightforward than the pain of insulate internally - AFAIK you have plans to inhabit the attic, so cheap insulation at loft-floor level isn't an option?

If you're considering Tom's build up have a think about how your PV is going to fix to it....roof hooks have some pretty stout fixings run into the rafters so you'll need to replicate those through the insulation. It's likely been done but not commonplace. If you want in roof trays then speak to the manufacturer

Also check whether you'll need planning permission if you're going to raise the ridge height.

Posted By: philedgethe securing and loading is quite different to the tiling battens

I don't quite understand that, but what I was describing above doesn't involve the slating battens, but the downslope battens beneath them, which occupy the position that rafter-tops usually fill. The hooks fix to the downslope battens just as if they were the rafters.

I still don't understand e.g. how can tiling battens be @400c/cs across (along the slope?) and also @300c/cs up the slope? Start from the beginning - is this a modification of a standard tiled roof (before PVing)?

Posted By: philedgewind lift with only fixing to a 38x50mm counter batten

As I said, the downslope battens are secured against uplift by long-screwing to the rafters (through the 150 EPS/woodfibre). O'course it would all have to pass the installer's structural check - number and proximity of the long-screws to the hook etc.

Dead weight is not the main issue when specifying PV roof fixings. Uplift is the defining factor, and PV, being essentially large wings bolted to the roof - can leverage a considerable force - much more than individual slates or tiles.

If fixing to battens, this needs to be considered, and there are clear guidelines for batten fixing tray systems to include additional fixings.

Uplift is also much higher near the edges of roofs, and this should always be mitigated - either by keeping the panels outside the zone of maximum uplift, or increasing panel fixings towards the edges

In the case of woodfibre insulation above the rafters (at least for the woodfibre we provide) we would probably suggest the counterbattens be a minimum 80mm wide to avoid crushing the boards when the battens are screwed in.

The counter battens and screws hold all the insulation in place and need to be designed to withstand any wind/snow loads and carry the weight of the external finishes as well as any uplift. The counter batten width needs to be sufficient so that when the fixings pull into the structural frame, the batten doesn’t crush the insulation layer.

You will need to speak to a fixing manufacturer who can design a fixing methodology to consider all these factors.
(we often recommend www.heco-schrauben.com

For wood fibre on a pitched roof, a minimum ventilated airspace of 38mm must be allowed, vented at eaves and ridge. On roof slopes above 18-degree pitch, our woodfibre sarking boards do not need a membrane as they have a hydrophobic coating to repel any water that gets to the insulation layer

For flat roofs, the potential for condensation on the underside of the roof finish is always greater due to the restricted airflow - a minimum ventilated airspace of 80mm is required, and the addition of a breather / weathering membrane is recommended. When using a membrane with insulation, a nail-tight tape should be used between the counter battens and membrane. This seals around the fixing as it penetrates the membrane.

Be aware that insulation and battens will require long screws to reach the rafters - anything over 80mm depth will make it more likely that the screws will wander and miss the rafter centres. Back when I was installing, I made a special jig to ensure the screws went in straight and used a pull-out test rig to be doubly sure the fixing was mechanically sound. We were probably the only solar company to do this test, and only because I had the test rig from my industrial abseiling days!

We once fitted PV to a huge standing seam roof on a warehouse. The roof was installed by others, and we clamped to the standing seams. We used the correct amount of fixings to the seams - unfortunately, the zinc roofing company did not use enough mechanical fixings to the roof structure (about half of the design specifications). The entire thing, zinc sheeting, panels and all (about the zize of 2 football fields) peeled off in a storm and landed semi-intact about half a mile away!

Fantastic info from the coal-face - thanks sgt. Downslope (I prefer that term, less ambiguous) battens 80 wide - yes, I guess. Almost everything else is great to hear, as it corresponds with my thinking, but which can use all the practical feedback possible.

I thought of such a jig but never persuaded anyone to make one! Not difficult. What about those long helical headless drive-screws, which were the new thing a while ago but haven't seen mentioned for a while?

Posted By: sgt_wouldsspeak to a fixing manufacturer who can design a fixing methodology

I wouldn't ask the long-screws to prevent the downslope battens, and all that's hung on them, from sliding down the slope. Instead, as I say, I secure them at the top (difficult without bridging the insulation) so they 'hang' in tension, or better, if its a duo-pitch I fix the batten at the apex to its mate on the opposite slope.

Posted By: sgt_wouldsthere will still be 'downslope' forces exerted by the weight of the insulation etc.
Most of this will be countered by clamping friction and load spreading by the downslope counter battens.

I'd glue the insulation to the 11 OSB3 airtight sheathing that's gapfilling glued and screwed on top of the rafters, all edges supported.

The OP's question included Integrated Solar Panels. I guess they'd not act as wings, would exert just normal roof uplift?

Posted By: fostertomThe OP's question included Integrated Solar Panels. I guess they'd not act as wings, would exert just normal roof uplift?

Cross posted with FT
There was a recent thread here about the woes of integrated panels and the problems of the ability to change the panel type in the event of a failure. Essentially the OP was stuck with the same type of panel to avoid large changes to the roof.

Posted By: fostertomI still don't understand e.g. how can tiling battens be @400c/cs across (along the slope?) and also @300c/cs up the slope? Start from the beginning - is this a modification of a standard tiled roof (before PVing)?

Posted By: philedgewind lift with only fixing to a 38x50mm counter batten

As I said, the downslope battens are secured against uplift by long-screwing to the rafters (through the 150 EPS/woodfibre). O'course it would all have to pass the installer's structural check - number and proximity of the long-screws to the hook etc.

If rafters are at 400mm centres with counter battens sat above them then each tiling batten is fixed to the counter battens at 400mm centres. Each run of batten is roughly 300mm from adjacent run so tiling battens are fixed at 300mm(ish) centres going up the slope. Lots of fixings to to carry the load of the tiles and resist wind lift as opposed to far fewer contact points for an on roof PV mounting system.

The wind lift concerns were around 80mm screws for the PV roof hooks set into 38mm timber. If you're only fixing into half the intended depth of timber then that needs verifying as being adequate

What about actual roof coverings?

Slates or Tiles?

Posted By: Peter_in_Hungary

Posted By: fostertomThe OP's question included Integrated Solar Panels. I guess they'd not act as wings, would exert just normal roof uplift?

Cross posted with FT
There was a recent thread here about the woes of integrated panels and the problems of the ability to change the panel type in the event of a failure. Essentially the OP was stuck with the same type of panel to avoid large changes to the roof.

Yes, that was me - Hipped roof, Welsh slate, in roof solar thermal panel and a bank of 6 Veluxes below it. Thought the panel would be a fit and forget, 10 years ago, but it needed replacing. I think what I learned most was that, if you can, fit your solar at ground level rather than on the roof. Makes any maintenance or replacements so much easier. When re-roofing, I picked Welsh slate for its longevity, which it has got. Just let down by mixing it with a solar panel.

If I had to do it again, and on the roof was the only option, would I go in-roof again? Probably, as it is so much sleeker looking and in an exposed location like here in Orkney, avoids the wind getting under the panels. But its a major thought thinking about what happens when replacements are required, and the manufacturer no longer makes that size. I was lucky, as they still do.....

Its a shame there is no standardisation of sizes / fixing methods across all manufacturers. You can replace a double socket from manufacturer A with one from manufacturer B and it will fit. And will likely still do in 30 years time. A shame solar manufacturers have not adopted similar standardisation.

'Its a shame there is no standardisation of sizes / fixing methods across all manufacturers.'

Unfortunately, the domestic installation of solar panels is infinitesimal in the overall picture. The vast majority of solar panels are manufactured for commercial solar farms, and this dictates making panels as cheaply as possible whilst maximising performance. Like cars, they seem to grow a bit each year, and I cannot see them ever being standardised.

If using tier 1 panels, they are likely to survive for at least 50 years whilst maintaining a useful output. The original 1950's Bell lab prototypes are still working after all these years, and they were incredibly crude by modern standards. I have an old Astropower poly panels from the mid 1990's that is still producing over 80%

The cheapest option is to buy a few extra panels and store them for any physical damage that may happen. This is more common than panel failures - e.g. storm damage, or panels being shot at by idiots.