Whole host of pitched to flat roof questions

18 Jul 2005
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United Kingdom

Hi I’m in the process of building an orangery on the back of our house which will have an outer pitched roof with a flat top which will then have a glass lantern on top.
I’m hoping someone will be able to point me in the right direction and answer a few questions about the roof makeup, insulation and ventilation. I’ve already had a steep learning curve with regards to foundations, soak ways etc so now it’s time for the beginning of lessons in roofing.
For the pitched section of roof the architect has specified a unventilated makeup of concrete plain tiles on 25 x 38mm preservative treated s.wood battens on proprietary microporous sarking membrane (avoiding need for ventilation), on 47 x 150mm C16 rafters at max 400mm crs. 80mm Celotex GA4080 between rafters, overlaid with vapour control layer and 45mm Celotex PL4000 at ceiling height to give a U-value of 0.18W/m²K.

However looking at the Celotex website it seems for an unventilated roof it suggests I would need to use Celotex XR4120 between rafters and Pl4000 in 25mm depth under the rafters.

I would like to keep the ceiling height below the rafters as high as possible and the height of the tiles as low as possible so it looks like the Celotex suggestion looks the better option but are there other pros and cons to each build up? I’m also a little confused as it looks like I will have a 30mm gap above the celotex to the top of the rafter, as it’s meant to be unventilated do I need this gap and would I need to counter batten above the rafters? Is there an option to full fill to the depth of the rafter with no insulation below? Maybe I should mention at this point I would like to opt for exposed rafter tails if possible so I’m very unsure of what I need to provide in the way of ventilation and where it should be.

Now onto the flat roof section.
The architect has specified a flat roof (warm construction) to comprise 13mm thickness of white spar chippings on 3-layer built-up felt roofing (or proprietary high performance roofing system), on 126mm CelotexTD4126 insulated decking with integral vapour barrier. Felt dressed up beneath existing tile hanging at rear and perimeter of rectangular lantern light. Decking to be secured through s.w. firings, (1 in 40 fall ) into 47 x100mm C16 roof joists at 400mm crs. Roof void to be unventilated and to be provided with ceiling of 12.5mm. “Duplex” plasterboard or standard plasterboard with polythene vapour barrier. Vapour barrier to be sealed at joints and perimeter. Roof to fall to 4no code 4 lead-lined outlets discharging over lead apron to gutter below tiled roof.

As you can see from the diagram the flat roof section will also act as the gutter/drain for the lantern so would it be better to use epdm or felt to cover the roof, I would need access for maintenance and cleaning from time to time? I’m also wondering about the firings to give the slope to the sides along with the front and back at the same time, is there a better way to do this? And is the Celotex ok just sitting on the firings or would I need a further layer of ply/osb?

All ideas and suggestions welcome.
Many thanks.
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Unventilated roofs require insulation across the rafters and ON TOP of the rafter line. Below and between is not a warm deck roof and may require additional venting, i.e. breather membrane etc.

Just read a bit more of your loooong post......

Yes, you will need to fit an insulation underdraw to the rafter underside to prevent shadowing etc.

Pre-insulated 9mm ply roof deck can be fitted directly to the furrings depending upon your joist spacings and noggin arrangement. The safer bet is fix a 12mm ply first.

What's your gripe with furrings?
There is no point what so ever in discussing your roof until who ever did the drawing designs a sky hook to support the top 89x152 USB and the double bolted timber bressummers supporting the roof light.
When you have a properly designed practical drawing then suggest you come back
Regards oldun
Hi thanks for the replies I think.
Oldun, not sure what you mean about a skyhook but that upper beam is really the other side of a cranked steel beam which roughly on that drawing goes from where it says 50X100mm then down to a steel post in line with the rafters. To the left of the 50x100mm it goes all the way back to the wall and sits on a padstone. The bolted bressummers are again off of the upper ring beam. I'll just add a pic, might make more sense.
Hope that helps?

noseall maybe I should have just asked various questions rather than a long post but I thought it might get over more of what has been designed and what the possibilities are.

I'm in danger of another long post so here goes.

1, In contrast to what the architect has specified the celotex website says that for 150mm deep rafters "unventilated between and under insulation" I could use TB4000 in 120mm with PL4000 under it in 25mm to give 0.17 (W/m²K) this would leave 30mm of spare rafter space. To improve things further is there a way I can fill this space to avoid insulation below the rafters?
2, After a bit of a search it seems I don't need to cross batten if the breathable membrane has a 10mm or so droop between rafters is this why I need the 30mm gap between the insulation and the membrane?
3, If I do it this way will here be a problem as I would like exposed rafter tails, I see there are products from timloc called roof vent Mk3 cottage Style which look like they are for exposed rafter tails but do I need them if I use a breathable membrane? How do I finish at the edge of the roof?
4, Flat roof, would it be better to use felt or epdm as it looks like an awkward shape with lots of sharp bends etc?
5, Looks like I can just put the celotex on the firings but as the flat roof slopes to the front, back and both sides will this cause an issue with the insulation not butting up? Any better way of doing it?
6, I take it the flat roof section doesn’t need any ventilation?

Sorry for another long post, I’m sure what the architect has designed would work but I’m wondering if it can be improved upon.
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Your new drawing is the sky hooks we were referring to. If you had posted it in the first place, then we would have known what Architect had in mind.
General comments.
Straight forward job providing you have a decent steel fabricator.
You really need a timber wall plate on top of eaves USB or noggins between each rafter with twisted down holding strap every rafter.
There is a very good chance that cranked steel will be below bottom of rafters. Either increase rafter size or gun 25x50 batten to underside rafters.
Your half round ridge is useless a shown. Either use a mono ridge tile or change detail st back of half round.
Your span on front appears to be bigger than flanks Suggest you alter that to save yourself extra work.
Your questions.
1. TB4000 is only made up to 45mm thick and main purpose is to eliminate thermal bridging. Suggest you check calculator. To avoid insulation under your rafters you will need 165 of XR to give you 0.19W/m2K or 200 XR to give 16 W/m2K.
2. You are about right.
3. Never used them so do not know, If you want exposed tails with no facia you will want rafter brackets and rise and fall brackets.
4. Our opinion on yours three layer hot bonded felt.This an go straight on to 126mm TD4000. If using single layer EDPM then TD4000 needs an additional capping of 12mm Ply
5.Lay 11mm OSB or 12mm ply on top of firings, before celotex
6. Correct.Flat roof is warm deck not cold deck
Regards oldun
Thanks for your comments Olden, all taken on board. I didn't post that diagram first as I had a rough idea the structure was ok and my questions referred to the roof covering and insulation.
Celotex calculator checked and on the left hand heading under construction it says TB4000 but looking again at 150mm deep rafters it does indeed say XR4120.
Rafter brackets, is this because it looks like the birdsmouth cut will be too deep?
What do you mean by "change detail st back of half round"?
Thanks again.

PS would epdm or 3 layer with chippings hold up better to occasional foot traffic for cleaning etc?
Exposed rafter tails are the overhang from wall plate to facia plumb cut and do not collect a facia and soffit.
Rafter brackets are fixed to the side of the exposed tail and support your rise and fall gutter bracket. Gutters normally in cast iron, not cheap.
Half round ridge tile, first good gust of wind blowing across and down box gutter will lift them. Use mono tiles, drill two new holes further up bead mastic to back, lightly screw home with mastic in screw hole, mastic two original holes.
Felt or EPDM, sorry your choice. What ever you choose, you will have hell of a job to make those lead outlets water tight both in gutter and tiles. If pocket allows suggest lead apron up and over both sides of box gutter
Regards oldun
Thanks once again Oldun, any reason I couldn't use plastic gutters and mount the brackets on the front of the tails?

Mono tiles it is then.

Re the lead outlets, just been having a look online, any experience of flat roof drainage outlets? http://www.whitesales.co.uk/products/flat-roof-accessories/drainage-outlets/
I can see them working well to collect the water but not sure how I would manage to get them to pop out at gutter level and feed the gutter? They would make the roof line look much nicer than having 4 big bites out of the ridge.
You have hit the nail on your head, that is a fundamental problem with this type of roof, unless you come up with some kind of hotchpotch method of getting your rwp back through the external wall and into a drain pipe you will have to drain your roof through downpipes inside your building, which should always be a last resort. Any problems with it backing up and the joints will just blow and your house will flood. If you were to run internal rwps inside a commercial building BC would insist on steel rwps and proper pressure resistant joints, that can cope, no plastic stuff. You may hit the same here with your BCO. Even if you can get them through the wall they will still have to go internally before they hit the external wall. Bad idea. You will also need an overflow anyway for if the outlets ever block up.

Or do a proper job, if ever there was a call for a lead roof it is this, you can then fit some tidy lead outlets that your lead roof can be welded to and these can outlet through the tiles, even then its a bit odd but at least a good lead worker could make them look slick.

Your architect should not have designed you into this hole and his drawings are a bag of ol spanners.
Thanks for your input but to be fair the design was always going to be a compromise as the upper floor windows are quite low and I didn't want them too obscured with the new lantern on the roof. That's why the flat roof has been set below the level the ridge of the pitched roof section.

Not sure what you mean about the pipes needing to come through the external wall as I will only have glass walls. I thought I might be able to get the pipe to exit lower in the tiled section and into the gutter some how? Keeping a check on any blocking shouldn't be a problem as it will be overlooked by the upper windows.

I just need to know which of the 2 options is the lesser risk, as you might imagine a fully lead roof isn't an option unless my numbers come up.
A pipe traversing through the pitch and spilling onto the pitched roof is (IMO) just as bad as trying to get it through the wall, which you have said is not possible anyway as your walls are fully glazed.

Something like this maybe: http://drains.roofmaster.com/products/157508 still a have a daft pipe through the roof.

You'll have to compromise somewhere. what does your dopey architect think you should do? Its his design?
I'm sure you're trying to help despite the insults.
My architect has suggested and designed for lead lined outlets through the parapet, level with the box gutter as in his drawing in my first post. we have the same sort of arrangement on a larger area of flat roof which can just about seen on a photo in my gallery which have worked well since we moved to the house. Although in that case they can't be seen as they are on the side.
I was just wondering if It could be improved upon. I have seen those parapet drains you have linked to and they look ideal for the roof side but not sure how they would look or be sealed tile side? At least the ridge tiling would be constant using them.

How about this but with the pipe end going into the tiles rather than the normal out and with the parapet drain flat roof side.
http://www.roofingsuperstore.co.uk/...e-450mm-x-450mm-x-30-degree.html#!prettyPhoto silly idea?
Having a lead box tube coming through the pitch is no problem at all and quite often used for that detail.

Just make sure it's a fully welded box so if it does block, it won't run into the building.

A good lead man would make them up no problem with a nice soaker detail where they discharge onto tiles.

You will get staining where they discharge onto tile, but won't hurt owt.

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