New timber frame extension roof detail

[pudding]

So, concerning the following roof junction between the existing pitched slate roof and a new flat roof as sketched in the photo below:-

 

 

I've come up with the following detail. It was advised to me that we should aim to keep the rain water runoff from the slate roof off the new flat green roof, so i've come with this detail to keep the pitched roof water separate in the gulley shown.

 

 

Again, any questions or suggestions would be great

Edited October 2, 2019 by pudding

[Russell griffiths]

Why the green roof ?

cross sectional area of hidden gutter is far to small, make the bottom flat to increase volume, will block up in a heartbeat. 

[Russell griffiths]

TBH the more I look the more I hate it, will cause you years of misery, you need to create a large volume flat bottom box gutter. 

[Russell griffiths]

Who is doing your calculations on loading, some of those timbers look small for a green roof. 

And connecting to the rafter tails looks optimistic. 

I would install another wall plate on top of your stone wall, then send your new rafters through to sit on plate. 

Edited October 2, 2019 by Russell griffiths

[pudding]

1 hour ago, Russell griffiths said:

Why the green roof ?

cross sectional area of hidden gutter is far to small, make the bottom flat to increase volume, will block up in a heartbeat. 

Green is due to my other half being an ecologist and wanting wildflowers up there. Luckliy we currently get no debris in the gutters at all. Lived here for nearly 5 years and they almost spotless. Guess that'll change with a green wildflower roof next door though.

 

1 hour ago, Russell griffiths said:

TBH the more I look the more I hate it, will cause you years of misery, you need to create a large volume flat bottom box gutter. 

LOL, at least it's invoked some reaction! No problem with changing the design to a larger flat bottom box gutter. Would you get a gutter of the shelf for that, or make up with OSB and EPDM as I've currently drawn?

 

1 hour ago, Russell griffiths said:

Who is doing your calculations on loading, some of those timbers look small for a green roof. 

And connecting to the rafter tails looks optimistic. 

I would install another wall plate on top of your stone wall, then send your new rafters through to sit on plate. 

I'm meeting an engineer next Friday. He'll do all the calcs, The timbers I've currently drawn are just me guessing and the sketches show the main design principles I've got in my head.

 

Connecting to the rafter tails would be the optimistic and cheap solution, but like you I don't think they'll be chunky enough to cope, and we'll end up sticking another wall plate on top of the stone wall.

 

Thanks for the replies

[pudding]

How about this new design, with new wall plate, now 200mm joists and then only 100 PIR on top, and then a larger box gutter:-

 

 

I've found a nice u-value calculator online and its coming out with the following for that buildup:-

 

 

IS that shown dew point being in the middle of the timber/frametherm section a concern? It seems to do the calc based on a 20deg inside/-10 outside.

[PeterW]

That will need a VCL to make sure you don’t get problems. You could lose the first OSB in the middle, and just screw the top board through the PIR. 

[mvincentd]

I don't think thats where you want the dew point to be.  You've designed a hybrid roof which is generally undesirable due to it moving the dew point down.

600mm joist spacing.....12mm osb deck for a green roof?  You may not necessarily need the top osb though, depending on your edpm spec.

Your 1st gutter was scary, your 2nd is better but i'd strongly suggest if you want a worthwhile flat roof warranty get a supplier to collaborate with you on this design detail at the outset....they'll have critical details you'll have to adhere to, like having a robust stop-edge where the edpm rolls into the gutter...so replace that vertical osb with a 4x2.

As Russell said, years of misery if you get this roof junction wrong.

fyi, most recommendations are for 100mm minimum growing medium for wildflower....we're doing ok with 70mm which is a money and weight saver. Charles Fentiman here http://www.greenroofsubstrates.co.uk/ultralite_substrate.html is good.

[pudding]

For the VCL, the insulated plasterboard is foil backed. Enough? Or a proper VCL needed?

 

The structural numbers can all be taken with pinch of salt, i've just made them up. So the 600mm joist spacing may become 400, and the top 12mm OSB may very well become 18mm, once i speak to a structural engineers next week and he actually calculates the loading.

 

Any suggestions on a flat roof supplier? I hadn't thought of that at all and was simply thinking of forming the gutter shape ourselves and lining with the EPDM. The robust edge you mention, is that to ensure there is no movement whatsoever which may tear the EPDM?

 

Thanks for the link to the lite substrate, will come in handy when working out loading and maybe trying to minimise the extra weight.

 

 

[pudding]

The whole idea for the box type gutter in between the old pitch roof and flat roof was to avoid the green roof taking all the pitched roof water, as the builder friend suggested it would be a good idea to keep it separate. Seeing as it produces a few potential issues using the box gutter, I could slope the flat roof away from the house or towards the gable end in the other direction, so that the water run off from the pitched roof does go onto the flat roof and away. Like this:-

 

 

Then in order to avoid the green roof being waterlogged, use some of these :- https://www.sky-garden.co.uk/shop/drainage-products/rigid-drainage-per-roll.php

 

Seems like that could potentially avoid the need for the box gutter between the 2 roofs and make everything a lot simpler.

Edited October 3, 2019 by pudding

[mvincentd]

1 hour ago, pudding said:

Any suggestions on a flat roof supplier? I hadn't thought of that at all and was simply thinking of forming the gutter shape ourselves and lining with the EPDM. The robust edge you mention, is that to ensure there is no movement whatsoever which may tear the EPDM?

No idea in your area, and i'd hesitate to recommend the ones I used anyway.

I'm no expert but many/most membranes are fixed under a flat metal bar wherever they turn over an edge and that bar needs to screw into something robust, plus the edge needs to be stable so a boot while cleaning out the gutter cant crush or otherwise damage it...you might argue 12mm osb bonded to the pir and so lining it does the job...and your edpm manufacturer/fitter might be happy with that.....i was just using it as an example of details to not guess at, but consult early with the edpm, because If you're just buying firestone rubber and sticking it up there yourself thats one thing, but if you're after a long term insurance backed warranty it'll come at the cost of the preparations for the membrane being done to a spec'.

 

[mvincentd]

Controversial but what might be easier and equally ecological is to scrap the green roof, doubling the runoff you could harvest with which to water the rest of the garden!  It's a lot of bother for what if you don't have any particularly good vantage points from which to enjoy it.

I have similar but 12mm deep drainage mat.  Combined with the gentle fall you're not going to experience great waves of water rushing back to meet that coming off the pitched tile roof.