Flat roof workshop design

[Tarkin]

Hi all,

After a very long hiatus from woodworking and this forum (I've been devoid of tools and living in Germany for the last few years), I'm finally getting back into it and have got the go-ahead from the other half to put a small workshop up in our new back garden. The only constraint (other than size) is that it needs to look acceptable - which in this case means a clean modern look. Here's an example of the kind of look I'm after: https://s-media-cache-ak0.pinimg.com/or ... 49c583.jpg

The idea is to have a (probably) larch-clad shed, with a flat hidden roof and no visible sofit, fascia or eves. I want the cladding to go right up to the top so that no roof structure is visible. I'd gutter it at the back side where it can't be seen - unless you happen to be my back neighbour! My issue is how to do the ventilation of the cavity between cladding and timber frame properly. Endless googling hasn't revealed a clear answer so I came up with my own. The idea is to use a metal right-angled flashing, maybe 4"x4", which covers over the top of the cladding. I could leave a gap between the flashing and the cladding so that air could get round behind it.

I've attached a cross-section sketch of the design I'm envisaging. I'd secure the flashing on the face at each of the vertical batons with a suitable spacer to stop it flapping. I've also not shown the furring in the diagram, but I would angle the deck towards the gutter and lip the other three edges to promote water runoff. Any opinions on this design? Am I reinventing the wheel needlessly?

I've also made this a 'warm deck' design because firstly it seems to be what's recommended for flat roofs in Scotland these days and secondly it made my ventilation issues easier. However I'm struggling for height inside the workshop (there is a strict limit to the external height), so if there were a way to do a cold-deck safely and get that insulation between the joists it would give me an extra 100mm head room which would be useful. But I've not got a nice design solution for that, yet... (I know I could get more headroom by using a concrete slab rather than a suspended floor, but for various reasons I'm not keen on the slab route.)

As always, any advice and ideas much appreciated!

 

[Fitzroy]

What's to stop you having a cold deck roof, with the wall ventilation from behind the cladding ventilating the space between the insulation and roof?

The photo in the link seems to have a much more involved roof structure, it must be sunken behind the cladding, hence the box-gutter flowing through to the down-pipe.

In my build I've learnt hard that the devil is in the detail, that's a pretty complicated structure, with lots of bespoke details you are planning to build.

F.

 

[RobinBHM]

The image you linked is effectively a parapet wall design with a hidden flat roof and a shoot discharging into a hopper.

I think the 3 non gutter sides would benefit from having a lip of ideally 50mm but at least 20mm to prevent water from blowing off all around the perimeter of the roof, which I think you would find irritating as it could result in water dripping over the door and will make the cladding often wet.

A warm roof can be a mixture of insulation over rafters and in between if height is really critical, say 50mm over, 50mm in between. The kingspan site does u value calculations for flat roofs, so you can compare and see how much impact the thermal bridging of the joists will have on u value.

A fall of around 1 in 40 is generally accepted as good practice with a felt flat roof. Allowing for overlaps and slight dips this should give you a practical fall of 1 in 80. If you are using EPDM, you may be able to do closer to the lower figure, but the flatter you get the more risk of ponding.

Im not sure Ive seen the cladding cavity open at the top generally, so dont know if thats necessary (tyvek website may be useful for such detailing).

Personally Im not such a fan of roof styles that have minimum overhang, especially with timber cladding as the overlap provides a huge amount of protection to the cladding, but I realise you are keen on the visual style in this case. Of course you are in a location of high exposure unlike us Southern softies!

 

[siggy_7]

If you are struggling for headroom then why have the joists below the insulation? Surely raise them up to directly support the deck and insulate between? 50x100 joists give a max span of 1.71m at 400 centres, you may want to up the cross section unless you have a really narrow building. https://www.broxbourne.gov.uk/resident- ... span-guide

Sent from my SM-N910F using Tapatalk

 

[Tarkin]

Cheers for the answers, guys.

I know the 'reference' picture was much more complicated than I'm proposing; it was more about visual appearance of the outside than anything.

I am considering a cold deck, and leave a 1" gap at the top to ventilate it purely for headroom reasons.

I think though, overall my approach with metal flashing will work so that's what I'm going to proceed with. I do intend to lip the three non-gutterred sides, but maybe I'll increase the height of it a tad as you suggest, Robin.

Sadly the width of the shed is only going to be 7ft-ish (gardens in central Edinburgh are not generous - at least not on my budget!), so I pretty much can get away with 4x2 joists.

Now I just need to locate a supplier of folded sheet metal flashings which pass the aesthetics criteria...