Setting Out Flat Roof Firrings

[Adrawk]

I'm building a lower ground extension, the roof of which will be a terrace with access from the ground floor. We've set out all the joists as per the engineer's design but as there will be a parapet around all sides, apart from the stairs down into the garden, the roof falls need to hit two parapet outlets. How to calculate/plan for the firrings to achieve this is puzzling me.

The joists run from the back to front of the extension and are a 2.7m span, the roof is about 9m across the back of the house. I've allowed for a 1:40 fall in the roof build up, so the firrings start at approximately 70mm. Can anyone point me in the right direction as to how to best set out the falls to ensure it hits the outlets?

 

[noseall]

I'm building a lower ground extension, the roof of which will be a terrace with access from the ground floor. We've set out all the joists as per the engineer's design but as there will be a parapet around all sides, apart from the stairs down into the garden, the roof falls need to hit two parapet outlets. How to calculate/plan for the firrings to achieve this is puzzling me.

The joists run from the back to front of the extension and are a 2.7m span, the roof is about 9m across the back of the house. I've allowed for a 1:40 fall in the roof build up, so the firrings start at approximately 70mm. Can anyone point me in the right direction as to how to best set out the falls to ensure it hits the outlets?

We use furrings (diminishing strips) in one direction, i.e. along the joist and then use a build-up method if the roof needs to fall in both directions - i.e around the back of a lantern for example. The build-up method basically means, prior to fitting the furrings, we fit varying thickness strips of material such as 4mm ply, or differing thickness batten, or 25mm planed etc - anything that will be suitable for the line we have set up from the highest to the lowest point.

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So for example - in the bottom image, the roof slopes from the window end to the front and also falls either side peaking in the centre of the lantern and falling towards the sides. We did the build up method first i.e. fitted a 50mm thick timber on top of the back joist, attached a line down onto the front joist, then attached various thicknesses of material, according to the line. We then fitted the furrings on top of these to provide falls in the other direction.

 

[Adrawk]

@noseall thank you very much for your insight, I lurk here a lot and always read your comments. Following a few conversations with work colleagues and reading your post I resolved to do the below, essentially splitting the roof between the parapet outlets. The dimensions noted on the mark up of the drawing below are additional to the firring on each joist, those are cut to achieve 1:40 for every joist from the house to the parapet wall. The idea is that the 4mm increments on the additional packers will achieve the fall needed, I need to add more dimensions to the remaining joists but hopefully the idea is sound.

 

[noseall]

@noseall thank you very much for your insight, I lurk here a lot and always read your comments. Following a few conversations with work colleagues and reading your post I resolved to do the below, essentially splitting the roof between the parapet outlets. The dimensions noted on the mark up of the drawing below are additional to the firring on each joist, those are cut to achieve 1:40 for every joist from the house to the parapet wall. The idea is that the 4mm increments on the additional packers will achieve the fall needed, I need to add more dimensions to the remaining joists but hopefully the idea is sound.

View attachment 348531

Yes, you get the gist. The lowest joist requires no additional packing, just a furring. This is your start point with the line. I mean, it'd be great if you could buy joists that went up in increments of say 3mm at a time, then we'd only need to buy furrings, lol.

 

[Adrawk]

Tell me about it! I spent a couple of hours last night with my track saw cutting 2.7m long lengths to those dimensions. The 4mm was tricky but it worked out well.

I'll do some more tomorrow.

 

[noseall]

Tell me about it! I spent a couple of hours last night with my track saw cutting 2.7m long lengths to those dimensions. The 4mm was tricky but it worked out well.

I'll do some more tomorrow.

We just buy a selection of stuff of varying thickness. The thinnest being some damaged plastic eaves trays (2mm ish). We buy the thinnest ply, some 50mm x 19mm PAR timber, varying thicknesses of batten etc. We then use a combination of the materials to hit the line a the desired height.