Work out 5 degree slope

cheif kaiser

OK my brain has gone to mush on this one so need help please.

I have a flat roof pergola and wanted to fit a twinwall polycarbonate roof. The recommended slope is 5 degree so if I am to put a firring strip on each rafter and each rafter is 300 cm long. What is the height that the firring strip should be at the back if the front starts at zero?

Any help please?

try this

https://www.omnicalculator.com/math/right-triangle-side-angle

cheif kaiser

BrownFinger wrote: »

try this

https://www.omnicalculator.com/math/right-triangle-side-angle

Tried those calculators but I am clearly entering wrong figures for something because this seems to be telling me that the back should be 26cm high and that can't be correct!?!

I've seen firrings installed on loads of youtube videos and they are definitely not 26cm high.

MayoForSam

10:1 slope should be close enough e.g. if your rafters are 3m long then your drop should be 0.3m (30cm).

Bonzo Delaney

262.47mm

Lumen

If you need more slope, just stack the firring strips.

e.g. https://www.builderdepot.co.uk/47mm-x-100mm-to-0mm-treated-timber-firring-3600mm-4in-x-2in

100/3600=2.77%, x2 = 5.55%, close enough and more is better than less.

edit: undo, undo! 5.55% is not 5.55 degrees! Triple firring strips would be close.

cheif kaiser

Bonzo Delaney wrote: »

262.47mm

Thanks Bonzo! So 26cm is correct? That's 10 inches at one end and 0 inches at the other?

That seems extremely excessive for a pergola. I've seen loads of pergolas with polycarbonate roofs and the slope is barely even noticeable.

There is no way to take a flat roof and slope it up to 10 inches at the top.

I'm completely confused now

cruizer101

Where are you getting 5 degrees from.

Quick google seems to recommend 1:80 (0.72 degrees) as the recommended min.
And you should aim for 1:40 (1.44 degrees) in order to achieve that to account for material fluctuations.

That would be 7.5 cm on 300cm.

Lumen

cruizer101 wrote: »

Where are you getting 5 degrees from.

Quick google seems to recommend 1:80 (0.72 degrees) as the recommended min.
And you should aim for 1:40 (1.44 degrees) in order to achieve that to account for material fluctuations.

That would be 7.5 cm on 300cm.

I get 5 degrees from a quick google of "twinwall polycarbonate roof slope", e.g.

http://polycarbonate.com.au/polycarbonate-guides-faqs/
https://stormbuildingproducts.com/installing-polycarbonate-sheeting/
https://findanyanswer.com/what-is-the-minimum-pitch-for-a-polycarbonate-roof

cheif kaiser

cruizer101 wrote: »

Where are you getting 5 degrees from.

Quick google seems to recommend 1:80 (0.72 degrees) as the recommended min.
And you should aim for 1:40 (1.44 degrees) in order to achieve that to account for material fluctuations.

That would be 7.5 cm on 300cm.

That would seem much more reasonable to me and the fact that firrings, which are designed for this purpose, are
approx those measurements, however, as Lumen says below, 5 degrees is what seems to be the recommended.

Don’t know whether to just risk going with the shallower slope or not.

Lumen

cheif kaiser wrote: »

Don’t know whether to just risk going with the shallower slope or not.

That seems, eh, inadvisable.

https://www.sundancesupply.com/polycarbonate/proper-slope/

cruizer101

Lumen wrote: »

I get 5 degrees from a quick google of "twinwall polycarbonate roof slope", e.g.

My bad I just googled flat roof not polycarbonate.

Wonder why you need more for the polycarbonate roof

Cerco

See previous post by Lumen. Shows very clearly the build up of leaves and results in algae staining which looks terrible. You can pressure wash off but eventually the panels become marked and lose transparency.

listermint

cruizer101 wrote: »

My bad I just googled flat roof not polycarbonate.

Wonder why you need more for the polycarbonate roof

I don't think these are ever completely flat the manufacturing process seems to leave divers and undulations in the surface. Alot easier for stuff to catch and slow.

Calahonda52

listermint wrote: »

I don't think these are ever completely flat the manufacturing process seems to leave divers and undulations in the surface. Alot easier for stuff to catch and slow.

The other issue in the tape and bar that goes at the end over the gutter which catches stuff

10-10-20

cheif kaiser wrote: »

I have a flat roof pergola and wanted to fit a twinwall polycarbonate roof. The recommended slope is 5 degree so if I am to put a firring strip on each rafter and each rafter is 300 cm long. What is the height that the firring strip should be at the back if the front starts at zero?

I swear that came up in LC maths in 1996.
I got the extra points for having my name on the top of the page.

h=26.25cm

cheif kaiser

Lumen wrote: »

That seems, eh, inadvisable.

https://www.sundancesupply.com/polycarbonate/proper-slope/

Judging by that mess, it would indeed be unadvisable

Think I’ll just scrap the whole idea

JustAThought

cheif kaiser wrote: »

Judging by that mess, it would indeed be unadvisable

Think I’ll just scrap the whole idea

I’d say they’ve invented powerwashers since this was built! Don’t give up!!

cheif kaiser

JustAThought wrote: »

I’d say they’ve invented powerwashers since this was built! Don’t give up!!

It’s only a small Pergola that houses my hot tub. Just wanted to use it even if it’s raining.

Might ring the plastics company tomorrow to see if something else would be more suitable.

I’m not really one for giving up

Lumen

Double pitch?

dodzy

Anyway you slice it, and irrespective of the quality of work that goes in before hand, any structure topped off with corrugated plastic looks poxy. It’s the devils material. And weathers poorly.

cheif kaiser

dodzy wrote: »

Anyway you slice it, and irrespective of the quality of work that goes in before hand, any structure topped off with corrugated plastic looks poxy. It’s the devils material. And weathers poorly.

It’s not corrugated and I’ve seen it done an imho, it’s looks quite nice.

cheif kaiser

Lumen wrote: »

Double pitch?

I’m not sure that would look great to be honest.

I have other ideas so I’ll do some more research and come back to it. Thanks for advice though!