Colorbond roof question

[Chumley]

Hi all,

Been looking for a while on the internet, but can't find any answers, so here goes... I've got a hexagonal gazebo roof to build - will use colorbond roofing but with timber ridges. I'll have six colorbond panels in isoceles triangular shape with a base of 1.6m and centre height of 1.7m (making the 2 long edges about 1.9m) -- the base will be supported by the wall plates, the long edges will be supported by cleats screwed to the ridges - in other words the outside 3 edges of each roof panel will be supported. Nobody will be 'walking' on this roof, but it may need to carry a person's weight if we need to get to the centre of the roof for some reason - probably crawl up the ridge, so no great issue.

I'm assuming I will need some supports for the roof panel, parallel to the wall plate, but how many will I need, and what type and dimension timber should I use?

Also, a simpler question, the colorbond guy says the sheets come in 760mm widths. Does anyone know how much of this gets lost with overlap when you add a second sheet?

Cheers,
Adam

[silentC]

What you are referring to as the ridges, I believe are the hip rafters? They run from the wall top plate and meet in a point at the centre of the roof. I think you will also need at least one rafter running from the same point in the centre down to the middle of the top plate. Then you should fix battens at right angles (parallel to the top plate) at 900 centres. You might get away without the extra rafter if you have extra heavy battens or closer spacings.

According to my tables, maximum rafter spacing is 1200 and that would require 50x75 battens (F5 pine) at 600, 900 or 1200 centres. You've got 1600 at the bottom, so I'd be looking at a centre rafter in addition to the hips.

[silentC]

Regarding cover, a corrugated sheet normally covers 760mm including lap, give or take a couple of mm, so check that with the supplier because he may have been talking about cover, rather than actual sheet width.

[Chumley]

Thanks silent, much appreciated.
I think I'll go with more battens as they will be easier to fix than another rafter - and with the triangle shape, the 1200 rafter spacing will be only 400 from the wall plate, so if I use 3 battens at 400 spacing it should be good.
Cheers,
Adam

[Barry_White]

Like Silent says the cover width is 760mm. The overall width of the sheet is about 830mm. If you watch how you cut the sheet you should be able to use your offcuts on the opposite side of the roof, you will just have to watch which way your overlaps go.

[Barry_White]

A cheaper & easier option may be to fit two short Jack Rafters on each side of the hip rafters so that you don't have too many rafters meeting at the centre at the top.

[Chumley]

Hi Barry,

Thanks for the explanation of the colorbond - now I can plan how much to buy.

Jack rafters won't work in this situation as my colorbond won't be laid on top of the rafters but rather will be sitting on framework (75 x 50s) attached to the bottom edge of the rafter - there will be a small cover above the edges of the colorbond so rain will be diverted away from the edges, and the rafters will appear above the colorbond. I'm trying to make a kind of Japanese-themed gazebo. See pic below (quick pic, sorry) shows a section through the roofing - the finished rafter will be curved and shaped kind of pagoda-like. I know I'm going to need a fairly wide rafter to cope with the support, but I'm still messing with the design.

Cheers,
Adam

[Chumley]

Oops, here's the pic.

[silentC]

I think you need to give a bit more thought to how you're going to flash that. I don't think those cover strips will be sufficient to stop water getting up there and you're going to have problems with rot down the track. I suppose if you turn the ends of the sheet up it will help. Usually a ridge cap covers the sheet by a couple of hundred mm. The optimum solution would be a wrap around capping that covers the top of the hip as well but I don't suppose you're going to like the look of that.

The other thing I'd say is that if you beef the depth of the hips up a bit, it will give you more room to get battens and other support underneath and still get the look you're after. Or are you planning on butting the battens into the side of the cleat in your diagram?

[Chumley]

Silent, I'm still thinking and redesigning as I go - probably a dangerous way to build something, but if you haven't done it before it's always a learning curve -- that's why this forum is so good!

re the wrap around ridge cap - yes, I don't really like that idea as it will spoil the look. I'm not really that fussed on water deflection, don't get much of that stuff here anymore anyway, and it's an outside structure with no wall coverings so the inside will get wet if it ever does rain. Also don't think rot will be a problem as I'm using H7 treated pine which is rated for ground contact and covering all surfaces with 2 coats of solarguard before assembly.

Good point anyway, I'll probably make the cover strips a little wider and try turning up the bottom of each corrugation -- roof pitch is only 20 degrees so I guess some rain will get blown upwards, but its only going to run back down or drip inside.

Re the battens - I'm thinking to reduce the overall size of the ridges I will join them to the cleats with half-lap joins - gives them something to rest on but keeps the profile lower. Given that the cleats will attach to the ridges and get plenty of vertical support from them, do you think something thinner would do - say 50 x 30? I guess I'd have to be fairly accurate with my colorbond cutting, but 30 should be enough for supporting the edges.

Are those "nibbler" cutters, the ones you put in a drill, any good for cutting nice straight lines in corrugated colorbond?

Cheers,
Adam

[silentC]

Regarding the timber sizes and so on, it's starting to get away from what you can say just by looking up tables and so on. If it was a house I'd say get an engineer, but for a small gazebo, well read between the lines.

I've got one of those Professional Nibblers they are advertising on the shopping channel at the moment. You can cut a straight line with it but it requires a lot of concentration! Because your cuts are angled through the corrugations, you might do it better with a good pair of snips.

The nibbler does give a good clean cut but is a little bit hard to control when you are going up and down between corrugations. I mean it is the perfect tool for the job really but requires a bit of practice to master in that situation. I guess if you mark your line clearly and take it slow. They're $269, so not cheap.

[Chumley]

Thanks - tin snips it is.
Cheers,
Adam

[Dirty Doogie]

You can cut corro roofing very nicely with a powersaw fitted with a metal cutting fibre disc.

Mini ripple iron may look better for this project as along each hip rafter you will get a lot of cut valley in the standard corro profile.

Just a thought

Doog

[silentC]

You just have to watch the sparks and swarf. Make sure you cut sheets away from the pile so they don't get covered in swarf and give them a good brush down afterwards. Any steel particles left on there will cause rust stains. They usually recommend a steel cutting blade rather than a disk because they produce less hot particles and don't leave as bad a burr.

[chrisp]

You just have to watch the sparks and swarf. Make sure you cut sheets away from the pile so they don't get covered in swarf and give them a good brush down afterwards. Any steel particles left on there will cause rust stains. They usually recommend a steel cutting blade rather than a disk because they produce less hot particles and don't leave as bad a burr.

I agree with silentC - and add that the off-cut(s) from the first sheet are good for protecting subsequent sheets when cutting - and as a bonus they can also be used as a template for marking out the cutting as well.