Need to save an underspecced roof

[OldKettle]

Hi,
I have constructed an attached pergola roof with the following spec

- C24 2x6 rafters with spans between 4m and 4.8m
- C24 2x6 wall plates
- C24 2x6 bressummer
- bressummer supported on 100x100 posts at 3m spans and fixed to wall at one end
- rafter slope is ~5 degrees.
- rafters sitting on joist hangers at bressummer and wall plate
- pairs of 2x6 noggins between rafters
- 2x2 purlins @ 400mm centres above rafters

Most of the structure has been stood for a year, just with a tarp over it (plus some loose poly underneath for rigidity).

I would now like to finish it with a polycarbonate roof on but consulting some flat roof span tables, I can see that C24 2x6 can only support a 3m span (assuming 0.75kn/m2).

I think that the bressummer is also underspecced.

I believe that it may be possible to save the situation, with the following changes.

1) Rearrange the bressummer posts to set max span at 2 metres
2) Replace bressummer with 75x225 C24 timber, still on 100x100 posts
3) Add a further line of posts at the midpoint of the rafters, with 75x224 C24 timbers, to reduce rafter span to max of 2.4 metres.

Below is a sketch I originally did, but the structure now goes further to the left and attaches to a wall that is 90 degrees to the rear of the house, plus the rafters are 4m to the centre of the house, but 4.8m to the left and right, where the rear wall is receded.

I have also attached a couple of recent pics of the structure.

I am really loathed to run the extra line of posts at the midpoint of the rafters, but if I need to, then I will have to.

Any thoughts would be much appreciated.

 

[Mr Chibs]

So is the worry it will sag when the sheets are put on?

I doubt the weight of a bit of corrugated plastic will bugger up what you‘ve built...

yes I know there is snow Load to contend with, just use an Acro/wood prop when needed.

I built a canopy around 6 x 3m out of 225x50 with Twin wall Covering, quite ott when I now look at it.

 

[noseall]

Racking and wind could be an issue depending how well the sheets are fixed etc. Some diagonals would help at the top of the posts. I wouldn't worry about sag on a pergola.

 

[OldKettle]

Thanks to both of you.

Yes my concern is sag under maximum (snow) load, using the 0.75kN/m2 span tables, and the same for the bressummer which is just a single 2x6.

I don't have any immediate concerns with the polycarbonate because, as Mr Chibs says, it will be hardly any extra weight. So I expect a carefree summer, but winter is the concern.

Don't want it coming down on my head when I'm eating my turkey.

Because it is fixed to the wall at one end, I had hoped that racking would not be a significant issue, but I can add some diagonals. I can also extend it further to the right, and attach it to the neighbour's garage, if he will let me.

Had considered changing the bressummer now, so that it is done, and then worry about the span later in the year.

I guess what I really need to understand is, beyond the span table figures, what the realities of the current structure (span and bressummer) would be under that worst case max load.

Is it just a question of sag, or would the structure be potentially compromised if I leave it as it is?

 

[Mr Chibs]

Think snow loading is different in different parts of the country, you may know this already.

Do you get snow every year where you are?

If we are talking corrugated polycarbonate, this isn't that strong... perhaps twin/multi wall may have been better.
I'll be honest, although my canopy is smaller... I wouldn't worry about wind or snow affecting it.

As noseall suggested so diagonals adding... then I think I'd leave it be.

Sag will be a given at that span.

Forgot to ask, how come you eat your turkey outside?

 

[catlad]

Don't loose any sleep over that construction! by any chance we get two foot of snow you could put a prop under it until its thawed.

 

[big-all]

my thoughts
if the posts are in concrete and all the noggins have 2 screws each end then the corrugated is well screwed on it will tend to be fine except in high winds when it may do the creak squeek and groan thing that wood does
now fully a guess but doubt it will be a great problem initially [first 5-10 years] but then as rot will then be a possible problem the increasing movement will need component replacement but purely a guess