Timber size needed for carport rafters?

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geekIT

2419 posts

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#304073 2-Apr-2023 17:01

Any builders out there able to tell this long-retired one what size rafters at what size centers are needed for a standard iron roof spanning 5m?

Framework will be connected to the house. Diagonally braced. Pitch will be about 20 degrees. I thought maybe 200x50 at 1m centers? Or is that over-kill?

The pic is similar to what I had in mind. Probably won't be as flash, though 😏

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nickb800

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#3057882 2-Apr-2023 17:13

Have a look at this guide to specifying verandas - basically the same structure wise as a carport. It references tables in 3604, which is free online, for the specifics (table 10.1 and 10.8)
https://www.buildmagazine.org.nz/index.php/articles/show/veranda-design-and-construction
https://www.standards.govt.nz/shop/nzs-36042011/

5m is quite a span for a simple rafter and will need a bigger member (hehe) and closer spacing than you think. If you think of a versatile style double garage, they achieve a similar span with engineer designed and factory fabricated trusses - not just a 90x45

Ruphus

465 posts

Ultimate Geek

#3057935 2-Apr-2023 21:02

Based off the 3604 standards, 240x70 at 600mm spacing will span 5.1m.

But that's a bit of an odd size so 240x90 (which can be made up of 2x240x45) can span 5.1 at 1200mm centers.

Where are you located? Do you know your wind zone?

geekIT

2419 posts

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#3058056 3-Apr-2023 09:47

Thanks, guys. I'm in Otautau, about 50k NW of Invercargill. We get some fairly stiff breezes at times, mostly from the north or south. This is pretty much the same orientation as the house, with the carport rafters pointed east. IMHO, there'd be virtually no uplift factor from the wind as it'd be passing through the flat plane of the roof.

Still, I agree the span is a problem. My intention was to utilize the full width of the drive space.

Maybe I should just forget the aesthetics and erect a real basic, flat roofed, min1mum width, free-standing structure on 4 or 6 posts 🙄

What's the max span\centers for 200x50?

'Those who can make you believe absurdities can make you commit atrocities.' Voltaire

'A patriot must always be ready to defend his country against his government.' Edward Abbey

nickb800

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#3058170 3-Apr-2023 12:36

So 200x50 would do 3.5m at 480mm centres. Doubled up gets you to 4.7m. That's quite a lot of timber and $$! If you can get a line of posts in the middle of your 5m span, then you can make it two spans of 2.5m, which can be done with 200x50s on 1200mm centres. Depends what you're doing with the space - 5m is tight for two cars side by side before there's a row of poles in the middle

It looks like Otautau is high wind zone but that doesn't affect the calcs unless you're in a very high wind zone. I wouldn't second guess the wind - if it lifts the roof it could take the side of the house with it, or if freestanding crash into the house.

aucklander

477 posts

Ultimate Geek

#3063240 13-Apr-2023 13:58

I finished a carport attached to a shed in December, DIY built in Auckland, rafter span 3.7m, rafters 200x45 @ 900 centres, rafters resting on 2x 250x50 bearer. This is less than what the Code would expect (maybe), but Gabrielle came and went with absolutely no problem... rafters were fixed to bearer with Z nails and CPC80 each side, the bearer sitting on top of the 125x125 posts and fixed with T brackets and M12 bolts, posts concreted 1.8m into ground 250mm hole due to proximity to a retaining wall 1.5m high approx 1m away, otherwise I would have stopped at 900 deep (and save few $$ as I could have dig / drill that by hand, no need for a digger...).

Your 5m span sounds the tricky part... but in saying that, the house (built 1982) has exposed rafters at 45 deg, they are 190x45 @ 850 centres, rafter length approx 5.5m from ridge to wall (approx 4.1m measured horizontally from under the roof ridge to the wall supporting the other end of the rafters). My understanding is "they" had access to better pine "back then", more naturally grown, which was much stronger than the newer genetically selected and modified pine, which apparently is now ready to harvest in only 12 years. But this is beside the point of your question, I guess... What I am saying is that you could look at a demolition yard for timber 200x50 in good condition, but make sure it is from an old house (at least 40 years old, or more), chances are that timber is stronger than its modern equivalent.

Qualified builders reading this - is this a valid point about the timber becoming "softer" in the past 20-30 years than what it used to be say 40-60 years ago? Timber sizes had to remain the same to ensure compatibility in the future (additions / renovations / repairs) but what they did was to degrade them (make smaller spans, reduce max load, stuff like this). Any truth in this, or was I mislead by whoever told me this story a long time ago?

TaksuDesign

1 post

Wannabe Geek

#3063260 13-Apr-2023 15:23

I have a carport 5.2metres span.

It has box / I (eye) beams made from 100 x 50's (M's below) with boards 100x20 by 250 long (n's below) sandwiched between them.

The 100x 50's are on edge with a 50mm gap then another 100 x 50 .

Looking at the end

N n N

The overall size is 250 x 150 at 1 metre spacings.

I intend to put a layer of ply each side for added - unnecessary - strength so we can walk around on top of the flat brownbuilt roof.

The roof was built by the owner / architect who had a mate who was an engineer. This was his personal house and the car port roof is fine after 40 years.

Jase2985

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#3063263 13-Apr-2023 15:28

use LVL, will give you more stregnth and greater spans for the same dimensions

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ZMF

12 posts

Geek

#3075284 12-May-2023 07:03

The simple answer is, it is not that simple. 😅

There are a couple of variables that will impact the design. I highly recommend you speak to someone with an LBP, builder or designer and because there are many variables and it is impossible to know what exactly what will be needed, below is just for information and needs to be confirmed with a designer.

Things that could potentially impact the design:

Windzone
Loadbearing impact on the existing structure(s) it is fixed to.
Uplift / Fixings / Footings
If it projects more than 2m from the building it is recommended that bracing is used for the roof (BRANZ recommendation).
Fixings for iron roof
Snow loads

Using NZS 3604 as a guideline, Lightweight (iron) roof only, 1.5kPa max. snow load;

High, Very High, Extra High Windzone,
240x90 @ 1200 max. centers, 5.1 max. span. Fix each rafter to support with 2 / 90x3.15 skew nails + strap fixing or alternative 7.0kN Fixing. **See BUT

Low and Medium Windzone,
190x90 @ 1200 max. centers, 5.2 max. span. Fix each rafter to support with 2 / 90x3.15 skew nails + strap fixing or alternative 7.0 kN Fixing. **See BUT

**BUT

You will also need to consider the Purlin Sizes, fixings and max. span depending on the Windzone and Rafter Spacing. Having the rafters at no more than 900 crs. will be the best for an Extra High Windzone to support 70x45 Purlins on their flat.

The beam size and fixing type will depend on the spacing of the posts which will be used to support the beam and the max. span of the beam. The challenge you have here is the 5.0 span of the rafters. A beam to support this will most likely fall outside of NZS 3604. A Prolam / LVL beam will be a better option here.

The posts you will be using will depend on the beam you will be using. As the beam will most likely fall outside of NZS 3604, you will probably want to check with Prolam / CHH what will be needed. Generally, a 90x90 should be fine, but it depends.

Fixing to existing structure????? Stringer? Stringer to House Fixings. Rafter to Stringers (Check the kN Fixing required). Joist Hangers should be fine.... maybe.

Also need to consider the Footing sizes for each post, which will depend on the Uplift requirements and the Windzone..... and Snow Load. And then the Post to Footing fixings.

/sigh, but back in the day..............

Bung

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#3075380 12-May-2023 10:40

ZMF:
/sigh, but back in the day..............

Part of our house added in 1986 was a bedroom over a garage. The bedroom floor is on 290x45x4800mm joists at 600mm centres across the width of the garage. The bedroom ceiling has exposed rough sawn rafters 300x50x4800mm at 1000mm centres. The plans were apparently the 1st paying job on his own account by an architect. Unfortunately the copy left in the house is old tech that has gone too pink for any notes to be read. Even though I can't match any of that to NZS3604 nothing in the years since it was built suggests that it was under specified.

Edit add old pic, left rafters of flat root, right floor joists.

ZMF

12 posts

Geek

#3075653 12-May-2023 21:06

Looks great. 290x45 @ 600 is not far off from the standards, as I think the standards take an over cautious approach. So would be interesting to see what it looked like that time around.