How to build a freestanding pergola?

[funky-millar]

My cousin is wanting to erect a 4.1 X 5.0m free standing pergola.  It will be single span with only four posts in each corner.

What kind of 100x100 posts would you advise to be exposed to the weather elements?

How do I read the span tables for this to determine the size of beams.  Im assuming 240x45 or there abouts would be rough size.

Can i simply notch out the tops of the 100x100 for the bearers and use bolts M10s for these and also use stirrups on the bottoms concreted into ground?

If the length of the pergola is 5m what space could i get away with for additional joists etc and can these simply be joist holders installed.

If there is overhang needed for a visual impact are you better to install the joists above the bearer and notch out or have separate pieces each side with joist holders?

And if installing a sheeted roof does it need to have 15 degrees minimum pitch for water runoff and there fore gutters installed?

[MitchellMc]

Hi @funky-millar,

Not having a continuous span will raise some issues with the amount of overhang you can achieve. At one metre, you'll run into issues with the timber bowing and warping, and I don't think that's going to work out well. Even if the rafters were a continuous length, I'd only advise their extension of 500mm. I'd use long landscaping screws to attach them to the beam as per the rendering I've constructed for you below instead of using joist hangers.

Please let me know if you have any questions.

Mitchell

[funky-millar]

What software are you using to draw above?

[MitchellMc]

It's called Tinkercad @funky-millar. There's a slight learning curve, and I've been using it for some time. It's actually a software designed for primary school students to create 3D renderings that can be printed on a 3D printer. My colleagues use SketchUp for their designs.

Mitchell

[funky-millar]

Would interlocking work also? 

So rather than the landscaping screws which is a lot of trust going into screws.

Could you cut notch out of the front cross beam and then notch out the bearer also the same distance and then lock them together essentially?

[funky-millar]

I think there is a better solution.

Notching both the back and front the same depth so it is a continuous span

[MitchellMc]

You wouldn't be able to notch out the beam if it's close to the minimum requirements, as that would reduce its span and structural integrity @funky-millar. You would need to go for a larger calibre beam so your notching doesn't reduce it below allowable limits.

A similar issue applies to the rafters. You won't be able to use a joist hanger when notching, so while you could take a small notch out of them, you couldn't take half the thickness out of them, as it would reduce their structural integrity.

To achieve a flat roof, you'd need to take out a 50% notch from the beam and the same from the rafter. I can't see that complying with building codes, but it might be best to run that question past a structural engineer.

As with the image you've just posted, a small notch would work if it doesn't compromise the rafters or beam. But that doesn't sound like what your sister was looking for.

Mitchell