How to attach pergola post to existing decking?

[jasonlee]

Trying to add 4 supporting posts of a new pergola onto the existing decking, current decking is around 400mm off the ground. Are all posts need to be fixed onto a post anchor then concrete into the soil which I need to remove some decking boards, or I can just fix them straight onto the decking substrate which I will reinforce some of the floor joists. 

Any suggestions will be appreciated!

Jason

[MitchellMc]

Welcome to the Bunnings Workshop community @jasonlee. It's wonderful to have you join us, and many thanks for your question about connecting posts.

It would be best to remove the decking boards, concrete your new posts into the ground, add some joists around the post bases, and re-install the decking. This would allow the pergola to be self-supporting.

You could connect the posts to the existing posts and deck frame, but it would be best to run that by an engineer to check if it's a viable solution and how you should go about it.

Please let me know if you have any questions.

Mitchell
 

[jasonlee]

Hi Mitchell,

Thanks for your input. 

How deep you need the post to be in ground? Do you use H4 timber for that ?

[MitchellMc]

Yes, you'll need H4 timber, similar to 90 x 90mm 2.4m Post F7 H4 Treated Pine Premium KD - 2.4m @jasonlee. Or, you can use H3 timber and concrete in post supports. 

The depth at which you need your footer or post concreted in the ground is dependent on your soil type. It can be up to 1/3 of the overall post height.

Mitchell

[jasonlee]

Thanks Mitchell.

Regarding the opening width between each posts, I want to keep them 3 posts all together over the 7m length of span. that means I will have around 3500mm opening width between each posts. According to the span table, they biggest opening I can find it 3000mm, and it suggests to use 2/170x35 timber. 

Can I use one 190x45 timber for 3500mm opening width?

Thanks

Jason

[JacobZ]

Hi @jasonlee,

Unfortunately, I wouldn't advise you to go outside of the recommendations of the span table you've linked to.

I have found in the Woodhouse Span Tables that a 185 x 42mm F7 H3 Treated Pine Primed LOSP can span 3920mm with a roof load width of 600mm.

This is likely a good option for you.

Outside of this, large spans often require engineered timber products known as LVLs which can be ordered from a few different suppliers including Wesbeam, Dindas Australia and Tilling Timber through your local stores Trade Desk.

Let me know if you have any further questions.

Jacob

[jasonlee]

Thanks for your reply Jacob.

For the waling plate what is the minimum timber size I need to use? There are two sides of waling plate will be attached to the current rafters behind fascia board. The height of current fascia board is only 100mm, Can I use 120mmx45 timber across the 7m length for waling plate? 

Option 2 for me is to use MKII roof extenda bracket. However, I need to have a fully close ceiling so might need to install another beam just against current gutter? How's the flashing and waterproofing can be properly done that way so no water will be coming into the ceiling in the future?

Thanks

[MitchellMc]

Hi @jasonlee,

It's difficult to provide exact details on the required calibre of your wailing plate, and this is really in the territory of needing an engineer or builder to specify. I would suggest contacting one of them for more information and recommendations.

Extenda brackets come with their own weatherproof seal that prevents water from leaking through the opening that has been cut for them in the roof.

Please let me know if you have further questions.

Mitchell