Pergola / Deck post fixing advice

14 Sep 2007
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United Kingdom

So, I've gone and drawn up a deck / pergola, designed using timber that's availble from Silva Timber Product's ( The design incorporates 5 3.66m 90x90 western red cedar posts which form pergola posts, railing posts and which would be concreted into 80cm holes in the ground. Top and bottom view images attached. All fine.



I go to order the stuff today, and they've not got the 3.66 posts in stock. 3m is the biggest - doh!

What with wastage, this would leave only 40cm per post available to concrete in. Is this enough?

If not, I guess my options are:

1. Extend shorter posts so that more can be buried into concrete by using a metal spike or similar (the spike would be attached to the post, then the whole lot concreted in - so the spike would already be under 30cm concrete).

2. Find another supplier - I've tried googling, but can't find one.

3. Redesign so that the nice cedar posts are attached only to the deck frame, rather than used for deck support as well. I'd use standard treated timber posts for this instead.

Any advice appreciated.

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cedar is a very soft wood, one i have found to be very poor for structural work (looks good as cladding). You might wanna use treated posts for support.

how are all those roof timbers going to squeeze into the corner of the roof frame? you may need to put a small timber piece at 45 deg across and near the corner, and attach them all to that instead, if that makes sense....
if the ground is firm and the whole structure is well tied together then you may get away with 400mm, but i wouldnt do it. I definetly wouldnt use metposts either. Try a local timber supplier as they can probably order you up some cedar posts. WHen i do a pergola its always attached through teh deck into teh ground, anything else is a disater waiting to happen.
@Wabbit - yes, good spot. The pic doesn't have it in, but I'm planning on doing exactly that for the roof beams.

@Thermo - The ground isn't especially stable, I'll try a few local-ish timber merchants

I've opted for cedar posts simply for the look, so the whole external is cedar. I guess I could go for different post material, perhaps pine, then try and stain it to match?

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The design incorporates 4 3.66m 90x90 western red cedar posts which form pergola posts

I count 5 posts... :)

could you not concrete in some concrete grandfathers ( is that what they're called, the short posts you bolt wooden post onto? ) and then stand the posts on top of the concrete ( that you concreted the grandfathers into ) and bolt through?
easier to replace if / when the posts rot through at the foot??
see edit above

seems the things I'm thinking about are called "concrete repair spur".. about 5 foot long..
no idea where I got "grandfather" from ..think I saw it in our local fence suppliers price list.

Yeah, I think I know what you mean. This sort of thing?


If I could find decent things I could concrete in, then attach the posts to, that could work. As long as they were strong as and corrosion resistant

However, looking through this forum, this approach seems to be not considered ideal, concreting posts directly into the ground seems to be the recommended way.
ah, godfathers was it then? I was close.. :oops:

while googling for them I came across a site that is selling UPVC fence posts and gravel boards.. surely they're not going to be that string are they?
Barnster, firstly that is a nice looking pergola. Good luck building it, and please come back with some photos of it going up and when finished. Cedar is one of the most beautiful woods when looked after, and it is a real pleasure to work with. Are you planning to keep it oiled up or will you let it fade to grey?

Secondly, have you actually spoken to silva to see if they can source the posts you want? I have used them extensively and very often the stock levels on their website might not be what they have available in their warehouse - if you haven't already then it would be worth calling them to check.

If you can't get the posts in the length you want you could always use quality pressure treated timber for the uprights (not the stuff you get from a DIY shed, but the stuff that is treated to the core) and then clad it above ground with WRC trim boards. You would have one on each face and either overlap each one onto the next or mitre the corners. To be honest I would probably take that option anyway because cedar in the ground will not last as long as the equivalent quality pressure treated timber. It may be resistant to rot but it needs the ability to dry out which it won't get underground. Alternatively you could set the base up on pressure treated timbers and then run the posts up the sides of them as suggested by others. Both would look good, and remove the potential weakness.
@ xdave - Glad you like it :) Designing it with Sketchup meant I could make all the mistakes 'virtually' rather than with expensive wood (like not accounting for joist spacing for 45 degree angled boards). It's all clear WRC and I'm planning on oiling it up, should look nice. Silva recommended a 3 step process, spread over 6 months, finishing with oil.

I did speak with Silva earlier today and they say they're out until maybe the end of the month. I like the idea of bolting the uprights above ground to concreted-in treated timber uprights or concrete repair spurs.

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