The Brew Cabin

Posted: **Mon Nov 06, 2006 8:55 pm**

Hi there,

I am building a pergola using the designs on http://www.pavingexpert.com/featur07.htm

I noticed that the posts are approximately 2.8m long, but only about 0.5m is in the
ground. For fence posts I have always been told that a quarter of the post should be in
the ground, and this is less than a fifth.

Is this because there isn't as much lateral force on a pergola due to wind as with a
fence so it does not need as much to stop the post moving ? I guess with all the
braces the pergola could be pretty much free-standing, so the main thing is to keep it
in the same position in the ground.

Also, would it matter if the post holes were tapered slightly so they get narrower at the bottom?

Finally, I will probably use a post fix concrete like Hanson PostFix or Blue Circle Postcrete
because I have too much money and not enough experience of mixing mortars. Do you have
any views on whether either of these is a superior product?

Sorry for all the questions! Thanks in advance for any help you can offer.

Al Reynolds

Posted: **Mon Nov 06, 2006 9:36 pm**

When I do fencing I also put more in the ground than 50cm so I would be inclined to dig deeper if you want to. If you have all this money, like you say, you could always put a spike on the end of the posts or you can dig deeper and chuck a couple of bags of postcrete in each hole.

When I do fencing though, I do look at the quality of the soil, how good my hole is as well as the height of each post to decide when to stop digging.

Posted: **Tue Nov 07, 2006 7:09 pm**

Ted wrote:When I do fencing though, I do look at the quality of the soil, how good my hole is as well as the height of each post to decide when to stop digging.

Cheers for that - the ground around is well compacted so my inclination is to go to about
600mm and wedge a couple of bricks in the sides before putting the concrete in.

Any views on the holes being tapered? At the moment my holes are round, tapering
from 11" across at the top to 7" near the bottom (2 foot down).

Al

Posted: **Tue Nov 07, 2006 9:57 pm**

i would make the holes vertical mate,the bottom part of the hole is where the strength lies, if you imagine a lever with the top of the post being the bit you pull and ground level being the fulcrum,then the farther into the ground (and more resistance at that point) the greater the force needed to pull the post over
see here for more info
cheers LLL

Posted: **Wed Nov 08, 2006 8:19 am**

lutonlagerlout wrote:i would make the holes vertical mate,the bottom part of the hole is where the strength lies, if you imagine a lever with the top of the post being the bit you pull and ground level being the fulcrum,then the farther into the ground (and more resistance at that point) the greater the force needed to pull the post over
see here for more info
cheers LLL :)

I see what you mean LLL, although my understanding was that it is the ground which stops
the post from falling over, and the concrete is to make sure it's a tight fit so things don't wobble.
The majority of the force on the structure will be vertically downwards due to the weight of
the wood - it won't have anywhere near the effect from wind that a panel fence has - more like
a post and rail fence.

I will probably widen out the bottoms anyway, just to be sure!

Thanks,
Al

The Brew Cabin

How much of posts in ground for pergola?