are cement posts bad for house foundation?

Submitted by cadisk t3_122tl0r in DIY

Thinking of putting in a pergola or gazebo in the backyard. Currently it's all grass but down the road might put in a paver patio.

Since it's grass, the pergola or gazebo would need cement posts for the installation. Is there any risk to this for house foundation? I read that a cement patio could sink and might direct water towards the foundation. is there same risk with posts? the house is 10 years old.

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so-very-very-tired t1_jdrovi6 wrote

The pergola post footings aren’t connected to the house so…no, no issue with the house foundation.

You definitely don’t want a patio sloping towards the house but that’s easy to deal with at install time by properly preparing the base layer (and grading it away from the house accordingly)

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cadisk OP t1_jdrspxg wrote

thanks! I've now read about wooden pergola posts possibly rotting. I'm in Alberta, Canada and we get a lot of snow. what would be the best way to prevent this?

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allangee t1_jdruadp wrote

In Manitoba here. You won't have any rotting in winter as everything will be frozen.

I'm assuming you'll have at least three posts for the structure so there's no worry about it tipping side to side. That means you can put in concrete piles with brackets for the posts on top. That way your posts aren't in the soil or in the concrete. The metal brackets are set into the concrete before it cures, and then the posts are bolted to the brackets. Use pressure treated posts and make sure you use end cut treatment on the ends. If you want to be extra cautious, there are inverted cone-like things that will direct rainwater away from post and footing -- but I don't think that's really necessary.

Make sure you grade things so the centre of structure's "floor" is slightly higher than the edges to prevent water pooling.

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cadisk OP t1_jds6bof wrote

thanks!!

>You won't have any rotting in winter as everything will be frozen.

haha I was mostly worried about in the spring when the snow starts to melt.

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biggerken t1_jdsb7f6 wrote

I think I screwed up. Been stressed about it all winter. In SK. Last year summer I set 6 6x6 12 ft posts PT in concrete for a future gazebo. 4 feet deep in sonotubes filled with concrete. Then I finished the pavers around them in the fall.

If I could do it all over again I would do as you said. Concrete piles and brackets. Have been debating all winter about starting over, but man that would be so much work.

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allangee t1_jdteqh7 wrote

The "best" approach is only an opinion -- mine in this case -- and not-best doesn't mean disaster. Stop stressing.

If the top of the concrete is above ground level, you're already off to a great start. If you want, you can get those inverted cone things, or install flashing around the bottom of the pole, about an inch off the concrete, so water is directed away from where the wood goes into the concrete. The gap allows air to circulate in that area.

Fence posts can last a long time, buried, in direct contact with soil -- so chances are your gazebo posts will outlive you. If you live long enough for the posts to rot, take care of it then and rejoice in the long life you lived!

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biggerken t1_jdtjh2l wrote

Thanks, I needed that. Top of concrete is above ground level, and I was careful to slope pavers so water runs away. You are probably right. I didn’t do enough planning/research and rushed it a bit given our season for outdoor projects is so short and had a few projects on the go. Then started second guessing when winter came and I got time to plan the roof and rest of finishing.

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allangee t1_jdtjuar wrote

Since you're in SK and I'm in MB, I'll stand on my roof this summer and take a look at your gazebo.

Have a good one!

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so-very-very-tired t1_jdrwj56 wrote

As allangee points out, the best way to deal with it is by not putting the posts in the ground/concrete. Instead have the posts set on top of the concrete piers.

This does require additional lateral bracing in the rest of the structure.

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icosahedronics t1_jds4ygp wrote

wood rots when in contact with earth. dry rot will happen when in contact with concrete. so the way to prevent rot is to use a concrete pier until floor is above the earth, and then use a resilient sill plate such as preservative treated lumber for pieces in contact with concrete. alternative is to use a raised post base bracket such as made by Simpson.

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