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Concrete Base Depth


pickup22
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I'm not an inspector but after finding this forum, I've poured over the threads. The hands-on experience has given a lot to think about as a dedicated DIYer. I work hard to only follow best practices.

Local rules require 4' of depth for the concrete footing for decks but on uneven ground what do think is required? I have terrain where ground level differs by as much as 2'. Drilling down 4' would require me to go 2' below the foundation of the house in some spots. Most areas are more even. I would prefer to go 3' in those shallow areas and leave 1' of above ground to keep the wood well above any spring flooding.

This would still be below the foundation which is below 4' for most but not all of the house. For other spots where the ground level is more even, 4' would be used.

Would this pass muster to you professionals?

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You say you only follow best practices. If so then don't go against the code. Go at least 4' down in all locations.

Maybe you didnt build the house foundation. You are building the deck footers. You know the code. Why not just follow it?

This is a bought house. It just seems odd to me that some footers will be 2' lower than others even though they are only 5' apart. In effect some posts will be 6' deep and others will be 4' deep.

The code require 4' below ground level. I'm wondering if 4' is average ground level or area-specific ground level. I'm not looking to cheat. The cardboard sleeves you normally find are 4' and I would prefer to keep 1' above ground (ie still lower than than the rest of the surface) to keep the wood dry.

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You say you only follow best practices. If so then don't go against the code. Go at least 4' down in all locations.

Maybe you didnt build the house foundation. You are building the deck footers. You know the code. Why not just follow it?

This is a bought house. It just seems odd to me that some footers will be 2' lower than others even though they are only 5' apart. In effect some posts will be 6' deep and others will be 4' deep.

The code require 4' below ground level. I'm wondering if 4' is average ground level or area-specific ground level. I'm not looking to cheat. The cardboard sleeves you normally find are 4' and I would prefer to keep 1' above ground (ie still lower than than the rest of the surface) to keep the wood dry.

Just to clarify, in this case I feel that following the code literally (depending on the reading of ground level) will lead to problems. Because of the uneven surface and water collection in the spring it will have a few posts sitting in water. Drilling 3' in some spots will mean that all posts are at the same depth and keep the lower-situated ones above water in the spring.

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. . . Just to clarify, in this case I feel that following the code literally (depending on the reading of ground level) will lead to problems. Because of the uneven surface and water collection in the spring it will have a few posts sitting in water.

No. You're rationalizing -- and doing it badly. The piers don't have to be only 4' tall. You can make them 5' tall and place their bottom ends 4' below grade. There's no reason why following the code needs to cause problems with surface clearance to grade. They're two separate concerns. (BTW, Sonotubes come in standard 12' lengths throughout North America. I once order them in special 24' lengths -- it was no problem. Even if 4' lengths were the only thing available to you, it's easy to cut 1' sections off of them with a circular saw or a sawzall and scab them together.

Drilling 3' in some spots will mean that all posts are at the same depth and keep the lower-situated ones above water in the spring.

OK, here's the deal. Your local code requires a 4' depth because that's how far down the frost line is in Quebec. Frankly, I'm surprised that it isn't deeper. If your footings are much shallower than that, the freezing ground will tend to heave the footings upwards. This tends to be a particular problem with cylindrical footings where the frost can actually grab the sides of the cylinder and heave it upward. You want the bottom of your footings at least 4' below the finished grade. If you're concerned about some footers being lower than others that are only two feet away, place them both at the lower depth.

Furthermore, I'd suggest holding your tube forms 6" - 8" above the bottom of your hole. When you fill the forms with concrete, this will give you a spread footing at the bottom. The spread footing will help to resist the upward thrust that the frost will exert on the sides of the pier.

The difference in time and materials for this issue is minor. If some of your footings start to heave, that'll be a major headache. It's nearly always best to do it right the first time.

If some of your house footings are less than 4' below grade and they aren't heaving, it's probably because they enclose a space that stays warmer than the exterior. The heat loss from that space is preventing freezing below those footings. The deck piers won't have that luxury. They'll be surrounded by freezing soil on all sides so the frost depth will be deeper there.

- Jim Katen, Oregon

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Depending where in Pincourt you are leaving 1 ft above grade is a good idea.

Generally all foundations and supports should be dug at least 4' below ground due to ground frost conditions. this will also prevent frost heave during freezing and thaw cycles hence eliminating the possibilty of movement and damage to your new deck. Keep in mind that the sonotube itself helps break the capillary action of the frost heaving. The fact that there is some risk of spring flooding mans your soil has the tendency to get saturated with water and hece deeper is better.

Do not hesitate to call me if u require more advice.

thx

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Furthermore, I'd suggest holding your tube forms 6" - 8" above the bottom of your hole. When you fill the forms with concrete, this will give you a spread footing at the bottom. The spread footing will help to resist the upward thrust that the frost will exert on the sides of the pier.

Thanks for the idea. That will both address my concern and add extra strength.

My concern was that posts of different depths, if any heaving did happen, would be uneven and might put more stress on the structure. Since this doesn't seem to an issue, I won't worry about. I've over-engineered the deck plans on purpose based on weight capacity that I never expect to come close to so I'm probably safe.

Thanks for the advice everyone.

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It sounds like you need to do a little grading too. If you have areas that will hols water above your footing fix that first. Make sure your grade allows water to drain away from the house.

Usually it is only a inch or two for a few weeks per year. In general the whole property slopes away from the house. Even this area is still above the general grade. I hope to use this depression for storage under the deck. If I find that it collects too much water next year, I'll add filler.

--

Thanks for the offer Joe. I might like a quality inspection next year once I get some basic renovations taken care of. I'm looking to get all the house in good shape and avoid surprises. My pre-purchase inspection missed a lot of problems that I've spent that last 5 years correcting.

I notice that your website doesn't seem to be working now. I'm a web developer so if you have any questions, let me know.

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This may help you with your Sonotube footing http://www.sonotube.com/products/tubeba ... chart.html

With this base you can backfill before the concrete is placed which also helps prevent the tube from moving or lifting when the concrete is vibrated.

Furthermore, I'd suggest holding your tube forms 6" - 8" above the bottom of your hole. When you fill the forms with concrete, this will give you a spread footing at the bottom. The spread footing will help to resist the upward thrust that the frost will exert on the sides of the pier.

Thanks for the idea. That will both address my concern and add extra strength.

My concern was that posts of different depths, if any heaving did happen, would be uneven and might put more stress on the structure. Since this doesn't seem to an issue, I won't worry about. I've over-engineered the deck plans on purpose based on weight capacity that I never expect to come close to so I'm probably safe.

Thanks for the advice everyone.

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