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jl2698 03-27-2012 03:40 PM

Footing for new deck posts
 
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I'm replacing an existing 28' wide 11' deep old sunroom with a 14' wide 12' deep 9' high wood deck (use 2"x6"x14' PT deck boards, 2"x10"x12' PT joists & 2"x8"x14' PT beams, 6"x6"x9' PT posts). The existing sunroom is cedar wood structure & frame connected to the brick house wall via a 2"x10" ledger board and rest on 8"x8"x16" concrete block foundation (>4' blew grade for cold area). To minimize digging and costs, ideally I want to place/secure the three new posts on the existing concrete block foundation. After talking to few contractors, some of them recommend to install new concrete piers for the posts instead of using existing foundation.

Is there any problem/issue with secure the 6"x6" posts on existing concrete block foundation? Does concrete block foundation provide enough structural support the the posts & whole deck? If this is workable, please kindly advise what I need and how, so I can discuss with contractors in a more knowledgeable manor. All openings on top of the existing block concrete are already filled with concrete and bolts for securing 2"x4" frame of existing sunroom. Other than removing the bolts, do I need to do anything to the concrete blocks?

Thanks for any advise given.

ratherbefishing 03-27-2012 03:54 PM

If you leave the existing foundation, with no roof over it, won't it fill with rainwater?

cibula11 03-27-2012 03:56 PM

Pictures would be helpful. I'm guessing (and this is just a guess) that in SOME cases you could use the block wall, but contractors probably are a bit cautious about building their structure on top of something that they didn't do.....or might not be structurally sound. Just my opinion, but pictures and the advice of people on site would be the best.

jl2698 03-27-2012 04:07 PM

The old sunroom has a concrete slab and nature stone finished floor. So I don't think there will be any opening. But I may re-seal the nature stone since it will be exposed to the weather.

jl2698 03-27-2012 04:09 PM

Quote:

Originally Posted by cibula11 (Post 886741)
Pictures would be helpful. I'm guessing (and this is just a guess) that in SOME cases you could use the block wall, but contractors probably are a bit cautious about building their structure on top of something that they didn't do.....or might not be structurally sound. Just my opinion, but pictures and the advice of people on site would be the best.

ok. added a picture.

jl2698 03-27-2012 04:09 PM

Quote:

Originally Posted by ratherbefishing (Post 886740)
If you leave the existing foundation, with no roof over it, won't it fill with rainwater?

The old sunroom has a concrete slab and natural stone finished floor. The finished floor is "flush" with the top of the foundation so there's NO sunken area (like a small pool). So I don't think there will be any opening. But I may re-seal the nature stone since it will be exposed to the weather.

Daniel Holzman 03-27-2012 04:36 PM

Typically the footing for a deck or sunroom in a cold area like Buffalo is going to be on footings set below frost depth, which is probably four feet in your area. The old foundation may be slab on grade, in which case it is probably unacceptable. However, this is really a discussion you need to have with your local building inspector, as local practices normally drive foundation design. The inspector will be happy to discuss this and many other design issues when you apply for your building permit.

LMHmedchem 03-27-2012 04:38 PM

Your local building department should publish specifications for such things. It should tell you what size framing members you should use for a deck of a given height and such. When deck posts are over 8', they usually require heavier cross bracing than shorter deck posts, etc. I would probably do 4' sonotubes for the main posts with good cast-in-place post bases and such. You don't want the thing sinking into the ground after a few years. There really shouldn't be any guesswork in this kind of thing, the building codes should specify what minimum code is, and then you want to exceed it some.

It will be very important to make sure that water runoff is well managed. You may want to put in french drain around the existing pad, etc, to make sure that you don't end up with standing water there. You may want to bring the grade around the existing pad down a bit so that the water has somewhere to go. If the existing pad isn't sloped at all away from the house, you could still end up with an issue, but it's hard to say.

LMHmedchem

jl2698 03-27-2012 04:53 PM

Quote:

Originally Posted by Daniel Holzman (Post 886767)
Typically the footing for a deck or sunroom in a cold area like Buffalo is going to be on footings set below frost depth, which is probably four feet in your area. The old foundation may be slab on grade, in which case it is probably unacceptable. However, this is really a discussion you need to have with your local building inspector, as local practices normally drive foundation design. The inspector will be happy to discuss this and many other design issues when you apply for your building permit.

The existing foundation is based on 8"x8"x16" concrete block with more than 4' in depth below grade as I had specified in original message. It has supported existing sunroom with roof for more than 20 years and in very good condition.

jl2698 03-27-2012 05:02 PM

Quote:

Originally Posted by LMHmedchem (Post 886770)
Your local building department should publish specifications for such things. It should tell you what size framing members you should use for a deck of a given height and such. When deck posts are over 8', they usually require heavier cross bracing than shorter deck posts, etc. I would probably do 4' sonotubes for the main posts with good cast-in-place post bases and such. You don't want the thing sinking into the ground after a few years. There really shouldn't be any guesswork in this kind of thing, the building codes should specify what minimum code is, and then you want to exceed it some.

It will be very important to make sure that water runoff is well managed. You may want to put in french drain around the existing pad, etc, to make sure that you don't end up with standing water there. You may want to bring the grade around the existing pad down a bit so that the water has somewhere to go. If the existing pad isn't sloped at all away from the house, you could still end up with an issue, but it's hard to say.

LMHmedchem

Yes, the area around the foundation will be regraded so it will be at least 6" above grade which will be part of the landscaping phase. All those 2'x2' concrete slabs outside the sunrrom area will be removed and the area will be grassed.

The post might be 8' since the joists will be sit on top of the beam. The bottom of the deck is 8' from the finished floor. My question is why can't I use the existing foundation and save money by not having to dig and build new foundation for the posts? Wouldn't an existing 4' below grade concrete block foundation more stronger than 3 concrete cylinder footings?

LMHmedchem 03-27-2012 05:56 PM

Quote:

Originally Posted by jl2698 (Post 886787)
Wouldn't an existing 4' below grade concrete block foundation more stronger than 3 concrete cylinder footings?

It depends. A hollow block wall is designed to support weight the is more or less distributed evenly along the length. The structure of a deck loads all of the weight on a few point locations. The block is hollow, so it's not designed to support a vertical post, but a horizontal beam. If it was a pored concrete foundation, that would be a different situation. You also don't know what kind of shape the old foundation is in and I'm not sure what you would use to attach the new post to the top of old block. The hollow end of the block will be pointing up. The surface may have been mortared, but that wouldn't be something you would want to load a post onto.

A 4' sonotube filled with new high test concrete can support a massive structure and there would be no questions as to its soundness. There are a wide range of fittings available to attache the post base in a way that keeps separation between the wood post and the concrete. This is a major rot point, so that is something to keep in mind. You really need to look at the code requirements put out by your local building department. They are there as a service to answer such questions and make sure your plan is up to code so that it is safe, and also to insure that contractor do what they were paid to do. These folks work for you and they are a resource you should take advantage of. If you aren't required to pull a permit in your area, there should still be minimum specifications available, probably as a download .pdf.

LMHmedchem

jl2698 03-27-2012 06:16 PM

Quote:

Originally Posted by LMHmedchem (Post 886823)
It depends. A hollow block wall is designed to support weight the is more or less distributed evenly along the length. The structure of a deck loads all of the weight on a few point locations. The block is hollow, so it's not designed to support a vertical post, but a horizontal beam. If it was a pored concrete foundation, that would be a different situation. You also don't know what kind of shape the old foundation is in and I'm not sure what you would use to attach the new post to the top of old block. The hollow end of the block will be pointing up. The surface may have been mortared, but that wouldn't be something you would want to load a post onto.

A 4' sonotube filled with new high test concrete can support a massive structure and there would be no questions as to its soundness. There are a wide range of fittings available to attache the post base in a way that keeps separation between the wood post and the concrete. This is a major rot point, so that is something to keep in mind. You really need to look at the code requirements put out by your local building department. They are there as a service to answer such questions and make sure your plan is up to code so that it is safe, and also to insure that contractor do what they were paid to do. These folks work for you and they are a resource you should take advantage of. If you aren't required to pull a permit in your area, there should still be minimum specifications available, probably as a download .pdf.

LMHmedchem

I did talk to building department while inquiry about permit. He told me that it's ok as long as I can provide a cross-section drawing of existing foundation and explain how it's going to be constructed. Since I don't have access to original building plan and contractors were talking about installing new footings, I won't be able to provide this without knowing for sure it can be done. I have dug up the soil in one area of the foundation and making sure it's a concrete block and more than 4' below grade.

Perhaps contractors want a easier job by not using any existing foundation and dig new ones (at my cost :(). I know that if I use 2"x10" beam I only need 2 posts. I thought by adding one more posts it would help spread the load on the foundation. I have used several online deck design tools (ie, from Lowe, ..) and they all based on sonotubes. So I'm trying to get more info on using existing foundation.

In reference to your concern about the hollow concrete blocks, they all sealed off with cements on top before the 2x4 was laid during original sunroom construction. Would this already help with spreading the load?

LMHmedchem 03-27-2012 07:24 PM

Quote:

Originally Posted by jl2698 (Post 886836)
Perhaps contractors want a easier job by not using any existing foundation and dig new ones (at my cost :().

Much of the time it's harder to fit new construction to old construction that it is to start over. It often costs more to try to retrofit than to build new. Labor time it a bit part of the budget. If they get part way though and discover that the can't use the existing footers, then they have to start again. This is the case with bathroom renovations in old houses. Trying to put new tile and square shower surrounds on walls and floor that are no longer square and level (if they ever were) is so frustrating and time consuming that it's hard to get someone to even try. In many ways, it is much easier to take everything down to the studs and joists and start over. That way you can level the new floor and walls and everything goes in nice and smooth. You also get a chance to look at the structure, plumbing, and electrical to make sure that all of it is up to snuff. I think the situation is similar here. Breaking, or cutting out a 2' section of concrete block wall and digging a 4' hole will take no time at all, and even less if the contractor has any excavation equipment. A 8" sonotube costs $5 and 3 80 pound bags of concrete to fill it costs $9, so it's less than $15 total for the supplies for each footer. If you want to save some money, tell the contractor you will remove the old block and dig the holes yourself. You only need to to this for where the footers go, the rest can stay in the ground.

Quote:

Originally Posted by jl2698 (Post 886836)
In reference to your concern about the hollow concrete blocks, they all sealed off with cements on top before the 2x4 was laid during original sunroom construction. Would this already help with spreading the load?

It could help a little, but that layer of mortar is probably only 1" thick unless they completely filled to top layer of blocks. I have idea why they would do that, so that's probably not the case. You have to drill holes to bolt down the post base, so you could be resting you post on 1" of old mortar with holes drilled in it and an empty space underneath. At the very least, the builder would want to chip off the old mortar and fill the blocks with concrete so you know what you are working with. At that point, you might as well go with a tube. Builders have to worry about liability and they are going to use a method that is established and they know is going to work. If they try something odd and the deck ends up coming down, they are screwed. There are enough contractors sitting in jail cells around here because a deck or porch they built collapsed and someone died to make the others think carefully about it.

Even if it wasn't a liability issues, contractors like to stick to methods they know well and have done many times. As soon as you wade into something new and non-typical, you have no idea where it is going to lead you. Contractors are just not willing to go there. A tube is not going to give worse support than a well, and will probably give much better support because it is part of a system that has been designed to do what it does, not a system that was designed to do something else and has been retrofitted. It's hard to say if the wall would work or not, so they are going to be properly reluctant to try. When the building inspector said they would need to know how the foundation was constructed, that is a clue. The builder will have to sit around and figure out how to construct and it will probably be an estimation at best. With a tube, there is no guesswork and the explanation is straightforward.

LMHmedchem

jl2698 03-27-2012 08:01 PM

Quote:

Originally Posted by LMHmedchem (Post 886891)
Much of the time it's harder to fit new construction to old construction that it is to start over. It often costs more to try to retrofit than to build new. Labor time it a bit part of the budget. If they get part way though and discover that the can't use the existing footers, then they have to start again. This is the case with bathroom renovations in old houses. Trying to put new tile and square shower surrounds on walls and floor that are no longer square and level (if they ever were) is so frustrating and time consuming that it's hard to get someone to even try. In many ways, it is much easier to take everything down to the studs and joists and start over. That way you can level the new floor and walls and everything goes in nice and smooth. You also get a chance to look at the structure, plumbing, and electrical to make sure that all of it is up to snuff. I think the situation is similar here. Breaking, or cutting out a 2' section of concrete block wall and digging a 4' hole will take no time at all, and even less if the contractor has any excavation equipment. A 8" sonotube costs $5 and 3 80 pound bags of concrete to fill it costs $9, so it's less than $15 total for the supplies for each footer. If you want to save some money, tell the contractor you will remove the old block and dig the holes yourself. You only need to to this for where the footers go, the rest can stay in the ground.

It could help a little, but that layer of mortar is probably only 1" thick unless they completely filled to top layer of blocks. I have idea why they would do that, so that's probably not the case. You have to drill holes to bolt down the post base, so you could be resting you post on 1" of old mortar with holes drilled in it and an empty space underneath. At the very least, the builder would want to chip off the old mortar and fill the blocks with concrete so you know what you are working with. At that point, you might as well go with a tube. Builders have to worry about liability and they are going to use a method that is established and they know is going to work. If they try something odd and the deck ends up coming down, they are screwed. There are enough contractors sitting in jail cells around here because a deck or porch they built collapsed and someone died to make the others think carefully about it.

Even if it wasn't a liability issues, contractors like to stick to methods they know well and have done many times. As soon as you wade into something new and non-typical, you have no idea where it is going to lead you. Contractors are just not willing to go there. A tube is not going to give worse support than a well, and will probably give much better support because it is part of a system that has been designed to do what it does, not a system that was designed to do something else and has been retrofitted. It's hard to say if the wall would work or not, so they are going to be properly reluctant to try. When the building inspector said they would need to know how the foundation was constructed, that is a clue. The builder will have to sit around and figure out how to construct and it will probably be an estimation at best. With a tube, there is no guesswork and the explanation is straightforward.

LMHmedchem

Very dramatic:eek:. The problem is that the contractor (sent by a major retailer) I talked to wanted to put posts 8' from the wall which falls inside the sunroom foundation, which means he needs to break up the existing concrete floor to install the new sonotube footings. I'm also trying to save the existing floor. To install the sonotube right outside the sunroom foundation (within 12') would require special permit to cause damage to a nearby large tree roots and planting new tree due to the property is backing onto a ravine and conservation area.

If I can remove the top cement of existing foundation block for the areas where the 3 posts going to be installed and back fill the holes with cement, would this provide the required structural support? With three 6"x6" posts, there's approx. 6' between posts. Or add one more post to spread the load more evenly? There are also concrete footings under the concrete block foundation.

Daniel Holzman 03-27-2012 08:36 PM

I missed that detail about the existing foundation being four feet or greater deep. But later you noted that you had to prove this to the building inspector, and show a cross section. And earlier I think you noted that you had dug up one section of the foundation, and found it was four feet deep.

There is nothing wrong with a concrete filled block foundation that is below frost line. As long as the block is in good condition, and there is no reason it should have deteriorated, I don't see why you can't use it to support some PT posts. You may be able to chip out some of the concrete inside the block so you can install a Simpson stand off bracket, or you may be able to epoxy a bracket in place. Either way, if the building inspector is OK with it, your contractor should play ball, and it certainly sounds like it will save you some money and aggravation.


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