Decks, Patios, Porches and Docks - Building Deck Supports Over Drainage System
Doityourself.com community forum was created to provide answers to all questions related to home improvement and home repair. Doityourself community can help you find information about how-to topics on small fixes to large remodeling projects. With comprehensive how-to content and expertly moderated community forums DoItYourself.com makes it easy to tackle even the most complex home improvement projects.View Full Version : Building Deck Supports Over Drainage System
Sequoia
08-22-07, 08:55 PM
We are in the process of installing a footing for a curb that will serve to divert water away from our deck and foundation and support the post bases for the deck.
When we reached the depth for the base of the footing, which is 14 below the grade, we hit what appeared to be drain rock. We removed the drain rock by hand and hit filter fabric next. Under the filter fabric we felt more drain rock.
We assumed there was a good size hard pipe drain system under this area for all the downspouts coming off the rear roofs for the townhouses, but were surprised to find what appears to a drainage system surrounded by drain rock and filter fabric.
Several years ago a landscaping company put in a surface drain adjacent to our property that supposedly tied into a deeper hard pipe system, but the people who put in that drain are no longer around to tell us exactly what they did.
The drain rock filled trench, which is at least 24 wide, sits directly underneath and runs parallel to our proposed 18 wide deck footing. I understand drain rock is self compacting and makes an excellent base for a footing. Is there any reason why we cant put our footing directly over this drainage system? I would imagine it would be a good idea to line the trench with 6 mil plastic between the drain rock and the base of the footing.
Any guidance here would be greatly appreciated.
Paste picture link: http://www.fototime.com/BA46975B07C01D1/orig.jpg
Paste picture link: http://www.fototime.com/4673E21F4F233B5/orig.jpg
When we reached the depth for the base of the footing, which is 14 below the grade, we hit what appeared to be drain rock. We removed the drain rock by hand and hit filter fabric next. Under the filter fabric we felt more drain rock.
We assumed there was a good size hard pipe drain system under this area for all the downspouts coming off the rear roofs for the townhouses, but were surprised to find what appears to a drainage system surrounded by drain rock and filter fabric.
Several years ago a landscaping company put in a surface drain adjacent to our property that supposedly tied into a deeper hard pipe system, but the people who put in that drain are no longer around to tell us exactly what they did.
The drain rock filled trench, which is at least 24 wide, sits directly underneath and runs parallel to our proposed 18 wide deck footing. I understand drain rock is self compacting and makes an excellent base for a footing. Is there any reason why we cant put our footing directly over this drainage system? I would imagine it would be a good idea to line the trench with 6 mil plastic between the drain rock and the base of the footing.
Any guidance here would be greatly appreciated.
Paste picture link: http://www.fototime.com/BA46975B07C01D1/orig.jpg
Paste picture link: http://www.fototime.com/4673E21F4F233B5/orig.jpg
chandler
08-24-07, 04:51 AM
I am puzzled as to where the water was supposed to go. Just having a layer of stone under and over a layer of fabric, doesn't make a viable drainage system. Surely there is something under this. Although you could conceivably do a pour over the drain, another concern comes up with the sprinkler line. Is it copper? If so, you don't want to surround it with concrete. The properties of concrete will eat away at the copper and cause a failure.
I would get the local inspector involved with the footing, so you can get some local advice.
I would get the local inspector involved with the footing, so you can get some local advice.
Concretemasonry
08-24-07, 07:02 AM
It sounds like it is a real traditional French drain as is installed on golf courses to lower the general water table in the soil over a large area.
They are still used today and many were installed long before plastic pipe was invented and they work well. Lining the trench with fabric could decrease the effectiveness of the drain and cause your immediate area to be wetter.
Dick
They are still used today and many were installed long before plastic pipe was invented and they work well. Lining the trench with fabric could decrease the effectiveness of the drain and cause your immediate area to be wetter.
Dick
Sequoia
08-25-07, 01:14 AM
Larry and Dick,
I believe there is a drainage pipe that is at least 6 in diameter underneath the gravel and filter fabric. It most likely connects with the storm drain system within our complex.
This weekend I am going to dig down around the surface drain one of the landscapers installed several years ago which is located right next to where the footing will be located. I think he tied into one of the cleanouts for the underground drain to install the surface drain. Once I can see straight down the cleanout I should be able to get a good idea just how deep and big the drainpipe is underneath all that gravel and filter fabric.
Paste picture link: http://www.fototime.com/BEEBA285FED8D01/orig.jpg
Regarding the sprinkler line, it is plastic and we are going to relocate it.
I think we will also remove all the soil on top of the gravel underneath the footprint for our footing. We were planning to make the footing 18 wide, but maybe we should increase the width of the footing to spread the load over the entire width of the gravel filled trench.
I will post more info and pictures over the weekend.
Howard
I believe there is a drainage pipe that is at least 6 in diameter underneath the gravel and filter fabric. It most likely connects with the storm drain system within our complex.
This weekend I am going to dig down around the surface drain one of the landscapers installed several years ago which is located right next to where the footing will be located. I think he tied into one of the cleanouts for the underground drain to install the surface drain. Once I can see straight down the cleanout I should be able to get a good idea just how deep and big the drainpipe is underneath all that gravel and filter fabric.
Paste picture link: http://www.fototime.com/BEEBA285FED8D01/orig.jpg
Regarding the sprinkler line, it is plastic and we are going to relocate it.
I think we will also remove all the soil on top of the gravel underneath the footprint for our footing. We were planning to make the footing 18 wide, but maybe we should increase the width of the footing to spread the load over the entire width of the gravel filled trench.
I will post more info and pictures over the weekend.
Howard
lefty
08-25-07, 04:28 PM
Sequoia,
The footings for the deck are going to have to be set in "native soil", or in dirt that has been properly compacted so that the local bldg. dept. will approve it. You've got a job on your hands.
The footings for the deck are going to have to be set in "native soil", or in dirt that has been properly compacted so that the local bldg. dept. will approve it. You've got a job on your hands.
Sequoia
08-27-07, 02:07 AM
Lefty,
Based on what I read at the link below, I believe we are dealing with engineered fill. Also, since the deck is less than 30 off the grade and not attached to the house, a permit and inspection is not required.
Paste link: http://www.concretenetwork.com/concrete/footing_fundamentals/why_soils_matter.htm
<snip>If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it wont settle to any significant degree.<snip>
NOTE: Our soil is a clay sand mixture and is dry and very hard right now. I also computed the PSF the footing footprint imparts to the soil underneath. It works out to 361 PSF with a 50 PSF loading on the deck and the weight of the concrete figured in.
To try and learn more about the drain under the large gravel I removed the surface drain the landscaper installed and discovered he connected it to a cleanout for the underground storm drain. I peered down the 4 cleanout pipe and could see the top of the drainpipe, which measured about 5 below the grade. Unfortunately there was standing water and debris in the pipe so I could not get an idea how big it might be, but it should be of decent size to handle all the downspouts for a large group of townhouses. I did run some water down our downspout drain and could tell the water was traveling past the cleanout.
Paste picture link: http://www.fototime.com/BEEBA285FED8D01/standard.jpg
To restore the cleanout I removed the elbow the landscaper put in and replaced it with a fitting that restored the cleanout stack and cover.
Paste picture link: http://www.fototime.com/6825421CB6BA56C/standard.jpg
Based on what I see on the surface and while looking down the cleanout pipe I believe the area under the footing and curb we want to install looks like this. NOTE: While excavating down to the filter fabric we discovered it was just laid on top with the ends upturned, not turned down as to surround the gravel below.
Paste picture link: http://www.fototime.com/C1F9EF5F573073D/standard.jpg
The gravel starts at the depth we planned to locate the base of the footing. Since the filter fabric is 6-8 below this, we decided to remove all the large gravel down to the filter fabric to verify the exact width of the original trench and remove several large tree roots. Next we plan to replace and level the gravel and move the sprinkler line.
Pictures taken today.
Paste picture link: http://www.fototime.com/0797482A599A051/standard.jpg
Paste picture link: http://www.fototime.com/B61B44B8B6853CE/standard.jpg
Howard
Based on what I read at the link below, I believe we are dealing with engineered fill. Also, since the deck is less than 30 off the grade and not attached to the house, a permit and inspection is not required.
Paste link: http://www.concretenetwork.com/concrete/footing_fundamentals/why_soils_matter.htm
<snip>If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it wont settle to any significant degree.<snip>
NOTE: Our soil is a clay sand mixture and is dry and very hard right now. I also computed the PSF the footing footprint imparts to the soil underneath. It works out to 361 PSF with a 50 PSF loading on the deck and the weight of the concrete figured in.
To try and learn more about the drain under the large gravel I removed the surface drain the landscaper installed and discovered he connected it to a cleanout for the underground storm drain. I peered down the 4 cleanout pipe and could see the top of the drainpipe, which measured about 5 below the grade. Unfortunately there was standing water and debris in the pipe so I could not get an idea how big it might be, but it should be of decent size to handle all the downspouts for a large group of townhouses. I did run some water down our downspout drain and could tell the water was traveling past the cleanout.
Paste picture link: http://www.fototime.com/BEEBA285FED8D01/standard.jpg
To restore the cleanout I removed the elbow the landscaper put in and replaced it with a fitting that restored the cleanout stack and cover.
Paste picture link: http://www.fototime.com/6825421CB6BA56C/standard.jpg
Based on what I see on the surface and while looking down the cleanout pipe I believe the area under the footing and curb we want to install looks like this. NOTE: While excavating down to the filter fabric we discovered it was just laid on top with the ends upturned, not turned down as to surround the gravel below.
Paste picture link: http://www.fototime.com/C1F9EF5F573073D/standard.jpg
The gravel starts at the depth we planned to locate the base of the footing. Since the filter fabric is 6-8 below this, we decided to remove all the large gravel down to the filter fabric to verify the exact width of the original trench and remove several large tree roots. Next we plan to replace and level the gravel and move the sprinkler line.
Pictures taken today.
Paste picture link: http://www.fototime.com/0797482A599A051/standard.jpg
Paste picture link: http://www.fototime.com/B61B44B8B6853CE/standard.jpg
Howard