Foundations for wooden decking

[naboo]

I am looking to build wooden decking over the area water leaches out of our septic tank.

 

The area is about 8ft X 10ft.

 

My plan was to lay concrete slabs for the foundation, but the brother-in-law says I should build a steel frame foundation.

 

What are the benefits and drawbacks of each? Any better ideas?

[johng]

I would go for concrete slabs foundations make sure to  keep the wood out of the wet ground and treat it to prevent rot and insect attack ....metal will likely last longer

If galvanised or powder coated but cost more and harder to install.

 

[chickenslegs]

I replaced a rotten timber deck approx 3m x 5m.

 

I used steel frame (primed and painted) resting on concrete blocks, topped with Shera (cement fibre) decking planks. 

 

No worries about rot or termites. Hardly any maintenance - coat of decking paint every couple of years.

 

Shera planks need a little bit more support than similar sized timbers. I have joists every 40cm for my 8" x 1" planks, but the 12" x 1" planks would need less support.

 

I don't have the tools or know-how to make a steel frame, so I had to get help with that, but everything else is fairly easy DIY.

 

I think your BiL is giving good advice.

 

 

[naboo]

9 minutes ago, chickenslegs said:

I used steel frame (primed and painted) resting on concrete blocks, topped with Shera (cement fibre) decking planks. 

 

No worries about rot or termites. Hardly any maintenance - coat of decking paint every couple of years.

 

 

Thanks, I think this is what I will do then.

[Crossy]

Yup, a wooden anything, unless it's (expensive) hardwood is going to become food for the local wildlife unless loaded up with Chaindrite or the like.

 

Steel frame topped with Shera or one of the plastic wood substitutes (how good are they on the UV front?) is the way to go.

[canopy]

Usually leach fields from septic tanks need grass planted over them to work properly where the roots draw the nitrogen out.

 

[johng]

I completely overlooked the  cement fibreboard lookalike wood...wouldn't  it crack/snap very easily without lots of support  ?

Edited September 25, 2017 by johng

[Farma]

I've had bad experience with Shera planks. They crack and break easily. I'm now in the process of replacing the whole decked area with Thaisun wood plastic composite boards. A little bit more expensive than Shera cement fibre. HomePro stock Thaisun. 13.5 x 240 x 2.5 cm boards were 499 baht.

[eyecatcher]

rather than concrete blocks as a support a better idea is to use the square stub post foundations with the metal ties on top.

this becomes foundation and upright supports in one and easier to adjust to your desired height.

when you get your 4 corner posts(and 2 in the middle) set up then someone can simply burn a frame together or you can do it yourself with timber (more expensive and requires more maintenance)

 

shera/dura is notorious for breaking but the product is not at fault its the spacings. most people buy the cheapest thickness to their cost.

always go for the 22mm or 25mm, and dont let the support centres be greater than 400mm....300 is perfect.

[naboo]

16 hours ago, canopy said:

Usually leach fields from septic tanks need grass planted over them to work properly where the roots draw the nitrogen out.

 

 

Is this really necessary? I can see the grass helping release the water, but Thailand is hot, so the water evaporates quickly anyway, and this area has been open for 4 years and the grass on the lawn surrounding has never managed to take hold there, instead I have a broad leafed weed.

 

Regarding the nitrogen build up, now you make me worried that there will be a build up of ammonia. Has anyone experienced this before?

[canopy]

It's probably a good one to search in google for good understandings and solutions suited to your particular case.

 

[chickenslegs]

On 25/09/2017 at 8:37 AM, johng said:

I completely overlooked the  cement fibreboard lookalike wood...wouldn't  it crack/snap very easily without lots of support  ?

For outside decking Shera floor plank should be used. It's a minimum 2.5cm thick and up to 25cm wide. My experience (built one small deck 6 years ago and a larger one 3 years ago) is no cracking or splitting, but the support spacing has to be correct.http://www.sherasolution.com/en/commercial/products/application/floor-application/series/29339/info/125/

 

I think that some people use the wrong product for their project. Shera make a floor plank which is thinner and meant to be fixed directly to a concrete slab - not suitable for decking. Also, I see many broken fences which have been made with Shera plank (0.8cm thick) or Shera skirting (1.2cm thick). These are really just decorative products and snap very easily.

 

Now I'm starting to sound like a Shera salesman ... 

 

[Pogust]

I would never build anything on a leach field. One thing as Canopy stated is you need to handle what comes from the septic tank with some plants reusing the nitrogen and minerals. The other thing is that leach field can get clogged with time and require redoing. That means dig it up and fill with new gravel again. This depends on how it was done in the first place, what kind of soil you have etc.

 

I suggest you spend a little time searching the net about sewage handling before covering the leach field, it could be well spent time....

[sometimewoodworker]

On 25/09/2017 at 10:52 AM, Farma said:

I've had bad experience with Shera planks. They crack and break easily. I'm now in the process of replacing the whole decked area with Thaisun wood plastic composite boards. A little bit more expensive than Shera cement fibre. HomePro stock Thaisun. 13.5 x 240 x 2.5 cm boards were 499 baht.

Correctly installed and supported Shera planks last extremely well 

 

6 years on still perfectly good condition 

Edited September 28, 2017 by sometimewoodworker

[Crossy]

@sometimewoodworker is that where your missus sends you when you've been bad?

 

Nice job, our OP could do worse than copy (sans roof).

 

 

[carlyai]

On 9/25/2017 at 7:27 AM, canopy said:

Usually leach fields from septic tanks need grass planted over them to work properly where the roots draw the nitrogen out.

 

Yes, I was thinking along the same lines. Don't you need to accommodate the leaching first?

Do you have seperate tanks, so all the grey water from the showers, wash basins etc go to a seperate tank?  Then you only have to worry about the toilet/s. If your septic leach drains are in clay type soil, you need to get rid of the clay and put down some aggregate.

I'd say get the leaching right before the steel and Sherra build.