Garden cabin with oak subfloor perimeter on concrete pilings

[Larry Bethany]

I'm installing a 5m x 5m garden cabin in my back garden. There are height restrictions. In order to accommodate these and to accommodate the depth of a 6x2 subfloor resting on concrete pilings, I've had to excavate the cabin area about a foot below grade. Since the life of the cabin depends on the life of its foundation, the perimeter of the subfloor will be oak sleepers. This is the only part of the subfloor that the cabin walls rest upon. These sleepers can't be typical green oak which is sold for garden purposes. They have to be air dried. I plan to coat just the top of the pilings and the area of the sleepers in contact with the pilings with bitumen and also have a layer of flexible, lead-like, roof flashing between them as additional moisture barrier that will also outlast the bitumen. I have a question: With no moisture getting into the area under the cabin other than what may come up through and around a thick roll-plastic moisture barrier, is there likely to accumulate any moisture in that minimal space between the flashing and the sleepers that would lessen the life of the oak? An air dried oak beam is said to have a life of over 50 years. There will be some air circulation from the 6 inch gap between the cabin walls and the surrounding retaining wall made of concrete paving slabs stood on end and bolted together. There'll be a board covering that 6 inch gap but there will be space under it for air to get through. Unfortunately, there won't be enough air circulation to air dry green oak, so I have to go with the more expensive air dried oak. What do you think of this plan? Does anyone recommend that I do any of it differently?

 

[Sachakins]

5m2, that's about a 8x6 shed area, why a 6x2 joist floor? Seems excessive.
Given what you suggest, would concrete posts be a better option than oak across the pilings?

 

[Larry Bethany]

5m2, that's about a 8x6 shed area, why a 6x2 joist floor? Seems excessive.
Given what you suggest, would concrete posts be a better option than oak across the pilings?

5m2 = 5m x 5m. Unless I've misstated it.

 

[Larry Bethany]

One timber salesman told me that if I use green oak sleepers, they'll never dry out in this use; hence, it would rot soon, lasting may just 15 years. Another told me that what rots wood is when the conditions go back and forth between dry and humid, that if I use green sleepers and they get little air circulation but are in a stable environment that doesn't fluctuate from dry to moist, then they should last their usual long life of 50 years. What's the general knowledge on this?

 

[akirk]

5m x 5m = 25m2
it is c. 16.5ft x 16.5ft or 272.25 ft2
reasonably large - my workshop is an old garage at 3m x 5m so this is about a 1.5 / 2 car garage size

 

[Doug B]

Don’t know if this will help, a couple of years ago I did what sounds like a similar thing only on a smaller scale.
The rear of our garden is all made up ground so when it came to building our garden room I put in concrete piles.

I left steel rod sticking out of each pile & welded more rod connecting up all the piles, I put weed membrane down then shuttered round to form a grid.

After filling the shuttering this is the finished base with a few rows of bricks laid.

Don’t know if that would work in your situation but it takes away any concerns about rot & makes a real sturdy base without as much hassle as pouring a concrete slab particularly as access to the end of the garden isn’t great.

 

[Glossopguy]

Your plan sounds pretty solid to me. As long as you maintain good ventilation around the cabin, you should minimize any potential moisture buildup.

 

[Sachakins]

5m2 = 5m x 5m. Unless I've misstated it.

Apologies 5m2 to me is the square area, ie 2.5 x 2m. So at 5m x 5m makes sense now, but still go for concrete lintel/posts spanning the filings.

 

[Larry Bethany]

Don’t know if this will help, a couple of years ago I did what sounds like a similar thing only on a smaller scale.
The rear of our garden is all made up ground so when it came to building our garden room I put in concrete piles.

View attachment 180085

I left steel rod sticking out of each pile & welded more rod connecting up all the piles, I put weed membrane down then shuttered round to form a grid.

View attachment 180086

After filling the shuttering this is the finished base with a few rows of bricks laid.

View attachment 180087

Don’t know if that would work in your situation but it takes away any concerns about rot & makes a real sturdy base without as much hassle as pouring a concrete slab particularly as access to the end of the garden isn’t great.

Looks like a careful plan well executed. How deep did you have to go on those pilings? What did you do for the actual floor?

 

[Jameshow]

When I built a 2 story extension we put a 600mm /300mm raft in and the cement chap said you building a runway! You guys are belt and braces too!!!

 

[Larry Bethany]

Your plan sounds pretty solid to me. As long as you maintain good ventilation around the cabin, you should minimize any potential moisture buildup.

Ventilation was a concern, but I'm confident that the six inch gap between the retaining wall and the cabin wall will be sufficient. I'm wondering what would be the longest lasting moisture barrier to lay down on the soil spanning the 5m x 5m. At present I'm planning to use a thick roll plastic. But I'm not sure what compound it is. I know that different plastic degrades differently. I'm also going to paint the bottom of the sleepers with bitumen but leave the other sides untreated so that they can breathe. I wonder if I should not coat the bottoms and just leave them untreated to let them breathe too. I can always reach down there with a roller and coat them years down the road after they've done what seasoning they're going to do.

 

[Jameshow]

Ventilation was a concern, but I'm confident that the six inch gap between the retaining wall and the cabin wall will be sufficient. I'm wondering what would be the longest lasting moisture barrier to lay down on the soil spanning the 5m x 5m. At present I'm planning to use a thick roll plastic. But I'm not sure what compound it is. I know that different plastic degrades differently. I'm also going to paint the bottom of the sleepers with bitumen but leave the other sides untreated so that they can breathe. I wonder if I should not coat the bottoms and just leave them untreated to let them breathe too. I can always reach down there with a roller and coat them years down the road after they've done what seasoning they're going to do.

No coat the bottom of the sleepers stops the damp coming up.

 

[truckeruk3]

When we were in a similar position we laid a grid of concrete piles then placed high density concrete blocks on top .We then put DPC over the blocks before placing our sub-frame of 5 x 2 C32 timbers. Timbers were then painted with Black Jack liquid PVC .

 

[Doug B]

Looks like a careful plan well executed. How deep did you have to go on those pilings? What did you do for the actual floor?

The piles were around 3’6” deep, below 2’ deep we are on a seam of sand stone so when I built my workshop 20 years ago the soil I dug out for the footings I spread around the rest of garden to make it level, the majority of that soil when to the bottom of the garden as that’s where it fell away most & where we later wanted our garden room. Our soil is very sandy with very little structure which is why I went for piles in this built up area.
As this was only a garden room the floor was straight off the concrete grid & consists of a damp proof membrane, 4” thick high density insulation then 22mm t&g floating mr chipboard floor & click vinyl laminate on top.

 

[paulrbarnard]

Apologies 5m2 to me is the square area, ie 2.5 x 2m. So at 5m x 5m makes sense now, but still go for concrete lintel/posts spanning the filings.

5x5 is 5 meters square. 5x1 or 2.5x2 is 5 square meters

Putting a superscript 2 after the m usually denotes square meters not meters square. Complicating it further not using a superscript turns the m into the position of the decimal point. So 5m2 can be read as a linear 5.2m distance.

 

[Larry Bethany]

The piles were around 3’6” deep, below 2’ deep we are on a seam of sand stone so when I built my workshop 20 years ago the soil I dug out for the footings I spread around the rest of garden to make it level, the majority of that soil when to the bottom of the garden as that’s where it fell away most & where we later wanted our garden room. Our soil is very sandy with very little structure which is why I went for piles in this built up area.
As this was only a garden room the floor was straight off the concrete grid & consists of a damp proof membrane, 4” thick high density insulation then 22mm t&g floating mr chipboard floor & click vinyl laminate on top.

Good use of that soil. I wish I could do the same, but raising the level of my garden isn't an option. I'm having to get rid of the soil I excavated for the whole area plus what I dug out for the pilings. I'd love to find someone who needs 12 ton of top soil but it seems that that's something you can't give away even though it's sold in bags at garden and diy centres.

 

[Larry Bethany]

5x5 is 5 meters square. 5x1 or 2.5x2 is 5 square meters

Putting a superscript 2 after the m usually denotes square meters not meters square. Complicating it further not using a superscript turns the m into the position of the decimal point. So 5m2 can be read as a linear 5.2m distance.

Thanks for the info. This system is so easy to misunderstand, I wonder why it's stood for so long. How is 5m x 5m meters denoted?

 

[Doug B]

Good use of that soil. I wish I could do the same, but raising the level of my garden isn't an option. I'm having to get rid of the soil I excavated for the whole area plus what I dug out for the pilings. I'd love to find someone who needs 12 ton of top soil but it seems that that's something you can't give away even though it's sold in bags at garden and diy centres.

Look locally for turf suppliers the ones local to me will pick up good top soil if it’s free, also before shutting down our local landfill would buy top soil but you had to take it to them.

 

[Larry Bethany]

Look locally for turf suppliers the ones local to me will pick up good top soil if it’s free, also before shutting down our local landfill would buy top soil but you had to take it to them.

Thanks for the info. I'll look into it.

 

[Sachakins]

Any local allotments to you, they sometimes club together to buy bulk soil. So maybe they could hire a grab hire and collect it in one hit?