Large Shed Base

[ShayneColbert]

Hi,

I am planning on building a shed at the bottom of my garden in a small area of woodland. I want it to last at least 10 years. There is a clearing in the middle of a group of 5 large trees and some smaller ones.
My shed will be 6.6 x 4.2m with a 2.5m eaves height and a dual pitched roof. The area is on a slight slope. Ground is about 8 inches higher at the far end of the shed. Also my plot is only about 2ft from a small tree and there are a few overhanging branches. But this is all I have to work with.

Currently I am working on the base and would appreciate some input on whether I am going about it the right way.

I have excavated to a depth of about 6inches at the low end and 14inches at the deep end to get it level. There are still a few roots deeper in the ground but I don't want the shed sitting any deeper.

I plan to build a shed base consisting of solid concrete blocks with 4 x 4 wooden skids placed on top. I went for this design as I want a wooden insulated floor and didn't want a concrete one.

I plan on making 9 rows of concrete blocks running the length of the shed, 6.6m. They will be placed approximately 0.5m apart from the block center. I'm going to support these blocks with 3inches of hardcore followed by 1 inch of sand, then damp proofing, then 1 inch of mortar to hold the blocks in place.

I'm not going to hardcore and sand the whole area, just the strips where the blocks will be sitting. Once the hardcore is in place I'm going to refill the gaps with soil then compact the whole lot down.

Does anyone see any problems with this plan.

Mainly do you think the damp proof will keep the roots back.
Is the hardcore and sand layer thick enough.
Will just placing hardcore strips with soil in between be sufficient.
Is my block/skid spacing close enough.

Thanks for your time and any advice.

 

[theprinceofdarkness]

My first thoughts are that its the shed joists that have to be supported, so the blocks should be under them, but not necessarily continuously. The other is if they are 9" X ?" X18" blocks, are they going to be laid flat or vertical like a wall.
If the joists are puny and at 12" centres (like my shed) then they need supporting like yours 4.2m/8 ~ .5m. if they are bigger then the gap can increase.
I would thump down an area slightly greater the the 18" X 9" at each spot and put down a bed of plasticised mortar to set the "foundation" block (high density 18" X 9" laid flat). This I would do right around the base, with some vermin proof air vents (gapped vertical joints). It would be good if the base is slightly smaller then the shed so water drips fall clear of the base. Also because of the height difference between the ends, it would be best to make up the height difference as low as possible, by "stepping" the "foundation" blocks and/or using a nothing for first 1/4 of length, then one course of bricks for next 1/4, two courses for next 1/4 then three for the end bit, should make up the 8" difference. Then a course of 9" med, density block for the top continuous layer. Thus giving a full 9" clearance between the base of shed and the highest ground level.
Lay a sheet of DPC over the whole base, to stop water vapour from the ground getting upwards into joists.
Frank

 

[ShayneColbert]

Thanks for your post.
The blocks are going to be lying flat. They are about 18" x 9" by 4". I'm going to be using 4" x 2" skids running along the blocks. With 4" x 2" floor joists running perpendicular to them at 12" spacing. Or less if anyone thinks I can get away with less. This is so I can make a double plywood floor for insulation to sit in the middle.

 

[theprinceofdarkness]

WOW!, a double skinned PLY floor, that's impressive. It will also make the floor difficult to make and lay (and maintain?), unless you do it in sections. Using ply just to hold up some insulation seems to be a very expensive way of doing things. I would be thinking more off using plastic netting used for gardens. Sort of 1" square mesh and plastic about 1/16" diameter, NOT the very thin netting to keep butterflies of your cabbages. if this is tyraped to the undersides of the joists to support glassfibre insulation under the ply floor, now you can afford a thicker floor!.
Frank

 

[ShayneColbert]

That's what I was thinking but all my friends thought it was a bad idea. But I couldn't see what the problem would be. Could use a damp proof membrane to keep it dry.