Pier Foundation for a Shed

by Bill
(Arlington, Texas, USA)

I have to build my 10' x 10' shed close to a tree with exposed roots so I am going to elevate it and I was thinking of using concrete blocks to build it on.

I bought a Heartland "Rainier" kit with 2x3 boards to make floor with but as it is rather large I am concerned about the blocks sinking into the ground and was thinking about putting the blocks on top of some sort of gravel after tamping down the ground with some sort of heavy object.

How many blocks should I use. Are blocks even a sound idea? I read about the plastic sheet under the shed and thought that was a good idea. does the plastic sheet go under the blocks or attached to the bottom side of the floor. There is standing water after it rains so that is also a consideration as I can not stop that. Also I have a clay soil.

Thank you for your consideration

Comments for Pier Foundation for a Shed

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Concrete block shed foundation
by: John - Admin

Hi Bill,
Thankyou for your question.
Firstly with regard to the option of whether to use concrete blocks or a plastic shed base I think that the concrete blocks sound better in this case as you will construct the base of the shed up of the ground so that it doesn't get soaked when water starts to pond.

For constructing the concrete block piers dig a hole 3" deep and 18" diameter, compact the soil in base with a tamper and place a 3" layer of gravel in the bottom. This will help drainage beneath the block and also help to spread the load into the soil beneath.

I would say that you would typically have 9 blocks arranged in 3 rows of three with a 6x2 on each row to support the 3x2 floor joists. However I am not exactly familiar with the type of floor you are using so you may want to ask a bit more on this or send me a sketch.

I hope this helps, let me know if you have any more questions.

All the best

John
Beef Up The Floor
by: ShedGuy

Bill:
Seriously consider upgrading your floor joists to 2x4 treated lumber instead of the 2x3 and space them 12"o.c. You will be thankful you did years down the road. As for the ground that you are building on, clay is pretty solid for the most part. If you are going to put down rock first, make sure you use a rock that is packable (CA6 or a 1" white rock). Avoid using pea gravel or river rock as they wont pack down and your building will eventually settle causing it to rack. Racking will cause a misalignment of your doors and may cause leaks when it rains. Laying concrete patio blocks along the length of the runners under the floor joists at intervals of 4' will also give you adequate support. On a 10x10 building, you should have three runners and you would have three points of contact with the patio blocks on each runner. The whole idea is to keep the floor from sagging as time goes by. Good luck!
building on a wet area
by: Anonymous

I have an area at the bottom of the garden prone to flooding. Although there is a paving stone base the water sits on top and the old shed floor was completely rotten. could i use patio blocks on the paving stones to raise it up?
Careful building in wet areas
by: John

The way that I understand it, it sounds as though this area of the garden only floods occasionally. Building up the shed foundation on blocks above the max flood/water level sounds a reasonable idea.

Two things to watch out for would be that the wet ground might be quite soft and so it is possible that the shed might settle a bit over time.

Also it would pay to build in a damp proof course (some bitumen or heavy duty PVC) into the piers to stop damp rising up from the ground and into the shed base timbers.

As you have found out timber and water will lead to early decay. However if this is the only space that you can fit a shed in your garden then you just have to make the best of it.

Let me know how you get on!

John
Packable material under pier
by: Simon

Hi,

I have a load of 20mm limestone chippings that I would ideally want to use as a base for my piers. Would these be considered 'packable' - I would hate to have any settlement down the line.

Thanks, Simon

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