Frequently Asked Shed Floor Questions

by John Coupe
(Admin)

Shed Floor FAQ – I get asked a lot of questions about building shed floors and so I have pulled together a lot of these onto one page for you to browse. Hopefully, you can find an answer to your question in amongst these.


If not please visit my Shed Questions page and ask away.

My only request before you do this is to take the time to submit several pictures and write a good summary your problem. I am keen to help but if I just get a one-liner saying - “I need some help with my shed roof!” it takes a lot longer to help you get to the root of the problem and find a solution.

I look forward to hearing from you if you don’t find an answer to your question below:

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Shed with concrete floor/base

by Stephen
(Surrey)

I intend to build a 12' by 8' larch ship lap shed with a concrete floor. What size of concrete base should I build and how do I attach the shed to the base to ensure it is waterproof.

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Attachment
by: Anonymous

Place threaded rod into the concrete just before final finishing.

Use galvanized steel fasteners; either 1/2-13 or 9/16-12 should be fine.

Ensure the the rod is sunk a minimum of 4 inches into the concrete and is exposed above floor level approximately 5 inches. This will provide about 1-inch of thread above the wall bottom plate if two 2 x 4 's are used.

After the walls are standing, use a flat washer and split lock washer under the nut. Be sure to layout the rod locations so that they are in the center of the bottom plate's width and fall between between the wall studs.

Place a rod between every other stud space and at each corner.

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Building Lifetime 8X10 shed on slightly below ground

by Minh
(Toronto, Canada)

I'm planning to build a Lifetime 8x10 shed tomorrow and just noticed that the concrete foundation is not completely above ground. It's an old foundation and I just removed the old shed that was on it. The foundation is still good to use as it is. On two sides of the foundation, the top of the concrete is above the ground but on the other two sides (long and short side) it's about 3 inches below the ground. I have my brother-in-law and father-in-law coming tomorrow morning to help me build it. What can I do to properly seal the shed from water? I'm thinking of using a thick plastic for underneath the base of the shed and another liner on the outside of the wall to the ground and then add gravel sloped away from the shed. Also, I'm thinking of adding caulking on the outside.

Any advice would be much appreciated.

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Does your shed have an integral floor?
by: John

Hi Minh,
Thank you for your question. I think that I can picture your concrete base and would recommend that you dig away any excess soil from the uphill side so that any water flowing down the hill will effectively flow around the shed. You could dig a small trench on the two uphill sides and fill them with shingle.

I think that Lifetime sheds come with their own plastic sub-floor so that any water that comes from the walls or lands on the exposed section of slabwill flow away under the shed and not find its way into the interior.

I hope that this helps and that your shed project is a success.

All the best

John

Thank you
by: Minh

Thanks John for your suggestion.

I ended up doing exacting what you said by clearing the soil around the concrete foundation to allow water to flow around the shed.

Cheers,

Minh

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Leveling off a shed floor on slabs

by Darren
(Essex)

Hi John, What's the easiest way to level off a tongue & groove floor which is laid onto Concrete slabs? What I mean is, in certain areas it dips slightly. I've tried packing little blocks of wood under the areas that need sorting out, but it just seems like an ever ending task.

Thanks
Darren :-(

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How to level a shed floor
by: Anonymous

Hi Darren,
Thanks for your question. I understand your difficulties. There are two parts to follow here, firstly the sequence for levelling the shed floor and secondly the method for achieving those adjustments.

The general method for levelling a shed floor is to get one corner at the correct height. Working from here the floor beam is then levelled using a spirit level to the next corner. Then keep on working your way around the building.

From your question I think that you generally get this part of the job. It is the method of packing the beams that is causing difficulty. I have two suggestions here.

The first is to use an adjustable shed foot (you can find these on Amazon). These feet are made of plastic and a screw thread so are continuously adjustable. They aren't too expensive and seem to work well. I used them to install this shed base.

The second method of packing the beams is to use premade plastic shims that are available from DIY suppliers such as Screwfix. These plastic shims are horseshoe shaped and colour coded in different thickness of 1, 2, 5, 10 mm so that you can very quickly get the right thickness of spacer that you need. A friend of mine reckons that they are one of the best inventions of this century (way ahead of all this tech crap!). I think that you might agree ;-)

I hope that the above helps, let me know how you get on.

John

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Strong Raised Timber Floor

by David
(Hertfordshire)

Because my plot slopes, it is rather impractical to build a concrete base for my shed, which would be the best option. I want to use my shed as a workshop to carry out some DIY woodworking, which means the use of some machines such as table saws, router, woodworkers bench and a lathe.

Some of this equipment weighs up to 50kgs and isn’t just a dead load (like a bench) but will effectively be a live load when used.

So - Jon, can you advise on the structure of the shed base (possibly sub base) which will be on pillars/posts up to 1m long out of the ground?

shed likely to be 24-30ft long x 12ft wide and 8ft to eaves with a shallow gable roof.

Thanks in advance

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Pad foundations for a shed
by: John - Admin

Hi Greenboy,
Not sure if it is the floor design or the foundation design that you are after here. A few figures that you might find useful:

The self weight of the shed can be roughly assessed as 22.5kg/m² for the walls, roof and floor.

The load for equipment and people in your shed would be typically around 150 kg/m². This is a typical domestic floor loading, the equipment you describe is relatively light. If you were to have a big lathe or industrial saw bench you would need to increase this quite a bit further.

A roof load of 75 kg/m² should be allowed for small buildings such as this to allow for snow and access for maintenance etc.

One you have assessed these you come up with a self weight of the shed of approx 2500kg. With an imposed load of 5850kg.

A poor soil for foundation purposes has an allowable bearing capacity of 5000kg/m² so my mental maths reckons that you need about 15 pads on a grid of approx 1.8m with each pad at about 400mm square.

Is this the sort of info that you were after or was the timbers for the floor?

Good to hear from you.

All the best

John

Timber Size
by: GreenBoy

Thanks John

Glad to see the competition was such a success - clearly the world loves shed too :)

Thanks for the advice on the foundations - this is a great help.

The timbers i was planning to use were 50mm x 200mm on the main grid with some intermediate 50x100 inbetween - i dont think even 18mm ply can span 1.8m without aditional support - i was therefore planning on placing these at approx 400 centres - does this sound ok?

Thanks again for the support - and such a great we site.

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Insulation and floor covering suggestions

by Caroline
(UK)

Hi, I'm an artist and I, or rather my husband, is transforming a 10 x 20 shed into a studio. The studio will be insulated using either Kingspan or Celotex. Can you advise on insulating the floor and what would be the best floor covering? I am a textile, mixed media artist so no paint splash etc on the floor.

Many thanks for your help.

Caroline

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Shed floor insulation and coverings
by: John

Hi Caroline,
Floor insulation can go below or above the shed floor as supplied. It will be easiest to have the floor insulation fitted below the floor as the shed is being built. Getting to it afterwards is tricky.

Installing the floor insulation above the existing floor is also possible but this will reduce the head height inside the shed. So it depends how tall you are and the height of the shed. When installing insulation above the existing floor you would probably install timber battens of the same depth as the insulation to support the new plywoo/boards that are fitted over the insulation. It may also pay to have a breathable membrane installed beneath the insulation to stop draughts and moisture.

With regards to the flooring much of it depends on your taste and how you see the shed floor wearing. Painted/varnished floor boards may be OK for you. Some cheap vinyl flooring may give more of a finished look with a wide variety of colours and styles available and be easier to keep clean.

Often vinyl flooring offcuts that are too small for a house room but perfectly adequate for a structure the size of a shed are available quite cheaply from flooring suppliers.

I hope that the above is of some help. Let me know how you get on.

All the best

John

Ordering of floor layers
by: Anonymous

Hi, I've finalised most of the details of a shed I'm planning to build but am still struggling to finalise the foundation/floor structure.
I'm thinking that I will use pier foundations with 2x4 joists, celotex insulation and osb boards on top.
I'm not sure how I will keep the celotex from dropping out (battens, chicken wire, membrane, another layer of osb?), and I'm not sure where a damp proof membrane should be layered (above/below joists, osb, celotex).
Any advice appreciated.

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Joists and bearers

by Gary
(UK)


Hi John, I'm wondering why advice is always to lay the treated wooden bearers at 90 degrees to the joists. If I lay the bearers directly under the joists wouldn't that give me a stronger floor?

Hi Gary,

Good question. If I read your question right what you are asking is; Why are the wooden bearers as shown as the image on the right rather than the image on the left?

Firstly, the floor joists in a shed should be designed to be strong enough to span between the bearers. So there is no specific need to have the two timbers one on top of the other.

Secondly and more importantly having the timbers and bearers at right angles to each other promotes air flow beneath the shed. This helps the shed base to dry out if it becomes damp, which will prolong its life.
Also, the contact points between the two timbers when they are at right angles is small so there is very little space for damp to be trapped between the two timbers.
If the timbers were one on top of the other this contact area would be large and moisture could become trapped.

Finally, having timber bearers running at right angles helps to iron out any small irregularities in the surface below. The bearers can be shimmed to be quite level, but if there is a discrepancy of say 4mm between two bearers which are 600mm apart (typically) then the level difference is less than 1:150. And most DIY shed builders should do much better than this.

So there we have it the main reasons for having the bearers at right angles are:

-Airflow
-Minimising transmission of damp
-Part of achieving a level floor

Hope this helps, would be good to see some pictures of your shed when you have it finished.

Regards

John

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Shed Floor Construction

by David
(Stoke-on-Trent)

I am building a shed and am wondering about the merits of a plywood or tongue and groove construction. Any tips?

Regards

David

Answer:

Hi David,

I have always used plywood when building shed floors. Obviously, T&G will provide a stronger floor. The other factor you need to consider is whether or not the plywood should be pressure treated or not, and I always use pressure treated. It's a little more expensive but well worth the extra expense in the long run.

And, should it be 1/2", 5/8", or 3/4" thick? Here again, the more the better. Don't put down a shed floor that you know you may have to replace in 5 years because it just wouldn't stand up to the weather with moving in mowers, bicycles and the like with water being a heavy duty culprit to decay and rot.

Take the extra precautions.

Go with T&G, pressure treated, and 3/4". Its just a shed, probably not huge, so your extra expense is going to save you headaches down the road.

All the best,
John

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Is a floor required?

by RJ
(Athens GA)

How necessary is building a floor on your shed?
I have very hard red Georgia clay. It doesn't really snow at all here. I'm trying to save money building the shed so if a floor isn't necessary and won't cause any big problems down the road I was considering leaving the floor out.

Suggestions?

Cheers

RJ

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Reasons for installing a shed floor
by: John

Hi RJ,
Thanks for your questions. I am not familiar with the ground conditions where you live, what the weather conditions are or what you intend to use this shed for. It may well be that your project will work well without a floor. I will outline below the reasons why floors are more often than not installed in sheds and then hopefully you can come to your own decision.

Damp
An important function of a floor is to stop dampness from the soil getting into the shed. The floor does this in two ways. Firstly by lifting the surface of the shed off the ground, air can flow underneath the floor allowing any moisture vapour to be vented out to the atmosphere. Secondly some shed floors may have a physical barrier in the form of a PVC Damp Proof Membrane that stops water penetration. A shed with a damp internal atmosphere is not a good place for storing many items that are susceptible to rust, mildew etc.

Vermin
Lifting the floor off the ground also helps to protect the the contents of the shed from vermin (insects and animals). Vermin such as rats and mice get easier access to the shed if there isn't a floor, also burrowing creatures can come up through the floor. In terms of insects having a floor is a good barrier to ants, slugs etc.

Flatness
Depending on the use of the shed having a good flat floor may or may not be important. However it is easier to achieve a flat floor using timber or concrete than simply levelling the ground.

Those are my top three reasons for having a floor in a shed. I think that they are fairly compelling. Though if the shed was a run-in shed for animals a floor is not often used and also some storage type barns or pole sheds are often constructed without floors.

If budget is your main concern it could be that you design your shed so that if you really need a floor that you could install one later as finances allow.

Let me know how you get on.

All the best

John

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