new shed on concrete slab

AliceW

I've recently had an old brick garage (20ft X 10ft) demolished and plan to have a 16ft X 8ft pressure treated wooden shed with floor bearers installed on the concrete slab -- the slab will be bigger than the shed. Pent style shed with a gutter along the short side next to the fence.  I've noticed that after heavy rain the slab has puddles. Does this mean that water is likely to get under the new shed and rot the timber bearers?

Exodi

In an ideal world, you'd address the pooling though I'm lazy so if it was me I'd just use a plastic shed base.

WIAWSNB

I would never have any timber in direct contact with the concrete base or ground.

There will be short pedestals designed for such a job, but you could just use slices of a composite fence post and slip them under the timber bearers, or even whole posts laid flat.

Something like this, for example, would serve two purposes - levelling the shed base, and keeping it clear from the ground. Check specs for weight handling, etc.

grumpy_codger

I'd never put timbers, even treated, directly on a flat concrete base. Instead, I'd put bricks, preferably black, under the timbers. Ideally, full perimeter and few in the middle.

ButterCheese

There are pros and cons of raising it on legs.

Pros:  easy to do, will keep your wooden structure pretty much dry.

Cons:  Expensive depending on what you keep in there.  If you're in there working all the time and have lots of heavy equipment, I'd opt for a full base rather than plastic legs, it will be stronger and more stable.

Creating a void encourages rodent to nest under there (keep an eye on that).

The other option would be to grade the top of the slab so water can run off, but then you'll have more of a job to get the legs/base level on what is now a slightly sloped base. 

I'd probably use a plastic full base, it might be £150 - £200 but it's probably the best solution for belt and braces

Section62

AliceW said:

I've recently had an old brick garage (20ft X 10ft) demolished and plan to have a 16ft X 8ft pressure treated wooden shed with floor bearers installed on the concrete slab -- the slab will be bigger than the shed. Pent style shed with a gutter along the short side next to the fence.  I've noticed that after heavy rain the slab has puddles. Does this mean that water is likely to get under the new shed and rot the timber bearers?

Ideally shed bases should be slightly smaller than the shed to eliminate that problem, but as you have the base already all you can reasonably do is to minimise the risk.  How much of a problem you might get depends on which way the base falls, the direction of the bearers and the amount of treatment the bearers are given.  'Pressure treated' wood in contact with the ground won't necessarily last that long anyway.

If it were me I'd put the shed where the red line is on this picture -

The reason for that is you'll have some overhang on two of the most exposed sides, so they won't be a problem.  Having a ~2 foot gap along the back next to the fence will help with maintenance, but with a 4" gutter (don't use the 3" 'mini' type as they are nothing but trouble) the 'open' gap to the fence will be ~20".  This means the rain would need to be near-vertical before it gets to the concrete base along that edge.

The downside of that position - if the door is in the near short side - is having a puddle immediately in front of the door.  But if that is an issue you could put some slabs or decking boards down on the concrete to keep your feet dry.

Are you putting the shed up yourself?  Again, if it were me I'd give the bearers a couple of coats of something like 'creocote'.  It isn't as good as traditional creosote, but is much better at protecting damp wood long-term than typical pressure treatment chemicals.  If the shed is being put up for you by the suppliers then they probably won't agree to use creocote, and will claim their pressure treatment is all you need.

Raising the bearers off the ground by using bricks or slabs would be another option - but you need to use a lot and place them very carefully, otherwise the bearers won't be supported enough and the floor will 'bounce' and sag.

ButterCheese

Section62 said:

AliceW said:

I've recently had an old brick garage (20ft X 10ft) demolished and plan to have a 16ft X 8ft pressure treated wooden shed with floor bearers installed on the concrete slab -- the slab will be bigger than the shed. Pent style shed with a gutter along the short side next to the fence.  I've noticed that after heavy rain the slab has puddles. Does this mean that water is likely to get under the new shed and rot the timber bearers?

Ideally shed bases should be slightly smaller than the shed to eliminate that problem, but as you have the base already all you can reasonably do is to minimise the risk.  How much of a problem you might get depends on which way the base falls, the direction of the bearers and the amount of treatment the bearers are given.  'Pressure treated' wood in contact with the ground won't necessarily last that long anyway.

If it were me I'd put the shed where the red line is on this picture -

The reason for that is you'll have some overhang on two of the most exposed sides, so they won't be a problem.  Having a ~2 foot gap along the back next to the fence will help with maintenance, but with a 4" gutter (don't use the 3" 'mini' type as they are nothing but trouble) the 'open' gap to the fence will be ~20".  This means the rain would need to be near-vertical before it gets to the concrete base along that edge.

The downside of that position - if the door is in the near short side - is having a puddle immediately in front of the door.  But if that is an issue you could put some slabs or decking boards down on the concrete to keep your feet dry.

Are you putting the shed up yourself?  Again, if it were me I'd give the bearers a couple of coats of something like 'creocote'.  It isn't as good as traditional creosote, but is much better at protecting damp wood long-term than typical pressure treatment chemicals.  If the shed is being put up for you by the suppliers then they probably won't agree to use creocote, and will claim their pressure treatment is all you need.

Raising the bearers off the ground by using bricks or slabs would be another option - but you need to use a lot and place them very carefully, otherwise the bearers won't be supported enough and the floor will 'bounce' and sag.

Good call.  That's why I would use a full plastic base, but it depends how well you want to do it and how long you think you'll be living there.  In my experience, the more traffic the shed gets (i.e. the more times you go in and out over a given week/month) the more succeptable it will be to wear and uneven pressure; once the base or sides start to move, they will only get worse over time.

I would also advise against a pre-fabricated shed as they don't use very thick wood for the supporting structures.  Perhaps buy the timber yourself and get someone to put it together if you can't.  We have asked our local roofer to build us a shed for this reason.

Lastly, I've seen a lots of landscapers use damp proof course or damp proof tape on shed bases as well.  Might be something to consider - I have no way of comparing them to a shed used without anything, but for a small outlay it's probably worth it.  My MIL has a small wooden shed which was put in 10 years ago, dorectly on top of paving slabs.  One side of the shed has now rotted at the bottom and is leaning

EssexExile

My wooden shed has been sitting on concrete for over 20 years with no problems.

As some have said it's a problem that the shed is smaller than the base, would it be possible to have the shed made to fully cover the base? A shed is never too big!

TELLIT01

ButterCheese said:

I would also advise against a pre-fabricated shed as they don't use very thick wood for the supporting structures.  Perhaps buy the timber yourself and get someone to put it together if you can't.  We have asked our local roofer to build us a shed for this reason.

Lastly, I've seen a lots of landscapers use damp proof course or damp proof tape on shed bases as well.  Might be something to consider - I have no way of comparing them to a shed used without anything, but for a small outlay it's probably worth it.  My MIL has a small wooden shed which was put in 10 years ago, dorectly on top of paving slabs.  One side of the shed has now rotted at the bottom and is leaning

Claiming that pre-fabricated sheds don't use very thick wood is a massive generalisation.  Mine came from a local company and the framework is 50x38mm, boarding is 18mm T&G and the roof 18mm boards. The shed is 3 metres long, with pitched roof and rafters every 600mm.  I'd suggest that is reasonably thick timber.

I agree about using damp proof course or tape.  It certainly can't do any harm. 

Section62

TELLIT01 said:

ButterCheese said:

I would also advise against a pre-fabricated shed as they don't use very thick wood for the supporting structures.  Perhaps buy the timber yourself and get someone to put it together if you can't.  We have asked our local roofer to build us a shed for this reason.

Lastly, I've seen a lots of landscapers use damp proof course or damp proof tape on shed bases as well.  Might be something to consider - I have no way of comparing them to a shed used without anything, but for a small outlay it's probably worth it.  My MIL has a small wooden shed which was put in 10 years ago, dorectly on top of paving slabs.  One side of the shed has now rotted at the bottom and is leaning

Claiming that pre-fabricated sheds don't use very thick wood is a massive generalisation.  Mine came from a local company and the framework is 50x38mm, boarding is 18mm T&G and the roof 18mm boards. The shed is 3 metres long, with pitched roof and rafters every 600mm.  I'd suggest that is reasonably thick timber.

I agree about using damp proof course or tape.  It certainly can't do any harm. 

Just needs some care and attention to the details - if the DP extends all the way to the outside of the shed (or beyond) then the upper surface could catch some rainfall.  If the DP is curled up or dished then the bearer could end up sitting in water which is unable to drain away through the DP. In that situation it might be better to have no DP and the bearers directly on the concrete as the porous nature of concrete would drain the water off sooner.

ButterCheese

TELLIT01 said:

ButterCheese said:

I would also advise against a pre-fabricated shed as they don't use very thick wood for the supporting structures.  Perhaps buy the timber yourself and get someone to put it together if you can't.  We have asked our local roofer to build us a shed for this reason.

Lastly, I've seen a lots of landscapers use damp proof course or damp proof tape on shed bases as well.  Might be something to consider - I have no way of comparing them to a shed used without anything, but for a small outlay it's probably worth it.  My MIL has a small wooden shed which was put in 10 years ago, dorectly on top of paving slabs.  One side of the shed has now rotted at the bottom and is leaning

Claiming that pre-fabricated sheds don't use very thick wood is a massive generalisation.   

Just putting it out there, having watched several comparison vids on YouTube and also advice from our roofer who put up sheds full time for 6 years.  Yes there are proabbly good ones out there but the majority of people would buy one from a "sheds r us" type of website