Concrete Base Question

[John Smith Esq.]

Oi oi!

I'm constructing a shed on to this concrete base next weekend. It is slightly too small but providing the trench you can see is filled with something, it will be big enough. It has had bricks of some type in the trench which I have removed because they were slightly proud of the concrete surface. I've cleaned the underlying hard surface of the mortar bed and the idea was to put the bricks back. They are still slightly proud though. The thing is, I'm planning to put seven pieces of 3x2 down length ways across the base, and then put the shed on those (the shed's bearers are width ways) to protect it from moisture and hopefully deter rodents. One of these seven timbers will need to go along the full length of the trench as is this is where the front all of the shed will be.

So, how best and easiest to fill the trench?

- I don't think the bricks are going to be even enough because of the underlying surface.

- could mix concrete on the base and then scrape it in to the trench. But will it set level with the existing surface or shrink a bit?

- just fill it with pea gravel and put the timber over the top. Edit: This will not work as those flags stones on the other side of the trench are lower.

- put smaller driveway blocks on beds of mortar and have one every foot or so through the trench like piers.

The trench is 80mm wide x 100mm deep.

Thoughts?

 

[Alastairreid]

If it was me I would build a couple of courses of brick round the base at the size you need and use the existing base as your shed floor.
A course of dpm on top of bricks and build your shed from that.

 

[John Smith Esq.]

I don't have time or room for that, so will have to use the floor it comes with. Interesting idea though.

 

[robinbanks]

Concrete or driveway blocks, whichever you prefer.

 

[noseall]

Oi oi!

I'm constructing a shed on to this concrete base next weekend. It is slightly too small but providing the trench you can see is filled with something, it will be big enough. It has had bricks of some type in the trench which I have removed because they were slightly proud of the concrete surface. I've cleaned the underlying hard surface of the mortar bed and the idea was to put the bricks back. They are still slightly proud though. The thing is, I'm planning to put seven pieces of 3x2 down length ways across the base, and then put the shed on those (the shed's bearers are width ways) to protect it from moisture and hopefully deter rodents. One of these seven timbers will need to go along the full length of the trench as is this is where the front all of the shed will be.

So, how best and easiest to fill the trench?

- I don't think the bricks are going to be even enough because of the underlying surface.

- could mix concrete on the base and then scrape it in to the trench. But will it set level with the existing surface or shrink a bit?

- just fill it with pea gravel and put the timber over the top. Edit: This will not work as those flags stones on the other side of the trench are lower.

- put smaller driveway blocks on beds of mortar and have one every foot or so through the trench like piers.

The trench is 80mm wide x 100mm deep.

Thoughts?

Stick another layer of slabs on top of those old ones, and you have either a bigger shed or a porch hardstanding.

 

[John Smith Esq.]

Hello, thanks for the responses. I think there has been a misunderstanding.

There is a concrete base there already. I am going to put the shed on to that. However, the base needs to be widened slightly. There is a trench you can see in the picture which I have removed bricks from. They stood proud of the concrete surface. So how best to quickly and cheaply extend the base into that trench? Bearing in mind I will have timbers running length wise to put the shed on to, including along the top of where that trench is.

My thinking is ready mixed concrete prepared on the base and then scraped into the trench. But will it shrink and end up being lower than the rest or the best? Or have driveway bricks on beds of mortar through the trench, to the same level of concrete surface.

Thoughts?

 

[Deluks]

Sheds last much longer when raised off the ground.
If you buy a bog standard, off the shelf shed, it usually sits on timber bearers built into the floor. These are usually quite small sections of timber and can often end up sitting in a puddle or standing water, especially in a concrete base.
If you can get some cheap paving slabs under the corners and middle of the shed, these will keep it clear of the ground and provide ventilation beneath. You could cantilever them over that small gap and it wouldn't be a problem.

If the bricks are frogged you could trim them down with a hammer and bolster or grinder and put them back in the hole.

 

[robinbanks]

I think there has been a misunderstanding.

No misunderstanding. I approve of your suggestions 1 (concrete at relevant points or full fill) and 3 (driveway blocks on mortar at relevant points)

ready mixed concrete prepared on the base and then scraped into the trench. But will it shrink and end up being lower than the rest

Readymix needs no prep; it turns up in a wagon and pours out ready to use. You wouldn't order readymix for this small of a job, you'd use a self mix of 3/2/1 gravel/sand/cement

Concrete doesn't shrink appreciably enough to bother about here; your 100mm fillet might end up 99.95mm. If you're building a shed to the level of accuracy where the slab shrinkage would present an issue, it'll the most intensely engineered shed in the world.

Personally I wouldn't have bothered taking the bricks out, I'd have just ripped down the piece of wood that was due to sit on them so its top is level with all the others, but we are where we are!

Side note, you can get plastic joists/fence posts/batten etc these days; may make for a better material to sit a shed on than wooden ones if you're concerned about longevity/the slab has any hollows. If it has no standing water, less of a concern

 

[John Smith Esq.]

No misunderstanding. I approve of your suggestions 1 (concrete at relevant points or full fill) and 3 (driveway blocks on mortar at relevant points)


Readymix needs no prep; it turns up in a wagon and pours out ready to use. You wouldn't order readymix for this small of a job, you'd use a self mix of 3/2/1 gravel/sand/cement

Concrete doesn't shrink appreciably enough to bother about here; your 100mm fillet might end up 99.95mm. If you're building a shed to the level of accuracy where the slab shrinkage would present an issue, it'll the most intensely engineered shed in the world.

Personally I wouldn't have bothered taking the bricks out, I'd have just ripped down the piece of wood that was due to sit on them so its top is level with all the others, but we are where we are!

Side note, you can get plastic joists/fence posts/batten etc these days; may make for a better material to sit a shed on than wooden ones if you're concerned about longevity/the slab has any hollows. If it has no standing water, less of a concern

Thanks for the response and I like your idea about just ripping the timber and leaving the bricks as they were! good to see the different thinking people offer which is why I like this forum. Yeah, I didn't mean ready mixed, I meant like bags of pre mixed concrete.

I've actually gone with Deluks's suggestion in the end. A lot less mess and hassle, and I'm limited for time. I just bought 15 cheap concrete flags and have put them over the base, equally spaced apart. Five of them overhang the trench by about 50mm. This approach helps to iron out some unevenness on the base surface; will get the shed off the ground (I'm going to staple DPC to the bottom of the shed's bearers as well), and will mean lots of ventilation beneath the shed. And also, it gets past the trench issue very easily. I'll just fill that with gravel at some point. I suppose the only downside would be if the cantilevered flags broke off, but this doesn't seem likely as only 1/9 of each flag would have nothing under it.