Small outbuilding foundation where it's wet and windy - pier & beam or slab?

[sniederb]

I'm planning to put up a bird hide in our back yard, Western Isles (Scotland). The structure itself will be timber, approx. 3m x 2m and 2m high, slanted roof (maybe with solar panels). The structure doesn't need insulation, but damp and rot is a concern (did I mention Western Isles?) The ground seems to be a mix of gravel and soil about 50cm deep, then solid rock. (The image shows roughly the intended size, using some spare clay liners to indicate the corners)

 

I'm struggling to decide on the type of foundation. We're quite a bit away from the road, so the amount of concrete needed is fairly relevant.

 

My understanding so far is that I could do pier & beam, which would mean to dig 4 holes, use some sort of sonotube and put in beam connectors. The alternative would be a monolithic slab, which would mean digging the entire area down about 20cm, add a frame on top and then fill it all with concrete.

The pier & beam option is attractive because it needs so much less concrete, but what I read on this forum is that most people put down a slab.

 

What are the things I need to consider to reach a decision?

[MikeSharp01]

I was thinking just yesterday about how to provide foundation for our lean too shed on the garden room for batteries, the solar panels are on the roof of the garden room. My analysis was that the whole decision came down to permanency and how much and material I would need, there seemed like three options. Slab, Post and Concrete beams with topping of screed, Post and Wooden Beam (which I assume is what you are considering - but I may have that wrong). In the end I decided to go for the concrete slab approach as it is only a small area (1m²) and will give me a simple install but I can wheel barrow the concrete in, albeit with difficulty. The alternative concrete beam approach was expensive and complex for such a small space and the wooden approach, although the simplest was not the most enduring. 

 

So I am going for concrete slab but with your bigger space and difficult access I would go for the wooden post & beam approach and make sure you have good ventilation below to keep it as dry as you can.

[Conor]

The 3x2m timber structure you describe is basically a large shed. Assume no need for running water, tiled floor etc? Basically box, opening on one side and a bench seat or similar? It's not going to weigh much. You could get away with putting it on timber stilts, sitting on paving slabs. Or even on a trailer chassis if you ever need to move it. No concrete needed either way.

Edited January 13 by Conor

[sniederb]

5 minutes ago, Conor said:

The 3x2m timber structure you describe is basically a large shed. Assume no need for running water, tiled floor etc? Basically box, opening on one side and a bench seat or similar? It's not going to weigh much. You could get away with putting it on timber stilts, sitting on paving slabs. Or even on a trailer chassis if you ever need to move it. No concrete needed either way.

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

[JohnMo]

My pallet shed is just on concrete blocks levelled. Our summer house is on heavy concrete 3x2 slabs.

 

Or do screw piles - no concrete

[AartWessels]

40 minutes ago, sniederb said:

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

I've built a shed slightly bigger on 6 concrete piers of 400mm high. Dug them in, and put 2 4x4 beams in parallel on top of them.

Then, built the floor frame on top of these beams. It gives some ventilation underneath to allow things to dry. I've built a large decking in more or less the same way. Saved me from hammering in hundreds of stakes 😂

[Conor]

59 minutes ago, sniederb said:

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

I don't think you need to pour anything. A stack of pavers or concrete block to a couple hundred mm above the ground. Maybe one in each corner and a couple on the long length. Then just set the timber base on top. If it's exposed you'll want some sort of tether sunk in to the ground. 

[sniederb]

30 minutes ago, Conor said:

I don't think you need to pour anything. A stack of pavers or concrete block to a couple hundred mm above the ground. Maybe one in each corner and a couple on the long length. Then just set the timber base on top. If it's exposed you'll want some sort of tether sunk in to the ground. 

Conor, I'm worried about the strong winds we get. So the "tethers sunk in to the ground" is why I'm thinking to pour concrete piers.

[sniederb]

51 minutes ago, AartWessels said:

I've built a shed slightly bigger on 6 concrete piers of 400mm high. Dug them in, and put 2 4x4 beams in parallel on top of them.

Then, built the floor frame on top of these beams. It gives some ventilation underneath to allow things to dry. I've built a large decking in more or less the same way. Saved me from hammering in hundreds of stakes 😂

Cool, that sounds pretty much exactly like what I have in mind.

[Galileo]

On 13/01/2024 at 09:32, sniederb said:

Conor, I'm worried about the strong winds we get. So the "tethers sunk in to the ground" is why I'm thinking to pour concrete piers.

I was thinking the exact same thing, if the Western isles are anything like Shetland, if it's not fixed firmly to mother earth it will make its way to Norway rapidly.

 

Built a storage shed on my croft back end of last year, made concrete piers formed with an upturned plastic bucket with the bottom cut off, cardboard concrete formers are not common/cheap in the UK for some reason. I used 6 piers, so I could save on timber thickness for the beams and use what the builders merchants had in stock, getting chunky Douglas Fir off the shelf is not an option up here.

[kandgmitchell]

A few years ago in order to build a deck over very soft wet ground I set short lengths of 4" x 4" treated fence posts into the ground surrounded by post mix. Once cut to the right height I then coach bolted 7" x 2" joists to the faces to make a frame, infilled with smaller joists and decked over.

 

Could you use the same system and just frame your walls off the deck/floor so the hide is essentially sat on timber piles which provide support and anchorage.

[sniederb]

An update to this thread. I've put in 4 piers, each about 50cm deep and filled with concrete. 4x4s are the supporting beams, onto which our timber frame is built. The roof ridge is at about 2.5m.

 

Now a local builder is telling me that those pier won't withstand the winter gales here (80mph wind is not uncommon, two years ago a gust was measured at 100mph). He recommends weighing down the frame with either large concrete blocks or gabions. I definitely won't disregard advice from a local builder, but the same time those concrete piers were the whole point of securing the shed. Any thoughts on this?

[George]

I think it is unlikely the piers would pull out (although would have to run the calculation to be sure), but there is a lack of bracing in the walls and roof, and no holding down / restraint straps in the superstructure (roof, walls) down to the foundation. 

 

For 100mph gusts on a lightweight roof every rafter will want some sort of strap.

Edited June 10 by George

[ProDave]

As well as the above, panel in the space underneath the shed all around and tightly, so the wind can't get underneath it to try and lift it.

[sniederb]

16 minutes ago, ProDave said:

As well as the above, panel in the space underneath the shed all around and tightly, so the wind can't get underneath it to try and lift it.

Thanks Dave, I'm definitely planning on doing that. I'm leaving the underneath space open for now because I want to keep all options open as to the "weighing down". 

[saveasteading]

All the above are good. Many 'temporary' cabins have sat on paving slabs for decades. 

I would let the wind whistle underneath it. Then tie it down.

So have steel straps under or into the concrete bases, and fix them hard to the structure.

 

[joe90]

Isle of Lewis 🤷‍♂️I would want straps/cables holding the whole thing down, roof on, perhaps ground screws?

https://tgsupplies.co.uk/product/heavy-duty-ground-screw-pergola-ground-anchor-gazebo-tent-metal-pegs/?utm_source

 

or similar.with cables over the building.

Edited June 10 by joe90

[sniederb]

8 hours ago, George said:

I think it is unlikely the piers would pull out (although would have to run the calculation to be sure), but there is a lack of bracing in the walls and roof, and no holding down / restraint straps in the superstructure (roof, walls) down to the foundation. 

 

For 100mph gusts on a lightweight roof every rafter will want some sort of strap.

George, thanks for the feedback. Currently there are brackets on all vertical joists (one each). The rafters are screwed on vertically and horizontally. Would you add more brackets / straps?

[SteamyTea]

Here is a way to work out the loads, then you can calculate how much mass to add to counteract them.

 

https://www.structuralbasics.com/wind-load-calculation-on-walls/

 

Wind speeds at ground level are very low, and short of the shed being at the top of a hill, are significantly lower than the Met Office shows, which are measured at 10 metres.

Edited June 10 by SteamyTea

[saveasteading]

We have a big shed laid on a slab. Not an especially exposed site, with hills and trees around.

it should have been fixed down.

In the wind it blew across the slab about 2m, and that was just a windy night, not a gale. 

We put car tyres on the roof for dead weight, but that wasn't enough. It is now bolted down to the slab.

 

As there isn't risk to life, you must make your own judgement, but overdo it, don't underdo it.

These brackets are not strong in themselves, and could bend  at the screw or rip out at the first screw.

I would suggest using a heavier gauge of thread with a wider head or a washer. 

eg a decking screw. 

 

[sniederb]

1 hour ago, SteamyTea said:

Here is a way to work out the loads, then you can calculate how much mass to add to counteract them.

 

https://www.structuralbasics.com/wind-load-calculation-on-walls/

 

Wind speeds at ground level are very low, and short of the shed being at the top of a hill, are significantly lower than the Met Office shows, which are measured at 10 metres.

Using https://skyciv.com/wind-load-calculator/ I get the attached chart. Bit lost with that, I confess. What does that mean for my shed?