Foundations for garden building

[leem2837]

Hi, I've seen numerous opinions on this, but still not sure which way to go.

I'm going to build a garden room at the end of my garden - 7m x 5m. I know many recommend a full concrete slab as a base, but it's not practical in terms of either making it or the cost. It's 120 feet from the house, with no easy access from there anyway.

So I'm looking at alternatives (have been for months TBH) but could do with some guidance...

One option is piers - concrete piers and a timber frame beneath the main building - or there's strip foundations.

To be honest I've never done either of these, but making my own piers sounds like an affordable and workable solution. I've seen pre-cast piers from companies like Swift, but that still comes to around £1200 for my size. My other concern is it's suitability for a structure of this size?

I'd appreciate any advice from anyone who's done this before... Or if there are any other suitable approaches...

 

[theprinceofdarkness]

This is a huge shed, what are you going to put in it? if its a tractor then you need a very solid floor to take the ton or more of weight. If its going to have a table tennis table in it , the the weight should be a bit less.
I don't know where the breakeven point is, but the more piles you have the lighter/cheaper the wooden frame work can be for a given load. The other thing is the floor, if it is laid inside the strip foundations or inside the piles and not part of them, then again it can be lighter, resting on hardcore on the soil, and being designed to "settle" (drop) inside the walls.
Frank

 

[jeds]

I'd look at precast concrete pads. You'd install them just below ground - depending on grounds conditions - possibly on a ballast bed, with timber posts on top and build everything off of that. This is a very old fashioned system which easily allows you to adjust settlement later. It does depend on ground conditions and how much settlement is acceptable though. I wouldn't try it on soft, peaty mush.

 

[leem2837]

@theprinceofdarkness:

It's a garden room rather than a shed. So a mix of home office / gym equipment.

I was intending to build the joists off the wooden base frame so that it would be fixed?

@jeds: I saw the Swift plinth system, but it was coming in at around £1100 for the size I need! I can't seem to find a decent alternative, but I'm sure there must be one...

 

[theprinceofdarkness]

You say the "Swift" system is £1100, what is this based on? I once built a garage on piers I cast in situ, 22 off at 9" diam with steel rebars X 1.2m deep. Much cheaper system. The only advantage the Sift system has is that it has the variable height adjustment. This can be don on a DIY basis by terminating the pier in a big stainless steel thread (bit of studding). the other advantage is that it looks neat, this can be done by terminating your pier with a 9" X 9" precast wall coping stone, with a hole drilled through it for your studding. ( http://www.marshalls.co.uk/homeowners/view-precast-wall-caps-and-copings.
I used a 9" post borer with an extension to dig the hole, through a length of 9" sewer pipe to stop the earth falling into it. A couple of big black bags were then taped together with the bottom cut out of the top one. This was then dropped through the sewer pipe, the reinforcing put in, then filled with concrete and the sewer pipe wriggled out. In your case, the concrete must terminate a couple of inches below soil level and you must have wired your studding to the reinforcing and made sure that it is central and upright. Splodge a bit of mortar on the top of the pier, and thread the capping on and do a nut up enough to squeeze out a bit of mortar.
Cost?, all depend on the size of the piers, in my case 1m X .225 Diam ~ .04 Cu m or 40 l (2 1/2 bags of ballast)@ £2.5 each + steel £2? + cement £1? - much cheaper if you but a dumpy bag of ballast £50 for .7 Cu m.
Frank

 

[leem2837]

That sounds like a good approach. I'm not overly concerned with appearance as they shouldn't be visible once finished anyway, but the way you've done it sounds great.

I think I'll make the piers myself in the way you've done yours, then maybe add these ( http://www.tuin.co.uk/Timber-Frame-Base-Pads.html ) to give me an adjustable element on top if I really need it. On the other hand if I make sure the piers are level these aren't really needed are they?

One question - why would I need the concrete to finish below soil level? I want to build a wooden frame on top of the piers, and provide some air flow, so wouldn't I need the piers to be at least an inch or so above soil level?

 

[theprinceofdarkness]

With my suggestion, the coping stones would be sitting in the grass, so hiding the pier tops. The timber base pads have been discussed here before, I do not like them, they do not look very robust.
Frank

 

[garyo]

With my suggestion, the coping stones would be sitting in the grass, so hiding the pier tops. The timber base pads have been discussed here before, I do not like them, they do not look very robust.
Frank

This is something I'm considering myself, and I like the stainless steel studding approach suggested above. How do you interface the studding to the timber frame, Frank? Simply a stainless nut and then a flat stainless place with a hole in the top, or do you fabricate some stainless 'cradles' to hold the joists more securely.

 

[theprinceofdarkness]

I would have thought that the decking just sitting on the washers would do. In fact thinking about it, I think that the tops of the studding will need to be chamfered, unless your drilling is exceptionally accurate. If you are worried about the decking blowing away, drill right through and counterbore the top of the joist to take a nut. So when the height nuts are sorted, tighten the top one with a socket then cut off the excess studding with a very thin disc in a angle grinder.
Warning:- stainless nuts on stainless studding will cause "galling". The threads actually bind, there are many threads on the web about "anti galling compound". Using any other sort of nuts on stainless will be OK.
Frank