Mark Oliver's World

Posted: 19/05/2021

Installing A Garden Office _ Part 6

See the full series of posts, to find out how we got here.

Research

Now I know where the office will sit, and how it will affect the garden, neighbours and fauna, its time to figure out how to get it erected.

The first decision to make, is to self build, buy prefab, or "get a tradesperson in".

Due to the size I require, most prefabs wont fit or will be too narrow. They also don't give the lighting I want without significant cost increases. For something about 3x4m is going to cost in excess of 7k for just a shell. To kit it out with electrics and decent finishes inside and out, its several thousand more. Also most prefabs wont have the insulation levels I require. I need to work in this Office every day of the year.

I decided then to build it myself. It is a real challenge for my adequate DIY skills, and I love that I can may it bespoke for me.
Getting someone in to build it for me is always an option still, but for now, is a last resort!

First up is the choice of base!

My initial thought is that I don't want a concrete base. I have limited access to the garden (I can only get a thin wheelbarrow through the gap, and only at the right angle).
This means its either difficult to get all the sand and cement in the garden, or it needs to be pumped in. My office location is about 200ft away from the road, so cement pumping gets expensive with all the extra hosing required.

This lead me to 2 other options:

  • Ground Screws
  • Pillars

I like these options as they are low impact on the environment they are placed in.

Ground screws

These are as the name suggests, a giant screw (somewhere between 500mm and 1m long), which using a tool, you screw into the ground.
For a building 3m x4m you would need 12 of these.
They are a great idea, but as I found out, they get expensive at that number. I was also concerned about the weight of an office, compared with a shed which they seem to be aimed at.

Pillars

These are great, there are lots of options, concrete shaped pads that just sit on the ground, with an adjustable height.
A great idea, unfortunately again, cost got in the way.

So after talking with my Very Wise Mum who has experience in the building trade, I decided on building block piers.
These are just 2 concrete blocks sat on top of each other, cemented into position and made level.
These cost less than £2 each, so 18 of them (I need 9 piers) with some sand and cement and ballast and hardcore cost less than £100. A significant saving!

9 Pillars you say?

But how do you know you only need 9 pillars?
In the building trade, there is something magical called a "Span Table" which is some mathematical witchcraft that tells you how far apart supports can be between spans of timber of certain dimensions to accommodate certain loads.

These magical tables tell me that with a 47x125mm (2x5") C24 graded timber, I can have a gap of a max of 2.82 metres length between piers.
I have a 3mx3.6m building, so I need supports in the middle of the floor too, hence 9 pillars!

Why did you not just use 2x8" timber instead to give a 4.55m span instead? Simple - the answer is height! I need to keep the building below 2.5m in height, so those extra 3" make a big difference to the internal height. Who wants a low ceiling!

Oh right, smarty pants - Then why did you not use 2x4" instead then, that would work upto 2.24m width. Good spot - I could have, but I wanted a bit more rigidity in the floor, and 100mm insulation, so to keep the insulation off of the blocks, it need to be higher.

What is C16 vs C24 timber?

This basically gives you a level of strength. The higher the number is, the better the wood, and therefore stronger.

Take a look here for a full list https://web.archive.org/web/20180203020533/http://www.wickes.co.uk/helpadvice/buying-guides/timber-buying-guide

Researching all of this was great, and here are a few sites that really helped me:

Whats next?

Building the floor!

Thanks for reading this post.

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