Scott's workshop/office/shed in Surrey - in progress

[Scott]

My dream was to build a workshop which would be big enough to give me a great working space, and somewhere I could create a small office, plus another storage space that I could use for the usual sort of shed storage like the lawn mower aswell as put a chest freezer and tumble dryer in. We'd had a few kids in a relatively short space of time and had been running out of space ever since.

I had a decrepit old shed at the end of the garden which was filled with junk, was falling apart and was on a weak concrete base that was cracked and wonky. On the plus side it did have electricity and a fuse board. The first job was taking it down. It had been built pretty solidly and proved a lot more stubborn than I had expected. I made great use out of a Milwaukee reciprocating saw, a sledge hammer and an enormous chisel drill I'd managed to pick up cheap from Aldi. I needed to hire a couple of skips to get rid of all the wood and concrete.

The before picture! Not my new garden room!

 

[Scott]

I had thought a lot about overall size in the months leading up to starting the build and had finally concluded that 7.2m by 4.2m would be the best. I wanted something as big as I could get while keeping under the building regs 30m2 maximum, and I think these dimensions seemed sensible to me because it meant that I would be able to use a minimal number of 1200mm wide sheets of OSB on the walls by doing it that way and have a more even stud positioning. So, with that concluded I came up with a design for my base.

I drew this up in Sketchup, referencing the appropriate span tables, and deciding on a joist spacing of 600mm. In hindsight, after I gained experience with the rest of the build, I wished that I had simply used 400mm spacing, but having said that it hasn't had any adverse effect.

You'll notice that the spacing of my foundation piers (I had already decided to use ground screws) is not identical on the horizontal. This was simply to take account for my 600mm spacing and keep the number of ground screws to the bare minimum.

 

[Scott]

Next came the ground screws. This was one of my favourite parts of the build, perhaps because I didn't have to do anything other than make some tea every now and then and just sit back and watch!

I had been in touch with Gil at Gravitas for a while, having seen him feature on one of Robin Clevett's builds on YouTube, and discovering he wasn't a million miles away from me in West Sussex. He's a very nice guy and was happy to chat about different options re screw lengths, etc., and what would work best for my garden room, prior to getting the green light to go ahead.

The reason I opted for ground screws was because I didn't want to have to do any more digging myself to level the plot, and I didn't really want to put a massive concrete slab down. I do like the idea of a concrete slab, with the PIR insulation underneath it, as it can save a huge amount of height which is useful if you're trying to come in below 2.5m for permitted development. However, being in the south east and with my plot about 100m from the road, all the quotes I was getting to lay a concrete slab were coming in around £5k-£6k. The ground screws came in around £1500, took comparatively very little time to install and allowed me to start building on them immediately. Additionally I had already been granted planning permission to go to 3m height so that wasn't a concern for me.

On the thankfully beautifully sunny day, Gil turned up with all his equipment. When you get your first look at the screws it does feel a bit like you're in the land of giants, as they do look exactly like giant versions of normal screws.

My son standing next to one of the 1.25m screws.

I gave Gil the plan for my base frame and he marked out the area with line spray and put crosses where each of the screws should go, checking each one and the whole area for square as he went.

He then pre-drilled a hole with a massively long chunky drill bit, before getting all the ground screws into position. Most of the screws I had gone for were 1m long, with a couple on the near left hand corner where the ground sloped slightly 1.25m. This meant that each screw was buried at least 900mm in the ground.

Then it's a matter of getting them all in - Gil uses a number of traditional levels, plus a laser level to ensure everything gets to the correct height while his high powered drill unit slowly drives the screws into the ground. Each screw took about 3 or 4 minutes to get in.

After which you're left with a neat pattern of obstacles you can enjoy tripping over until you get your base on.

Before the install I was worried about how precisely the screws would be positioned and whether they would end up in exactly the right place. There has to be a bit of leeway, and occasionally the screws can hit problems. One of mine reached a point where it simply wouldn't get down to the depth required. Gil had to remove the screw and try to pre-drill the hole again to sort out whatever rock or object the screw was being blocked by. In the end it worked and the screw went in, a bit wonky but nonetheless in the right position. Also, because the screw heads are about 200mm, there is quite a bit of wiggle room when it comes to how you position your base frame on top of them.

 

[Scott]

I'd already started building the base frame based on the sketchup drawing above so was able to test it immediately in-situ once the ground screws were in. My plan was to build 4 rectangular pieces for ease of movement, then fix them all together and surround them with an outer layer to make the two width thickness I was after.

I opted for 5" high C24 timber for the base frame, which had an actual height of about 120mm. The span tables suggested this was fine with even a 600mm joist spacing so that's what I went for.

Once I had the four pieces onto the ground screws I could shimmy them about a bit to a) get everything square, and b) make sure that the heads of the ground screws wouldn't be sticking out visibly on any of the 'good looking' sides. I then fixed the border timber to give me that double outside edge, checked again for positioning and square (using this Faithfull 24" folding square), then bolted each corner to the ground screw using the angle brackets provided. I know many people say there's no need as the weight of the building will keep it in place, but while you're working on the base it can defintely still move, especially as you're banging in joist hangers or the like, and it was so easy to secure I felt it was a no brainer.

I then fitted little wooden stops to the inside edges of all the holes to support the PIR insulation, cut and fitted that, then sealed all the edges with foil tape. I then covered each length of timber with Green Glue joist tape, which I've used with great success inside my house too. It does a great job of preventing squeaking and, while it probably wasn't necessary here, I thought better to use it than not.

I was then able to lay the floor with plywood. I used 22mm sheets as I thought this was better with the 600mm joist spacing. If I were to do this again I'd go for 400mm spacing and use the caber boards instead of ply. Caber boards apparently offer better water protection and my floor definitely suffered after we had an absolute deluge of rain in July and August, even though I had attempted to cover it up.

If you do want to cover up your floor for a while, I'd definitely suggest you use a proper damp proof membrane (DPM) plastic sheet rather than tarpaulin because tarpaulin will leak water through (as I discovered) whereas something like this thick plastic DPM which I bought from Amazon didn't let a drop through and I was able to utilise it again later in the build as a DPM within my roof.

The last thing I did once the plywood was laid was to varnish the entire floor in yacht varnish. It took about three coats and then looked as good as a dance floor. While the varnish did offer some water protection, water did manage to ingress the layers of ply (even though I'd 'sealed' the edges with loads of varnish) and this is again why I'd go with caber boards if I started again.