Last week in Building a Workshop, Part 4 I’d just finished
erecting the 14 x 6 metre workshop. The walls were up, the roof done and the two
roller doors in place. In fact, the construction was all finished, eh?
The council needed to come and inspect the work,
and before they’d give the OK, I reckon quite a lot more work was needed. What
sort of work, then? Try drains and stormwater collection!
Now I never thought I’d be writing in AutoSpeed
about digging trenches and installing drains, but if my workshop was to get the
council tick, I figured they were a necessity. To put this another way, it’s
very likely that anyone building a home workshop will also need to consider this
The workshop is situated on land that has been cut
and filled. That is, the original land was sloping and to level it for the shed,
the soil on the hillside was scooped out and that soil placed lower on the
slope. To hold up the upper slope, a ~1 metre rock retaining wall was built.
When this wall was installed, the contractor
placed a perforated plastic drain pipe behind the bottom rocks, back-filling
over the pipe with coarse gravel. However, no attempt was made to give this pipe
a ‘fall’, so consequently, I have never seen any water come out of its end!
Instead, what I have seen is water pooling between
the base of the retaining wall and the shed slab, a distance of about 600mm. The
soil is impermeable clay and because this strip is always shaded from the sun,
after rain the standing water could remain present for a week or more.
Furthermore, in times of really heavy rainfall, I thought it possible the water
could reach the level of the concrete shed floor.
I could see most of this before the shed slab was
poured. I even said: “Hmm, I reckon that will need a drain – and it will be
easier to install it before the shed is built!” However, I did nothing – stupid,
Why ‘stupid’? Well, having now installed the
drain, I know for sure that I should have done so before the shed was erected –
it would have been far easier to dig the drain, and with a solid surface on which to work (rather
than mud), also far easier to build the shed!
Anyway, after the shed was built, I decided to
install a slightly subsurface drain. This comprised a 20 metre long trench dug
to an average depth of about 75mm. At the point where the water pooled, the
trench was only about 50mm deep, but at the end where the water would discharge,
it was about 150mm deep.
Digging the trench was a right pain in the bum –
what with rocks that had to be broken up with a sledgehammer and crowbar, and
tree roots that had to be individually cut with a saw. Not to mention shovelling
the sticky clay while working in a narrow space!
Along the base of the trench I placed a layer of
... then inserted a 20 metre long perforated
drainage pipe, 50 mm in diameter. Over the top of the drainage tube I placed
more gravel, in fact enough to fill the trench and then cover the surface soil
between the base of the retaining wall and the shed.
The gravel is both porous (there are lots of
spaces between the stones) and permeable (the spaces are connected). Water can
therefore flow down to the perforated tube, but in addition, water can also flow
down the slope of the trench by passing through the interconnected gravel pore
Putting the gravel in place took a long time. The
distance between the wall and the shed is too small to fit a wheelbarrow, so
each shovel-load of gravel had to be carried down individually. Perhaps 200
shovel-loads were needed.
So that was the drain finished on the southern
side – now, what about the other sides? On the north and west sides I couldn’t
imagine water accumulating – the steep slopes are away from the shed. However,
what I could see happening was these areas becoming very wet with run-off from
the walls of the shed. Run-off from the walls occurs when there is wind-driven
(If I seem to be talking about rainfall a lot,
there’s a good reason. Where I live in the Gold Coast hinterland, the annual
average rainfall is 1500mm; it’s not unknown to receive 200mm in one day!)
To prevent water making a mud bath at the base of
the walls, I decided to place gravel ‘paths’ (they’re not really paths!) on the
north and west sides. Because there was sufficient space to use a wheelbarrow,
and the gravel didn’t need to be as deep as that used over the drain, these
‘paths’ were fairly quick and easy to put in place.
The gravel and the drain tube cost AUD$220 and this
work took two days.
Another aspect I expected the council inspector to
be looking at closely was the stability of the slopes. Any slope shallower in
angle than 45 degrees is normally regarded as OK, but again with a high annual
rainfall, you don’t want steep slopes that are not covered in vegetation or
So much to the astonishment of my neighbours, who
haven’t seen me do a scrap of gardening in 8 years, I found myself spreading
mulch all over the place! I bought six bales of compressed sugar cane mulch and
spread this over all the bare slopes. This was a quick and easy job, taking only
a few hours. The mulch cost $30.
The gutters and downpipes were provided as part of
the kit for the shed. However, I chose not to use the square downpipes, instead
fitting adaptors directly to the gutter outlets and using 90mm round U-PVC pipe.
These pipes were run to a 13,500 litre rainwater
tank that had been installed on a pad made at the time of the original shed
earthworks. The tank cost $2163 but the Queensland government reimbursed $1000
of this cost. The 90mm pipe and fittings cost $130.
Where I live there is no reticulated water, so the
house is run entirely on rainwater. The workshop tank is not actually plumbed
into this system (although this could easily be done if the main, larger tank
ran low). Instead, the main purpose of the new tank will be to supply vegetable
The installation of the plumbing – and painting it
– took about a day.
After I’d made the appointment for the shed to be
inspected by the planning authority (in my case, the local council), I was
rather worried. This was primarily because I didn’t know what was going to be
Would the inspector run around with a plumb-bob,
assessing the verticality of every column and wall? Or would he have a torque
gauge and measure the torquing of every bolt and Tek screw? Or would he simply
glance at the shed to confirm it looked something like its plans, and then look
more thoroughly at the stability of slopes, heights of retaining walls and
handling of run-off?
I didn’t know – and so I was worried!
In actuality, the inspection was somewhere between
the two extremes. The inspector carried a tape measure and the engineering
drawings. He inspected every column and rafter to make sure it was the correct
size and gauge; he inspected the base of every column to make sure it was bolted
to the floor; he inspected every corner trim piece to ensure it was attached; he
measured distances from the boundaries; he looked at the rainwater tank and the
Tek screws and bolts. But he didn’t ask any questions about drainage!
After about 20 minutes of looking, measuring and
noting, he made two comments. The first was that the overflow of the rainwater
tank should be plumbed away from the base so that the falling water would not
scour the fill on which it was standing, and the second was that in times of
very heavy rainfall, I might need more downpipes. The second was a suggestion,
the first I took as a command!
The inspector then handed over the signed
approval. Interestingly, this document has just three categories:
And there was a tick in each box!
Now, could I say the workshop was complete? Nope –
not without electrical lighting and power.
Next week - lighting
Go here for the next in this series.
Interested in home workshop projects and techniques? You’re sure then to be interested in the Home Workshop Sourcebook, available now.