The Workshop Construction: External Cladding

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The cladding is Douglas fir from a local saw mill (English Woodlands). The planks are 8″x0.8″, sawn, and work out at about  £13 per m². Douglas fir is one of our cheapest UK grown timbers yet has many characteristics that make it superior to standard softwood. UK grown Douglas fir is classed as only slightly durable – apparently our weather makes it grow too quickly, hence it is often recommended that it is preservative treated, but this is not necessary if the design and detailing are good. An excellent example of untreated Douglas fir external cladding can be found in the ecological building of the Scottish Renewable Energy Office for Natural Energy.

Design features to ensure longevity of the cladding:

  • Ventilation is provided behind the boards by fixing them to tile battens. An air gap is left at the top and bottom of the cladding so that air can flow freely behind them
  • Water repellent stain keeps moisture from penetrating the wood – OSMO One Coat Only ebony stain is highly water repellant and was applied on all edges before installing, and on the front and back of the overlapping sections as these will be impossible to paint later. The end grain received additional attention.
  • Stainless steel screws prevent rusting. Note that the screws on one board are not covered by the board above, so it is possible to replace a board without having to remove those above or below. Screws were pre-drilled to reduce splitting.
  • End stops terminate each run, reducing exposure of the vulnerable end grain.
  • Roof water is carefully directed away from the cladding via downpipes so that water does not cascade down the boards. Good overhanging eaves help too.

The above detailing should ensure a very long life – in excess of 30 years I trust. Maintenance will consist of replacing any cracked boards and occasional touching up of the stain. Re-staining is easy with OSMO One Coat Only – it does not peel or flake and you can just put another coat straight over the top at any point.

Back of the building

As the back of the building is rarely seen and difficult to access for repairs I have used a maintenance free cladding – Coroline bitumen corrugated sheets. These were fixed with screws and special rubber/stainless steel washers to the tile battens. They overlap to ensure they are water proof. They are certainly not lookers, but they should never need painting!

The finished building

Here is the finished building… well, almost finished.  The cladding and colour help it fit in with the other garden buildings, and it is a damn sight better looking and better constructed than the tumble-down sheds it is replaceing!

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The Workshop Construction: The Green Roof

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The deck was laid with 16mm Smartply (OSB) decking boards. This was a nice job as these boards are matched (T&G) on all four sides and slot together tightly. They were then screwed down to the rafters, quickly making a firm roof deck that could be walked on whilst it was worked on.

Arris rail was then attached all the way round by screwing through fom the underside and 8″x1″ fascia boards screwed to this to form an upstand.

I fitted flat-roof rainwater spigots into each end of the front of the roof. To prevent pooling behind them they needed to be recessed flush – a router came in handy here.

Butyl Roof Liner

The one-piece, 1 mm butyl liner was supplied from a DIY flat roof company – you simply supply the roof dimensions and it arrives in one (extremely heavy) piece. Getting it up on the roof was one of the hardest parts of this whole project! However, once up it was relatively easy to install. First, glue was applied over the timber deck, then the liner was rolled out and pressed into place with a broom.

In the corners a fold was necessary. The only cuts made were for the rainwater spigots, and here the butyl was bedded onto a ring of mastic to seal it.

The edges of the liner were folded over the top of the 8″ fascia board then pinned in place by a 4″x1″ timber that was screwed flush with the top of the fascia all the way around. Any excess butyl was cut off, leaving a neat finish.

Pond underlay was then added as an overlay(!)

This protects the butyl from the gravel that is used to create the drainage layer that comes next. (Although it is called pond underlay it is just carpet underlay as far as I can tell!)

A treated timber retaining frame was constructed to eventually hold the soil. It is raised 1cm on short posts and it sits under its own weight on the underlay. Within the frame pea shingle was placed in a 20mm layer to create a drainage layer. Around the outside 20-40mm angular shingle creates a plant-free border that ensures free drainage to the spigots. Note that the spigots have leaf-guards, primarily to keep the shingle out, and these need to remain accessible so they can be easily checked and cleaned if necessary.

Before the growing medium, the inside of the frame was lined with woven landscape (weedproof) fabric. This prevents soil washing down and blocking the drainage layer. The growing medium was then lifted, bucket by bucket, up onto the roof. This was the most exhausting repetative job and took much longer than I expected! The growing medium I used was supplied by Aldingborne Nurseries and consists of 50% sand and 50% recyled green compost. Good green-roof growing medium should be low fertility and free draining, hence the sand, and water retentive, hence the compost.

Finishing touches included a 6″x1″ faschia board screwed to the 4″x1″ around the edges, and finally, a 4″x1″ capping rail that frames the whole roof – we painted it a contrasting white.

Here it is, finished with some of the early planting installed:

Green roof construction drawing:

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The Workshop Construction: Walls and Rafters

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I constructed the walls as a series of timber-framed panels. The concrete base provided a suitable level surface to work on.

The framing is nominal 2×3 regularised studwork. Many sheds and garden buildings from DIY centres are made with much smaller gauge timbers. 2×3 is much stronger and create a really solid, robust building. 2×4 studs are used structurally in modern timber-framed house construction, even several floors high, so 2×3 will be perfectly adequate for garden building purposes!

I chose 2×3 studs for a number of reasons:

  • Strength – I need to be able to carry the weight of a green roof.
  • Depth – plenty of room for insulation, electrical conduit etc.
  • Cheap – as this is a standard construction material it is relatively cheap.
  • Handling – ‘regularised’ timber is nice – semi-planed and with the corners rounded off. It hardly ever gives splinters and is easy to handle.

 As you can see in the photo above, I laid out the studwork and screwed it together flat on the ground. I then screwed a sheet of 12mm OSB board to it, which gives the studs great rigidity. The edge of the OSB acts as a square, keeping the panels true. The OSB overshoots the top of the frame as it will be trimmed to fit the rafters later.

You can nail the studwork frame together but personally I prefer screws – slower and more expensive, but easier to undo and adjust.

Next, I stapled a modern breather membrane to the outside of the OSB. This will keep water that gets through the cladding out of the building whilst allowing any moisture inside the building to diffuse out. The membrane was allowed to hang down a few inches below the bottom rail to cover the joint between it and the concrete base.

This level of construction is virtually identical to that of a modern timber-framed house. Practising it on a garden building is good way to get experience if you ever consider building your own home.

Erecting the walls

The wall panels were then lifted up from one edge until they were vertical. Temporary diagonal braces held the panel upright until an adjacent panel was raised and screwed to it. By erecting two corner panels they quickly become self-supporting. Note the OSB board used as a square to make sure the walls are a right angle.

Although this method of construction is relatively quick and most of the work can be done by one man, it is advisable to have a second pair of hands available for a few minutes to assist when a panel is being raised. It is also advisable to avoid windy days!

This image shows the walls at the almost-complete stage. The tile batten which will carry the cladding is clearly visible over the breather membrane.

The door and window frames were built in while the panels were laying flat on the ground simplifying the construction.

Although the structure is clearly not weather-tight at this stage, the materials are effectively weather-proof for even several months – little harm would come to them from even heavy rain: OSB is quite moisture resistant in the short-term. What you would have to worry about, though, would be strong winds which could wreck the structure at this stage as the walls are like sails. For that reason if you did need to leave it plenty of props and cross ties would be in order.

Raising the rafters

For a flat roof, and especially a green roof, there needs to be a fall of about 1:50 minimum. The usual method is to buy wide joists, say 2×7 inch and cut them along their length so they taper from 7 inches  at one end to 4 inches at the other. I find long rip cuts awkward and tedious. Also the timbers are expensive. With the vision of all this ahead of me I tried to take a short-cut: I made my rafters out of two 2×4 timbers screwed together, one set at an angle to the other, so that they effectively created a single rafter with tapering dimensions, without a rip cut in sight! This certainly saved time, and if I had not built a green roof on top of my building I might have got away with it. Trouble is, such composite beams are never going to be as strong as a single piece of timber: with the weight of the green roof the rafters bowed significantly! 

 

Before the rafters went on a 2×3 top rail was screwed all the way round along the top of the studwork framing to tie everything together and provide a solid bearing for the rafters.  The horizontal, ceiling rafters (joists?) rested on the toprail within the OSB upstands of the panels. The diagonal, roof rafters were then fitted into a notch cut into the upstand of the OSB board, using a jigsaw, so that they projected out at each end to create an overhang. The ends were cut to an angle for appearence as they would be visible when the building was finished. At the higher end the diagonal roof joists rested on 2×3 noggins fitted between their companion horizontal ceiling joists. This provided a consistant fall.

Noggins were placed between the rafters to create a level top edge, and the remains of the OSB upstand cut off flush. The breather membrane was then carefully cut and folded over the top of the building and stapled in place, and tucked in around the joists to make a tight fit. You can just make out all of these details in the image above.

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The Workshop Construction: Creating the base

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Creating the base

As this building was to have a concrete slab base, the first job was to clear the ground and level the site. I then marked out the perimeter carefully and edged it with treated 6×1 boards held in place with stakes. I took time checking the frame was square and the edges level – you can’t easily correct irregular concrete later!

Type 1 sub base (MOT) was then barrowed in and raked level. This was was then compacted with a vibration plate until it was rock solid. As the ground beneath was sub soil I only used 2in of sub base, if the ground had been softer I may have needed a greater depth or a layer of hardcore first. The compacted sub-base was then blinded with a thin layer of builders sand and whacked again.

Next I covered the whole area with a single sheet of polythene as a DPC, folding it up the inside of the boards, creating a huge waterproof  ‘tray’ ready to take the concrete. Placing a couple of additional boards across the site divided the base into three areas – the idea being that the concrete slab is less likely to crack if there is any settlement or expansion.

I reckoned the concrete needed pouring in one continuous operation – a job I did not trust myself to do alone and complete in one day. So I employed a team of  local builders who mixed, barrowed, poured and levelled the concrete for me in a few hours!

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The Workshop Design

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Planning Constraints

Which style would you find most intrusive as a neighbour? Current permitted development regulations only allow the flat roof solution.

 My original plans were for a long shed with a pitched cedar tiled roof like the Cabin which we built a few years earlier. Aesthetically this would have helped the new ‘shed’ fit in well visually in the garden. However, a few days before I was about to start building it I discovered quite by chance that the planning regulations had changed and my planned design would now be illegal!

The current* regulations stipulate that any garden building built within two metres of a boundary must be no more than 2.5m (8′ 4″) high. This meant that only a flat roof structure would be possible… Back to the drawing board.

These new regulations are, presumably, meant to limit offence to neighbours by minimising the height looming over the fence and blocking light etc, but they do not necessarily work: With a considerately designed pitched roof the eaves to the rear (close to the fence) could be as low as 1.8m (6′) – i.e.  level with the top of the boundary fence. Sure, the ridge would be higher, but the roof to it would slope up and away from the boundary, minimising the visual impact and shading potential. The requirement to limit the maximum height to 2.5 m makes it almost impossible to design a sensible pitched roof structure that complies – and a flat roof needs to be close to the full 2.5 m high to work, if that is, like me, you wish to use full height standard doors (2.1m including frame), and have a sensible pitch. This means the neighbours look out onto a vertical rear wall rising 60cm (2′) above the fence line – far less attractive and more intrusive.

Rant over.

The ‘shed’ I designed, therefore, would have to have a flat roof. However, I turned this limitation into a creative opportunity and created a green roof. This was no small matter – green roofs require considerable attention and planning to make them successful! (See Green Roof design details here) 

Designing for function

This shed contains three internal rooms to serve a range of functions:

  • A garden office room in the central section looks out over the garden and contains garden furniture and ample cupboards. It is used as an informal office and project room, but can quickly be configured as a garden room for evening meals or as a sitting area during a garden party, or as a den for teen sleep-overs.
  • A potting shed at the north end is used to store garden tools close to the vegetable garden, but also houses a freezer and open shelves for storing garden produce.
  • A woodwork shed at the south end contains basic woodworking tools for the kind of DIY projects I do around the garden and home.

 

External style

The external styling is based on a local vernacular – black clapboard with contrasting white windows and doors. Similar features can be seen on many local country buildings e.g. on the Goodwood estate. The colours harmonise with the other garden buildings (The Greenhouse and Cabin) and follow the theme of our house which has black window frames and off-white walls.

Eco-credentials

The design includes plenty of eco-features, including a green roof, local grown timber for the cladding and sheepswool insulation.

Drafting

This is one of the most structurally ambitious DIY projects I have undertaken, close in many respects to timber-framed house design. If you are thinking of building your own house one day, I would recommend you start with sheds and other garden buildings – no building regs to worry about, and less costly if you make mistakes.

Like all my building projects, I start by creating many quick designs on the back of an envelope. When I have one I like the look of I draw a scale drawing on the computer. Sometimes in MSWord (Insert drawing), other times in Open Office Draw. I draw each piece of timber, windows and doors and fit them together on the drawing to make sure it all fits properly – this is particularly important where you are working within height restrictions.

I literally build the structure piece by piece from the ground up on the computer screen, as detailed elevations. As I draw in a rectangle for, say, a 2″ stud I am thinking about the fixings, rigidity, material economy and sequence of work. During this process I often find I am not sure how to approach a certain detail, but having identified it I am able to seek expert help from the internet, books or the local timber merchants for example.

I find the process of creating accurate scale construction drawing an essential step, and would strongly recommend you do this yourself before you undertake any shed construction. The benefits are numerous and include:

  • Identifying areas of difficulty in advance, minimising mistakes.
  • Clarifying the construction sequence so you don’t waste time.
  • Enabling the creation of an accurate order list for materials.

A partially completed construction drawing in Open Office Draw

In practice I often deviate somewhat from the initial drawings as I find that once I am actually working with the materials new ideas and innovations suggest themselves. This does not mean the initial construction drawings were pointless – far from it – they provide most of the essential thinking needed to enable anything meaningful to start. Unlike house building, though, where planning control often restricts modifications to the specified design, with a garden building the design can be modified somewhat as one goes, in response to the reality in front of you.

I find the whole exercise of design, planning and execution intellectually challenging and stimulating. It lets you be architect, quantity surveyor and builder – highly recommended!

*Check your local building regs – don’t assume that what was true when I built this building will be true for you!

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