Build a roll-off roof garden observatory

Part 2 of 3: How to overcome the extra footprint that is needed to support the roof runners

Previously, the first part of our build looked at how to build the foundations, walls and flooring of our roll-off roof garden observatory, and now we are ready for the next phase, the roof Roll-off roof (ROR) observatories are the simplest observatories for DIY construction, but one disadvantage compared to rotating domes is the additional footprint that you need for the roof runners. If these are higher than head height, it’s less of an issue because you can walk underneath them, but our observatory walls are only 150cm high, so runners would reduce the usefulness of the area they enclose. In this article, the second part of our three-part series, we’re going to show how an ROR system can neatly sidestep this challenge.

By building the observatory close to a fence, the runner nearest the fence is not obtrusive and you can place potted plants below it. Our runner, which has a V-section sliding gate track on top of it, is a single length of kiln-dried timber that is continuous along the length of the observatory wall and outwards. It is supported by corner posts and a 100mm square fence post.

For the other runner furthest from the fence, we’re using a gateleg system, keeping it as simple as possible through careful design – the less complicated it is, the less there is to maintain.

We calculated that the roof, with solar panels, would eventually weigh nearly 85kg, which would be distributed along the length of the two runners. The gateleg was designed to be sufficiently robust that it securely supports the roll-off roof’s weight.

We needed our roof to slope down to the north, but solar panels need to slope the other way. This meant that a small slope angle was essential. To prevent pooling of water, the minimum permissible slope for a roof covered with EPDM (roof membrane) rubber is 0.72°. We incorporated a small safety margin with an angle of slightly more than 1°.

The lower cladding boards overhang the walls on all sides, except where there are cut-outs for the runoff runners. This reduces the likelihood of rain or snow getting blown inside – while acting as a useful stop for the movement of the roof.

Taking precautions

A removable roof is a potential security risk, so we installed internal hold-down latches with a combined holding capacity of more than a tonne.

The thermal properties of a black EPDM roof covering could potentially cause big temperature changes inside the observatory. But the solar panels shade much of it and we installed insulation board between the joists. This helps to keep the inside temperature more equable to the outside.

See the step-by-step for details. Next month, we will install the low-voltage solar-PV (photovoltaics) electrical system and consider options for controlling your telescope from the comfort of your desk.

What you’ll need

Tools include: spanners (for the lag bolts) and screwdrivers; a try square; a circular saw and a jigsaw; a hammer; a drill and quick-grip clamps.

Materials include: 9mm OSB3 orientated standard board; 95mm x 45mm kiln-dried timber and 25mm insulation board; 100mm ² treated fence post and socket; a bolt-down sliding-gate track and six wheels; gate hinges, tower bolts and hold-down latches. Plus EPDM (rubber roofing) sheet and adhesive; an edge and gutter trim kit for EPDM roofs; assorted screws, nails and lag bolts.

Step by step

Step 1

The gateleg is built from 95mm x 45mm timber. The hinges are potentially the weakest link in this system, so make sure you get good quality ones. Use at least three hinges and use screws that go through the jambs and cladding into the corner posts.

Step 2

Connect the roof joists to the sides of the roof with lag bolts. We used rabbet (recessed) joints at the back of the roof to give a 1° slope away from the front. Two of the joists are spaced to accept the solar panel mountings.

Step 3

The roof has three wheels on each side, attached with lag bolts, which run along sliding-gate tracks. The track is narrower than the walls, so it is possible to compensate for slightly non-parallel walls. (The buffer at the end of the gateleg track is temporary.)

Step 4

A tower bolt secures the gateleg in its closed position, where it bears against the roof cladding, locking the roof closed. Another bolt holds the gateleg open and the cladding cut-out ensures that the gateleg is in the correct position before the roof can open.

Step 5

The cladding on the far side of the roof to the runners prevents it opening too far. And to ensure that the roof cladding can clear it, the top of the wall on this side is hinged and folds downwards. When this is up, it helps to lock the roof in its closed position.

Step 6

The roof deck is 9mm OSB3 (orientated standard board) sheet. We covered it with EPDM rubber roofing membrane, which lasts longer than felt and is a material of choice for low-slope roofs. After we glued it down, we secured it with gutter and edge trims.

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ALL PICTURES: STEVE TONKIN