Build a roll-off roof garden observatory

PART 1 OF 3: In part 1 we look at how to locate the observatory and build the floors and walls

Setting up a telescope and a mount for an evening’s observing can take over an hour – more if you’re imaging and add a camera in the mix – and nearly as long to put it all away again. However, if you have even the most basic observatory, setting up can take less than five minutes; as the cliché goes, it’s a game changer.

In this series of three articles, we’ll look at creating a roll-off roof (ROR) observatory that has minimal impact in a small garden. We’ll cover the basic structure, the design and installation of solar panels (photovoltaics or PV), and also how to add a remote control system to it for an imaging setup.

Let’s begin with the structure itself. A roll-off roof is easier to construct than a conventional dome, and the roof is more suitable for solar panels. A small observatory is classed as ‘permitted development’, but there are exceptions, so you should contact your local planning authority if you are in any doubt.

The movement of a telescope on an equatorial mount describes an (imaginary) sphere, so this will determine the minimum internal dimensions of the building. A simple way to work out the radius is to use a piece of string to measure the distance from the intersection of the right ascension (RA) and declination (dec.) axes to the most distant point on the scope (usually the front of the objective lens or back of the eyepiece/camera). If your observatory needs to fit you in as well, make ample allowance for that.

Making foundations

Unless you are competent with groundwork, you may need a builder, and they will need a plan and elevation drawings of the work. In our design, the walls and roof are supported by the four corner posts, so the footings for these must be substantial. The stem walls support only the floor’s weight, so shallow footings are fine here.

We had a sturdy tripod, so our floor design included three concrete pads for its feet, but you may prefer to have a central pier set on a pad. You must isolate your pads from the floor so that any vibration in the rest of the observatory is not transmitted to the scope.

It’s a good idea to incorporate security at the design stage, including an alarm and a padlock: we used security bolts and stud hinges on the door.

A bugbear of many observatories is condensation. You can incorporate passive control of this with a combination of insulation and ventilation. The insulation reduces large, rapid temperature changes and ventilation reduces the likelihood of trapping warm air that will condense as it cools.

Next month, we’ll show you how we overcame a disadvantage of ROR observatories, which is the extra footprint needed to support the roof-runners.

What you’ll need

Tools include a circular saw (follow all the safety precautions and always wear appropriate protective equipment); and a staple gun for securing the vapour membrane.

A portable workbench and a trestle; a green laser level; a tarpaulin to protect the unfinished observatory from the elements.

Materials include a damp-proof membrane; OSB3 orientated standard board (11mm and 18mm); 25mm insulation board; vapour barrier; shiplap cladding; 95mm x 45mm kiln-dried timber; 25mm x 50mm battens; security hinges and bolts; plus screws and nails.

Step by step

Step 1

Once you’ve chosen your observatory’s location, the next step is to calculate the internal size of the building. We verified our calculations by making a framework of canes and ensured that no part of the telescope moved outside the framework (except at the top).

Step 2

We engaged a local builder to do the groundwork and install the footings for the posts that support the main structure, as well as the concrete block pads on which our tripod would stand, and the stem walls of two courses of engineering bricks that support the floor.

Step 3

We damp-proofed the concrete pads and covered the ground with weed-block fabric on which we laid polystyrene insulation board. The floor is 18mm OSB3 board, supported on 95mm x 45mm timber joists, resting on damp-proof membrane on the stem walls.

Step 4

The inside walls are 11mm OSB3 board, which we fixed to the corner posts with wooden blocks and reinforced in the middle with vertical battens. Angle brackets are useful for securing the battens to the floor. (The batten at the top of the wall is temporary).

Step 5

Insulation will help control condensation inside the observatory. The walls have a layer of 25mm polystyrene insulation board, which is held in place by friction, then there’s an air-space between this and a sheet of vapour barrier that we stapled to the posts.

Step 6

The outer layer of the wall is pressure-treated shiplap cladding, secured over the vapour barrier layer. Any cut ends of timber will no longer be water resistant, so use a wood treatment on them in accordance with the manufacturer’s instructions.

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Download a plan and measurements for your roll-off roof observatory build.

ALL PICTURES: STEVE TONKIN