Weatherproofing a Backyard Observatory

Building a backyard observatory is the fun part. Keeping it dry, tight and working through years of New Zealand weather is the part that decides whether you still enjoy it a decade later. A roll-off roof or dome has moving parts and deliberate gaps, which makes it harder to weatherproof than an ordinary shed. If you are still at the design stage, our notes on planning a backyard observatory cover siting and layout; this post picks up once the walls are up.

Start with the roof

Water gets into observatories in three main places: where the roof meets the walls, where a moving part meets a fixed part, and anywhere a fixing goes through the cladding.

Roll-off roofs. The gap between the rolling roof and the fixed walls is the weak point. A few approaches work well in combination:

An overhang on all sides, so the roof skirt sits outside the top of the walls and rain runs past rather than into the joint.
A brush or rubber wiper seal along the top plates, which closes the gap without stopping the roof rolling.
An upstand or small parapet on the wall top behind the seal, so wind-driven rain has to climb before it gets in.

Domes. The shutter slot and the base ring are the usual leaks. The shutter needs a generous overlap at the sides and a drip lip at the top. The base ring needs a skirt that hangs outside the wall, plus a brush seal to keep out draughts and insects.

Flashings and drip edges

Whatever the design, flashings do most of the work. Head flashings over doors, corner flashings on the walls, and a proper drip edge along every roof edge all keep water moving away from the timber. A drip edge with a small kick-out stops water tracking back underneath by surface tension, which is one of the most common hidden causes of rot in small timber buildings.

Seal every screw through the roof with a neoprene washer, and check them each year. Sealant on its own is not a substitute for a well-lapped flashing; exposed sealant breaks down in strong sun and needs regular renewal. For general background on how buildings are expected to manage water, see the building code compliance guidance on building.govt.nz.

Ventilation versus condensation

The surprise for many new observatory owners is that most of the damp doesn't come from rain. It comes from condensation. A closed building on a clear night cools quickly, and moisture in the air settles on the coldest surfaces: the inside of a metal roof, the telescope tube, the corrector plate.

Some ventilation helps by keeping inside air close to outside conditions. Louvred vents low on one wall and high on the opposite wall allow air to move without letting rain in. Fit insect mesh behind every vent.

Other measures that help:

Insulate or line a metal roof so its underside doesn't drip.
Use a vapour barrier under a concrete floor slab, or leave a timber floor raised and ventilated.
Run a small dehumidifier in humid parts of the country. Coastal sites and much of the North Island stay damp for long stretches.
Use desiccant such as rechargeable silica gel inside eyepiece cases and camera bags.
Cover the telescope with a breathable cover rather than a plastic sheet, which traps moisture against the optics.

Keeping insects and animals out

Spiders love the shelter of an observatory, and wētā, moths and the occasional mouse will find any gap. Brush seals, mesh on every vent, a door sweep and sealed cable entries all help.

Exterior finishes for UV

New Zealand UV is hard on paint and stain. A few points:

Lighter colours reflect heat, keeping the building cooler by day so it reaches night-time temperature faster. White or pale grey is the traditional choice for domes for this reason.
Penetrating stains on timber cladding need re-coating more often than paint, typically every couple of years on sunny elevations.
Paint systems made for exterior timber last longer but need sound preparation. Peeling paint lets water in behind it.
Treat end grain and cut edges before assembly. That is where moisture gets in first.

Rails, tracks and rust

Roll-off roof rails and the wheels that run on them sit outdoors in all weather. Galvanised steel is a sensible minimum; stainless steel is better near the coast, where salt air corrodes ordinary steel remarkably quickly. Keep tracks clean of debris, lightly lubricate wheel bearings, and check for flaking zinc or rust spots each spring. Dissimilar metals in contact, such as stainless bolts through aluminium, can corrode at the joint, so use isolating washers.

Wind and tie-downs

A roll-off roof is essentially a large lid sitting on rails. It must be positively held down when closed, not just resting by its own weight. Use at least two solid latches or bolts on each side, and design stops at the end of travel so the roof can't run off the rails in a gust when open. Domes need shutter latches and a secure way to hold the dome on its base ring.

Exposed hilltop sites are also the windiest. Check the MetService severe weather warnings before leaving the roof open overnight.

After a storm

Make a habit of a short inspection after any heavy rain or strong wind:

Look for water marks on the inside of walls and on the floor.
Check the roof seals and brush strips for displacement.
Look at flashings and screw heads for movement.
Run the roof or dome through its full travel to make sure nothing has shifted.
Clear leaves and debris from tracks and gutters.

A few minutes after each storm, and a fuller check each autumn before the long winter nights arrive, will keep the building tight for many years and the telescope inside it dry.