So you've worked out which solar collectors you need, how many to install, their optimum tilt angles and everything else. Now you need to physically check that you have the roof space and plan for the installation.

When you install solar collectors on your roof, you need to be prepared. You'll need access to the inside of the roof, you'll need equipment ready to haul the collectors up and you'll need all your tools on hand. However, before you get to that point, you need to mark out where the collectors will go and prepare their mountings.

Before anything else, sit down and draw a diagram of the roof space. Measure the actual size of the collectors or get an accurate measurement from the manufacturer. You'll also need to allow space around the collectors:

• Collector dimensions vary between models and manufacturers, even though they are all "standard size".
• Most collectors have about 1.5 inches of manifold pipe sticking out on the sides, near the top and bottom for plumbing.
• Some manufacturers factory-fit a union to the manifold, adding an inch or so.
• Most collectors end up needing between 3 and 4 inches of space around them.
• You will also need about a foot of roof space on each end of the array for pipes and roof penetrations.

Now that you have the layout on paper, grab your chalk line , marking crayon, tape measure and square (and safety equipment) and head for the roof. You need to lay out where the collectors will be fixed, to be absolutely sure there's space and certain that no part of them sits on the roof overhang. You might want to cut out large pieces of cardboard to represent the collectors and required space, so you can see how things look.

Once you've got the area measured out, snap chalk lines around the perimeter of each collector's mounting location. You can also use the chalk line to check for sagging by pulling it tight and seeing if the roof surface stays equidistant along its length. This is very important for collector mounting because they have to be mounted in a straight line – any sagging will mess that up and make it very difficult (or impossible) to get them to seal together. If the roof is sagging, install shims between the roof and the mounting brackets to counter the effect.