Building wooden pergola over garden pond for shade

I would like to build a pergola over my pond to offer some shading for my Koi and try and combat blanket weed. However I am not sure what wood I should use or how to construct it. Can you please give me some suggestions or point me in the direction of where to go to find out? Suzy Richards, Monmouth

The best wood to use would be 'green' oak and it will still be there when you and I are pushing up the daisies.

After that, coming in order of preference:

• Cedar

• Redwood (Thuja)

• the American Pines and then

• all the other pines and spruces that are home grown.

Dont buy any imported stuff, particularly hardwoods from dubious sources or softwoods from eastern Europe. Oak and Cedar dont need treating. The rest do. If you buy wood that has already been treated, perhaps 'Tanalised',ie pressure treated with preservatives, check that the ingredients are not toxic.

Up until recently, extremely lethal toxins like chrome and arsenic were part and parcel of the mixtures used to extend the life of wood. All suppliers should be switching to safer solutions as of from now, but check that it is safe.

Dont take anyone's word for it, look for written confirmation.

The construction is fairly simple. It consists of upright timbers set down 45cm into concrete or bolted into 'metpost' holders at three to four metres apart along the length of the pool. These timbers should be in proportion to the scale of the situation and the garden. They should be a minimum of 8cm square( old 3x3s) and 220cm high. But stretching over a pool it might look more substantial at 10cm square (4x4s) or more.

Even brick pillars are a possibility for large schemes. Along the length, run joists that that can either be notched into the top of the posts or bolted to the outside. These are either 10cm x 2.5cm or 15 x 2.5cm, running from post to post, meeting and cut diagonally at alternate posts. On top of these sit the cross bars, which at the posts sandwich them with a double cross bar. These can be notched to fit snuggly down onto the joists or just nailed into little fencing brackets. These can be smaller 10cm x 2.5. If you need more information, look in your local library in the landscape architecture and garden design sections.

There are generally a few scraps of information to be gleaned from some of these coffee table books. Sometimes timber suppliers or garden centres themselves will provide information to people who want to do it for themselves.