Irish Timber

A carpenter told me as a rule of thumb to use foreign sourced timber for the roof (rafters beams joists etc)would anyone know is this the general rule of thumb.Thanks

Native Timber would not be the norm and would cost you a small fortune. If possible have your roof timber Vac Vac Pressure Treated if not available make sure to treat your timber with a good wood preservative and everywhere it is cut to size/angle put some preservative on the saw cut's. I am sure that there are some ozone friendly preservatives out in the marketplace now.

There is absoloutly no need to treat timber with preservatives- once it is protected against the elements.
Load bearing timber must not contain more then 20% moisture, better not more then 18%. At this rate there is no need for preservation. All roof structures older then 50 years have never seen any form of chemical preservation. Ask your local priest, or go to mass and look upwards.

There is a tremendious difference in the quality of the timber available 50 years ago and today. Vac Vac Pressure treatment would not increase the moisture content. It was probably all native timber that was used then and it would have been air-dried for months on end. The rat-race times we live in now doesn't allow for all the traditional old methods of times gone past - more's the pity. The houses that were built 100 years ago will still be here in another 100 years where will the houses be that are being built today?

but is Irish grown wood inferior in quality to timber grown say in Sweden?

i thought the use of local timber was that it was used to the climate and therefore less likely to have problems that imported stuff

stupid or what?

I'm afraid we all need some "timber" education ....
Timber realy needs no chemical preservatives as long as the moisture is less then 20%, better less then 18 %. That's acording to the building regulations, I hope these are correct.
Vacuum drying can demage timber because it can cause splitting, warping etc..Vacuum drying is the cheap way to reduce the moisture content of timber, certainly not the best way. But if done correctly it still delivers acceptable construction timber.
Furniture makers would certainly prefer naturally dried timber.

Timber quality has improved, especially as the Irish forestry industry has concentrated on growing a better quality product and using species which are more appropriate. All timber for structural use is now machine graded and stamped which gives the end user some assurances regarding quality and expected performance. However that said foreign timbers will tend to be stronger and perform better in structural applications. This is due to climate and temperature- where trees grow in colder northern latitude growth is slower and the annular growth rings will tend to be closer together resulting in a stronger timber. Softwood species grown in Ireland grow quickly but their strength suffers accordingly.