Elias, how big is the room and how wide are the boards? Are you planning to run the floor at 90 degrees to the existing floorboards?

You would normally secret nail or float.

If nailed still leave expansion gap. I think 50mm brads would be ok, bit worried about hitting any services.

I havn't thought about underlay, I havn't used it but now you have got me worried perhaps someone else knows.

I have just finished solid wood flooring 18mm two and half inch width in random lengths, some only a foot long,
I hired a nailer (Porter- nailer its called) and it secret nailed perfectly, have used it on regular red deal floorboards in the past nailed at every joist, have to watch out for the pipes though.

Elias,

Board width makes a difference as wide boards are very prone to cup and I would avoid them if poss.. Robbo's boards were about 60mm which is ideal. If you nail each board then the joints may open up slightly but this will help to accomodate floor movement.

The brads go in at 45degrees so 50mm brads should only just show through on the far side. If you use a Portanailer then it will use it's own nails.

the wood timber yards sell plained  comes out
at around 20 to 21mm wood from diy sheds
comes out between 18to19mm

Fitted some 22mm finish size solid oak a couple of months ago.

The boards were 8", 10" & 12" wide 3.0 - 4.5m long. I counterbored , screwed and plugged them as there was quite a bit og cupping and twisting which I felt nails would not hold into the chipboard below.

http://img51.photobucket.com/albums/v156/jasonballamy/work/PICT0014.jpg

Also make sure that you let the boards sit in the place where they are to be fixed for a couple of weeks to allow them to adjust to the enviroment.

Jason

It was laid onto a floating chipboard floor that was inturn laid on polystyrene.

I didn't glue the joints so that the boards could expand & contract in the grooves only used glue (Titebond)for the 700 odd plugs that I cut from matching oak.

Used a trend snappy 9.5mm counterbore/4.0mm bit combination set to just break through the oak and fixed with 30x4.0mm gold screws giving me about 20mm of screw into the chipboard which was also 22mm thick.

left the first two boards out to enable me to use sash cramps to pull the very tight joints up then once I got to over 8' had to resort to folding wedges, one screwed to the subfloor. It was not helped by the 10" boards having a narrower groove than the others!

Are the boards you have to lay prefinished or will you have to sand & finish them once laid?

Jason