Re: Deck board spacing problem; can I use a saw to create a space?
That's why I always use a nail as a spacer when laying deck boards.
Forget using the jig saw --- a circular saw would be the better choice. Make sure the blade depth is slight less than the thickness of the planks otherwise if it's too deep you will cut into the joists --- which will weaken them.
The two problems I see --- you will need to ensure the cuts are nice and straight --- otherwise is the are wavy it won't look very nice. The other issue ---- when pressure treated lumber is cut preservative ( end cut preservative ) should be brushed on to coat the exposed wood.
Sometimes it's easier ( especially if screws were used )and better to pull up the deck boards and relay them --- depends.
Start at one edge ( the most visable reference ) --- relay the decking with an appropriate spacing and cut the last plank(s) ( the least visable ).
"" an ounce of perception -- a pound of obscure "