worried about the weight of loft boarding

[joboatbuilder]

I've read lots of the posts about adding 1"x3" timbers crossways to the ceiling joist, which will add strength, allow extra depth of insulation, cables etc, before boarding with loft chipboard. We've got an 1860s cottage with original roof timbers. Which have signs of woodwork, so presumably not as strong as they were originally.
I've got a bit concerned when I read that ordinary 4" ceiling joists are not really up to heavey loads, using the roof as room, etc. Can someone confirm that they are strong enough to support additional 3"x1" timbers, boarding and extra insulation plus lots of boxes of stored stuff? We've got lath and plaster ceilings and I don't want to give them any excuse to start failing apart - ie joists bending at all

 

[joboatbuilder]

EDIT:

We've got an 1860s cottage with original roof timbers. Which have signs of woodwork, so presumably not as strong as they were originally.

sorry, woodwork should read woodWORM

 

[Deluks]

An 1860's cottage will probably have better ceiling joists than any modern house. You haven't told us the size of them, width and depth. Also what's the longest span?

Edit: just re-read, were you implying that yours are 4" also? If so then they should be ok, you can crossbatten with 2x2 which will probably work out cheaper as it's a dimension that they sell lots of.