Loft Legs vs Timber beams

Using 50x50 timber joists laid in opposite direction of ceiling joists will be sufficient for boarding out for storage

Laying additional joists in an opposite direction to the existing ceiling joists is a good alternative to stilts/legs. In my opinion additional joists spread the weight more evenly. But the following considerations must be taken into account. It is important to maintain the correct insulation thickness of 270mm, so 50x50 joists may not be adequate. Also consider the fixing method. It’s important not to pull the existing ceiling joists upwards when they are screwed to the new joists, this would disturb the plastered ceiling below. This can be avoided by packing where necessary, bearing in mind, existing joist are not always perfectly straight or perfectly level with each other. Also care must be taken not to trap or nip electric cables.

It will be ok for storage and walking around on also cross battening will take out the bounce of the ceiling joist

I'd like to offer a little advice after seeing so many sagging ceilings and having to fix one that failed. Firstly I'd check the size of the existing joist plus any support wall that the joists are positioned over/on. It's likely that the original timbers were not designed to take any loadings. Secondly if you were going to install new joists in the opposite direction you need to ensure that these are sufficient not to close off airflow around any downlight you may have. Most downlights specify 50-100mm clearance, so if you were to use 50x50mm you might cause yourself some issues... Thanks.