I want to strengthen the attic floor for use as another room (possibly bedroom). The joists up there now are 95mm x 42mm. I am quite confused by info found online about what I would need. To sister the joists would I have to use joists the same size? Or can I add larger joists? Or would I have to replace the joists with larger ones? Any advice most welcome.

Ive just seen other similar questions so Im going to measure the span of my joists and post them...

Currently 2 x 4's in American parlance. Look also to see if the rafters end on a plate on top of the joists or not. To sister with a wider (higher) board, there has to be room along the entire length. You don't have to replace joists. Be sure before you do an attic conversion that your local building code allows for the room height too. If a very low ceiling that can be opened to the peak, see what that is.

2X4 floor joists are very small. Is this a truss roof design or stick built? If it is a truss roof, is it designed for an attic? If not, you can't cut them! If it is stick built, depending on the span, you may sister with 2X6 or 2X8.

Thanks for your replies.
It is a truss roof. The rafters are much thicker and end on much thicker joists(13"x3"). There are some thinner vertical supports between the rafters which end on the 2x4 ceiling joists.
The span of the attic is approx 8 metres. I presume if I sister the joists I will have to stagger the lengths in various places.

I'm not sure if it was designed to be used as an attic.- it certainly has been used as one for the past 11 years.

Worryingly I have noticed (after 11 years!) that whoever put the last access hatch in simply cut through the joists and left them unsupported on one side.

I was considering placing a larger access area overhead a small box room (130cm x 130cm), thinking the walls of the box room will act as support- as well as reinforcing the joists above of course. - I don't know which walls are actual loadbearing / partition walls though. The box room has three stud walls against a back brick wall.


Thanks for any advice.

I understand what you are thinking about the larger hatch in the box room. However, it is still appropriate to header off all the cut joist rather than just allowing the cut end to rest on the walls. Headering spreads the possible loads to all of the joists.

If the ENTIRE roof system is a truss roof design, the only bearing walls are the outside walls that support the trusses. Using the attic space for storage is OK. However, if you want to floor it and use it as an office of other load bearing, you just can't cut the trusses. Are the attic trusses designed with a big hole in them 8X7 or so or do they look like a Y or W in the middle?

The alternative to trusses is collar ties. Either keeps the roof from splaying out. The vertical pieces are probably kneewalls. Get a contractor or 2 in there to look or ask the building inspector. Measure and draw it all out.