Sistering joists

ems2

We have two rooms upstairs where the floor bows in the centre aganst the wall by 3 inches(!!!) at its deepest point, we have had biulders in to quote and they have all said they need to sister the joints and relay the floor.
We are going to attempt this ourselves as could not find a builder we were happy with, price and persons.

We have done a fair bit of research, interweb and library, and are confident we can do this, but are having a problem working out how this makes the floor level .

So my question is; do we attach the new joists to the side of the old on an angle so the end of the new joist does not actually rest on the wall but bolt them into the existing joists and allow them to take the weight?
Or do we put some sand and cement, or something?, on top of the wall and have the new joists resting on this AND as they are attached to the old joists this will hold them in place?

TIA!

titewad_2

copy and paste this into your address bar.. https://www.hammerzone.com/.../floor/joist1/sister.htm

ems2

Thanks but we have looked at this and we dont want to raise the floor as we don't have one... any other suggestions greatly received.

milagman

Basically using joists almost as long as the originals will span the sag. You might have to shave the ends of the new ones so that there isnt a step. Do use bolts and star washers. A right angled drill or attachment is a worthwhile investment/hire for this job as there isnt much room between joists

ormus

sister joists are usually used for extra load bearing. ie using a loft as an extra room.
for sagging joists i would glue and screw them to the existing ones, and use joist hangers too. then level under the floor using fillets.

ideally you would jack up the existing joists before attaching the sister joist. but this is not always possible.

ems2

how do you jack up the existing joists?