Loft Conversion Joist Calculations

I'm planning a DIY loft conversion project. I plan to strengthen the (ceiling) joists by "sistering" with wooden joists. The ceiling joists are 4.5" by 1.5" whilst the floor joists in the house are 7" by 2" (actual measurements). From my knowledge of beams (joists), the stiffness varies with the width and the cube of the height. On this basis I calculate that theoretically I would need to sister five 4.5" x 1.5" together to achieve the same effect as a 7" by 2" . Given that I plan to use the space for storage only, I consider that it would be sufficient to sister two 4.5" x 1.5" on to each existing ceiling joist. I do not have sufficient space to install full length joists, so I plan to concentrate on the central half of the span only (the section most likely to sag, and the section carrying most of the additional weight).I plan to glue and bolt the joists together using large washers to avoid damaging the wood.

I would welcome any comments on this plan.

The existing joists will support sod all, they are normal size for a roof truss which is designed only to hold up a roof.
So you are only strengthening the middles. and leaving the ends as they are? this will mean that the centres will not sag, correct, but also means that the ends will be under even more stress than ever. Likely to break at the point where they bear onto the walls. This is an even bigger problem than sag, think about it...

Thank you for your advice, and I want to preface my response by saying that I agree with what you say, and that I intend to follow your guidance.

I've seen plenty of joists sag, but I have yet to see any that have sheared off at the points where they meet the walls. I understand this to be because beams (joists) are generally able to withstand shear forces that are many times greater than the weights that are placed on them, but that they are often not substantial enough to cope with the bending moment caused by the weight. However, since we are talking about the roof over MY head, I'm not going to put this to the test !

I had a builder look, and he wants to put in new 7" by 2" joists. Can anybody tell me whether it is standard practice to embed these in the walls ? I am concerned that he will just "sister" against my existing joists and thereby provide a solution that is little better than mine and is still susceptible to the problem that Deluks describes.

windsurfer99ie said:

I had a builder look, and he wants to put in new 7" by 2" joists. Can anybody tell me whether it is standard practice to embed these in the walls ? I am concerned that he will just "sister" against my existing joists and thereby provide a solution that is little better than mine and is still susceptible to the problem that Deluks describes.

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you must support the 7x2" seperatly as a seperate structure otherwise your reducing the strength down to below 30 percent

windsurfer99ie said:

I am concerned that he will just "sister" against my existing joists and thereby provide a solution that is little better than mine and is still susceptible to the problem that Deluks describes.

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If he does this don't pay him!!!

7x2 sounds a lot better but only if the spans are the same as the other floors in the house. Longer distances require even deeper joists.

hi there i just require some advice on my loft. basically i got my loft converted like a month ago and the builders who did my loft now seem to me that they have done a dodge job because they have used wood instead of steel to hold the stress of the roof. basically they have used 2(7"x2") joist and bolted them together and put it wall to wall. and then they put joist hangers on theses and put rougly 13-15 7x2 joist and on top of that my new floor is on it. my concern is that will the 2(7"x2") joist carry the wait of my whole roof. the only information i knw is that the length from wall to wall is 18ft so the span is 18ft i think. and now the beams 2(7"x2") joist have bended and are now resting on the existing joist basically the old joist in my loft which are 4x2. i knw this is trouble becuz my celling could start craking soon. so is there any suggestion plzzzz.

thank you i would really appreciate the advice

structural engineer required pronto

you should have complied with building regs with several visits during the build from the bco[building control officer]then he signes it off on completion