Loft joists

saj_chow

Hi all,
Hope this wasnt asked before. I am trying to board my loft for storage and I want it to be safe to walk arround aswell in case i want to use it as occational hobby room ( no jumping arround loads😂 though). I have 100 mm joists spaced at 17” centers. I want to ask if I lay 100mm joistson top of existing joists perpendicularly and boardon top of it will my existing 100 mm joists be able to cope with the storage and walking arround in the loft or will the ceiling bow? I am not sure if the weight of the new joist itself will outweigh the strenght it adds.

Another option is to screw new 100 mm joists on top of existing joists like sister joists but on top. 

Can anyone suggest which option is better for me please.

DevilDamo

You would not achieve anything that way so would be waste of time and money. The ‘normal’ way is to bolt new joists to the side of the existing. Your spans will determine the size of joists. Are you looking to insulate the room too? What’s the situation with the stair, pull/fold down loft ladder or more of a traditional fixed staircase? Assume you’re going to want some natural light in the form of some rooflights?

tacpot12

LED Light panels are a cheaper option than rooflights. Having lights in the loft is a brilliant idea even if you are only using it for storage. 

saj_chow

DevilDamo said:

You would not achieve anything that way so would be waste of time and money. The ‘normal’ way is to bolt new joists to the side of the existing. Your spans will determine the size of joists. Are you looking to insulate the room too? What’s the situation with the stair, pull/fold down loft ladder or more of a traditional fixed staircase? Assume you’re going to want some natural light in the form of some rooflights?

Thanks Devil damo. one more detail i forgot, there are two beams running perpendicular to the joists on top of them dividing the joist span into three. These beams run from wall to wall and size is 100x38mm. will it work if i put joist hangars and put new joists on the beam? Not sure its conventional to install beams on top of joists. I dont see how the joists are attached to the beam. The old owners started some loft work, not sure if  they started something and left it unfinished. The beam seems thin to support any loads.
There is a big fluorescent light in the loft so lighting is sorted will get an electrician to add some more led to fully light up the loft
Thanks again for the reply

 

williamgriffin

The beams are sat on top of the ceiling joists, those ceiling joists are designed to hold up the ceiling below and are not the basis for a sold floor. 

saj_chow

the joist span is 6.8 m with support wall in the middle (not sure if the joists are supported my the wall is there a way to check this?), i have hatch and pull down ladders. not looking for natural light will install led lights. i will be insulating under the boards and in the ceiling aswell.

Homer_home

How's about some pics , so people can see exactly what you mean?

Pics paint a thousand words 

DevilDamo

You need to seek the advice of a structural engineer. Don’t go hacking around at your loft without first instructing a professional.

saj_chow

The picture shows the beam on top of the joists there are two beams from wall to wall sitting on the joists. Thanks again for your time guys. 

Petriix

If in any doubt then consult a structural engineer. Span tables for various loads are available online. Typically 175 x 47 at 300 centres would be required and would, in your case, be sistered to the existing joists and notched around the perpendicular beams. However, a structural engineer will be able to tell you exactly what you should do and the expense of their expertise would be a sensible investment. 

saj_chow

Thanks petrrix. would it work if I bolt smaller ( in length)sister joists on both sides of beam and add another piece to attach both sister joists on both sides of beam instead of notching. might struggle to get that lengths into the loft through the hatch.