Roof / Joist query

15 Feb 2006
Reaction score
United Kingdom
I am currently working out a roof design as follows - 15' lean to roof, rafters spanning 3.8m, supported mid span by a purlin. I have sized the rafters and purlins using Trada tables based on the assumed loads, my questions are as follows:

Am i right in thinking that I DO NOT NEED joists to tie the feet of the rafters, due to it being a lean to roof?? i.e. I could form a vaulted ceiling if desired.

My main question however is that, if I decided to go with a flat ceiling - how would I install the joists, given that they are not required as part of the roof structure?

Any help would be great!!

Sponsored Links
the joists can either be included along side the rafters or omitted.

they will sit on the wall plate next to the rafters and be fixed to a bearer plate or pole plate against the wall.

at the wall plate they will but up against the rafter and be fixed to the rafter as well as the wall plate. the tops of the rafters will be chamfered to suit the roof pitch.

at the pole plate they will be fixed via joist hangers, the pole plate being the same depth as the joists and will need to be bolted to the wall.
excellent - thanks for that,

How are the joists fixed to the rafter, seeing as they dont perform any structural purpose?

also - do i need a pole plate if I am using joist hangers? or can the joist hanger just be attached to the wall direct?

Thanks again!!
i would not say the joists are completely devoid of lending any structural significance. this would not be true.

if they were included, then by triangulating the joist with the rafters at the wall end as well as the wall plate end, then you will be stiffening the roof somewhat.

by triangulating i am referring to the inclusion of vertical stiffening timbers linking the pole plate to the rafter bearer.

also they will be carrying the ceiling and insulation therein. ;)

you will need a pole plate for the joist hangers, definitely! jiffy type hangers require multi fixing points along their length and would mean a series of drilled and plugged holes in close proximity rendering the job almost impossible!

built-in hangers are a different story and could be used, but i would opt for the simpler pole plate and jiffy hangers every time.
Sponsored Links

however, for arguments sake - if I did want to install the joists on joist hangers on boh sides, i.e. lower than the level of the wallplate, would this be possible and would it cause any problems?
not at all. it would be a false ceiling in this instance and could be built as such.
........not forgetting;

........ you will still need to address the insulation/ventilation requirements whether you insulate the ceiling or insulate the rafters.
Thanks again Noseall.

I was planning to insulate ceiling 90mm between joists and 40mm above (Celotex) I was also going to use a Tyvek membrane under the tiles to eliminate the need for ventilation of the roofspace.

Am I right in thinking that I would also not need a vapour barrier at ceiling level if using a breathable roof membrane such as Tyvek?
sorry for the mass of questions,

but what size of joists would you reccomend for a false ceiling - its spanning 3.4m and I was going to use mesh to carry the insulation between and 12.5mm plasterboard (foilbacked) fixed to them.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links