Beefing up roof joist for walking

[trevorbayliss]

I am near to fitting my timber joists on my new roof extension.

It is a 10 degree mono pitched roof and has oak purlins taking the weight of the new roof above.

The roof is divided roughly into 4 sections by the oaks. So the actual spans for the joists are 1.6m and then 3 spans of 1.3m approx.

The first section is at the rear of the roof next to the back wall of the house and will carry my 4 roof lights. I am doubling up the joists in this area around the windows, but i am thinking i should probably double up all the joist in this section as i can see my self doing work in this area in the future.

eg, painting the facia board and so using some small scaffolding. OR if i was to work on the slate roof above scaffolding would need to be weighted on here.

Am i wasting my time doing this or is it a good idea ? The cost is minimal as it will be an extra 5 lengths of 3.6m treated joist.

I am using 125mm C24 joists. Plus will have insulation and osb3 sheets over the joists. The roof finish is metal.

 

[CJRatch]

125mm X 50mm C24 max span on a pitched roof 15 degrees with 400mm centres is just shy of 3 meters. Your max span is 1.6 but at 10 degrees so you're within the timbers limits. Definitely double up around the windows but it's not essential you double up elsewhere although as you say it's not going to cost much more to do so anyway.

Be careful what roof windows your using at 10 degrees.

 

[trevorbayliss]

Thanks CJR

Yes doing 400 centres as well so glad to know i don't need to do it, but i think i will as it will future proof any heavy loading possibly imposed on this area if new work is going on above.

I am making my own windows, with an upstand and then a triple glazed unit sat into it. Very simple, but non opening.

 

[John D v2.0]

I'm no se, but over that distance that would be over specced even for a ground floor. Doubling up improves the strength but unless you have some large localised loads the issue would be deflection anyway. And given your short spans you won't really be having issues.
It's not a lot of money so go for it if you like, cheaper than an se. Watch for increased thermal bridging too.

 

[noseall]

It is a 10 degree mono pitched

Pitched?
What are you covering it with? Metal?

 

[trevorbayliss]

Yes Tata steel system.

Thermal bridging may go up a little bit I have 50mm between the joists and then 100mm over them to make a warm roof

 

[John D v2.0]

celotex I assume in which case the extra thermal bridging probably won't be an issue for the overall u values.

 

[trevorbayliss]

Yep