Flat roof, no access to roof, trimming a joist

PaddyLondon · 18 Oct 2018

Hello. I wonder if you could help with a question to see if I need to invest in a Structural Engineer and progress with a small project

Situation:

three storey house
no access hatch
joists at 16 inch centres
Joist span c. 3720mm/12' 2"
Want to create structural opening c 860mmx860mm, requiring trimming of one joist for energy efficient access hatch

Complication - sistering of joists would require removal of ceiling and the cost/disruption would kill the project.

Questions - in practice do I need to sister the joists either side of the cut joists? Would building control insist?
Do the answers change if the structural opening is next to a brick supporting wall, changing spans?

Would be very grateful for any pointers -if an SE and/or Building control are likely to insist sistering is needed, I'll put it on the back burner . Many thanks in advance.

^woody^ · 18 Oct 2018

Form a smaller hatch?

Use timber binders in the loft to avoid doubling the joists

Tigercubrider · 18 Oct 2018

Can't access be obtained by cutting a 16x 16 (or one dimension bigger) hole through the ceiling, then adding sisters or other strengthening from above, then remove a joist part for the hatch ?

That way the ceiling stays and minimal mess.

PaddyLondon · 18 Oct 2018

Sorry everyone, I missed a key bit of info. It's a flat roof, no roof space other than between joists

PaddyLondon · 18 Oct 2018

Like the suggestions though..

^woody^ · 19 Oct 2018

So you want to create an external access to the flat roof?

That will be fun.

PaddyLondon · 19 Oct 2018

^woody^ said:
So you want to create an external access to the flat roof?

That will be fun.

Not if I have to take down 24m2 of ceiling to get access to
the joists....
But, yes, we need an access hatch

wessex101 · 19 Oct 2018

Am I reading that right 12"x2" joists at 16" centres spanning 3720mm? They are massive joists at close centres, especially over that relatively small span. It would be a relatively simple calculation to work out if you could insert an access hatch but my gut feeling is it would be fine without additional strengthening.

PaddyLondon · 19 Oct 2018

wessex101 said:
Am I reading that right 12"x2" joists at 16" centres spanning 3720mm? They are massive joists at close centres, especially over that relatively small span. It would be a relatively simple calculation to work out if you could insert an access hatch but my gut feeling is it would be fine without additional strengthening.

I've made a pig"s ear of my question... The joists are 8 inches by 2 inches in cross-section with a span just over 12 feet. The roof is, from the joists upward, woodwool board, concrete screed, insulation, OSB, GRP . Built in 1963, upgraded recently.

wessex101 · 19 Oct 2018

Ah, 8x2's that's more like it. Could be a close call, but like I said it is an easy calculation so I would be surprised if a structural engineer charged more than £100 (doesn't include site visit).

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Flat roof, no access to roof, trimming a joist

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