Internal Roof Bearers/ Wood Type

[Robbo777]

Hi All

So I'm rebuilding the whole of the interior of my Moho and I have go to the internal roof timbers, I have used a pressure treated 25mm x 35mm white wood but I'm just not happy with the strength of it.
The span is 2m and I have a fair bit of sag in the middle, I know that when I fit the insulation which will be celotex it will help support it and then when I fit the outer cupboards it will reduce the span by about 400mm but I rekon I could do with something a little bit stronger in the first place so what would you suggest bearing in mind it needs to be strong and lightweight?

 

[Squiffy]

I assume that the material above the trusses, is the aluminium skin of the roof. Do you mean that without the support poles that the beams visibly sag? I would imagine you have copied the beam spacing and sizes as per the original, if so then the possibility is that you have used soft wood as opposed to a harder wood used by the original constructors. However there is nothing to stop you either increasing the amount of beams or increasing their size or both to compensate for the sagging. Don't forget the original builders would have been working on the basis of profit where time and cost are paramount when building thousands of coach builds. But as a one off you are able to design more strength into it for minimal cost or loss of space. If it was me I would do both I.e. increase the amount of cross beams and also increase the depth to 50mm and use 50mm foam celotex or equivalent which should be just right for Heki's or other makes of skylight and would be far more insulated than 25mm. Phil.

 

[Robbo777]

I assume that the material above the trusses, is the aluminium skin of the roof. Do you mean that without the support poles that the beams visibly sag? I would imagine you have copied the beam spacing and sizes as per the original, if so then the possibility is that you have used soft wood as opposed to a harder wood used by the original constructors. However there is nothing to stop you either increasing the amount of beams or increasing their size or both to compete sate for the sagging. Don't forget the original builders would have been working on the basis of profit where time and cost are paramount when building thousands of coach builds. But as a one of you are able to design more strength into it for minimal cost or loss of space. If it was me I would do both I.e. increase the amount of cross beams and also increase the depth to 50mm which should be just right for Heki's or other makes of skylight and would be far more insulated than 25mm. Phil.

Yes, the material above is ally and this picture that you see there was an early one, I now have trusses a bit more frequently but it still sags, I can't go anymore that 25mm otherwise I would have to completely reconfigure the interior as the cupboards would no longer fit.
This is a more recent pic and when I'm done there will also be 4mm plywood above the trusses and below the aluminium skin.
I'd like to go with a harder wood I rekon but not sure what to go for, any ideas?

 

[Squiffy]

Was the original construction a complete ply sheets above the trusses which were stuck to the ply sheets and then sheets of ply stuck to the bottom of the truss which would act as a box section and vastly reduce the amount of sag and as you say the 25mm celotex would also increase the rigidity. If that is the case them increase the amount of cross pieces and that should be OK. Have you already secured the cross pieces or are they just dry fitted for assessment.

 

[Squiffy]

Sorry mis read your last, that you have already increased the amount of cross pieces, my view is with ply section top and bottom with 25mm celotex sandwiched between it would be plenty strong enough and I would even guess strong enough to stand on the roof to clean it . Phil

 

[Robbo777]

Was the original construction a complete ply sheets above the trusses which were stuck to the ply sheets and then sheets of ply stuck to the bottom of the truss which would act as a box section and vastly reduce the amount of sag and as you say the 25mm celotex would also increase the rigidity. If that is the case them increase the amount of cross pieces and that should be OK. Have you already secured the cross pieces or are they just dry fitted for assessment.

Hi Squiffy

Yes, so above the trusses it was 4mm ply then the trusses glued to the ply then rigid insulation between the trusses and then vinyl lined ply sheets glued to the underside, please see below images.

No, I haven't fitted the cross pieces permanently yet as I still have to take them back out to get the ply above them, do you think the strength will come when it's all glued together?
My main concern is that the sag remains once all glued together and then I've got a real problem on my hands as I'd have to wreck all of the insulation, the plywood and the finished ceiling to put right.

 

[Squiffy]

Faith and confidence Robbo, when all is glued into place and the glue has set it will be a rigid sheet and as I said strong enough to stand on. The original builders would have done stress loading tests to a mimimal build cost and you by putting in extra trusses have improved on that . Phil

 

[Robbo777]

Faith and confidence Robbo, when all is glued into place and the glue has set it will be a rigid sheet and as I said strong enough to stand on. The original builders would have done stress loading tests to a mimimal build cost and you by putting in extra trusses have improved on that . Phil

Thanks mate, that's what I was thinking and hoping for, just needed to hear it from someone else too

 

[Squiffy]

 

[xsilvergs]

I'm not a wood worker but interested in your post.

Having watched Grand Designs, would there be any advantage in plywood sheet cut in strips and bonded together? Kevin is often on about glulam beams.

I stiffened the fibreglass roof in my trailer using steel Unistrut.

 

[Sprinter 1 cup]

If your putting a roof rack on for panels could you integrate it , like said uni strut x2 on outside of roof, bolted though every beam ?