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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Carpentry Questions >> Adding Support to Purlin for New Roof https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1498342667 Message started by invisible-man on Jun 24^th, 2017, 11:17pm

Title: Adding Support to Purlin for New Roof
Post by invisible-man on Jun 24^th, 2017, 11:17pm

Hi there,

I wanted to add support to purlins in order to allow for a new roof tile other than slate type tiles at present......

All timbers are 4"x2" including purlins (join in middle), ceiling joists, rafters and binders and the length inside from one gable end to the other is 24'...... currently there is only one supporting strut/prop for each perlin, each one supported by the only two 90 degree supporting walls.....

The distance between the two purlins are 9'6" and sit approx 30" high measuring up from the ceiling joists. There are two central supporting walls running approx half each of the 24'......one pretty central, then the other offset from purlin 4' - wall - 5'6" purlin......

I sort of understand that you can add extra supports in pairs from one side to the other with a brace over a central wall, but the brace would have to be about 7' long in between and therefore I feel loading the ceiling rather than the supporting wall......

I was thinking along the lines of an additional 5 supports each purlin if that is considered to be sufficient, but then there is the execution.....

Any ideas or suggestions would be greatly appreciated, photos below.......

[album]invisible-man&1498339249[/album]

Title: Re: Adding Support to Purlin for New Roof
Post by woodsmith on Jun 25^th, 2017, 7:41am

I think to strengthen your roof would be quite an undertaking and the consequences of getting it wrong could be disastrous. To do any sort of work like this you would need to get your local council's Building Control involved. They will want detailed drawings and almost certainly calculations from a structural engineer, I fear this would be expensive as I think you would need to employ an architect to draw up the plans. Do you absolutely need to alter the roof?

Title: Re: Adding Support to Purlin for New Roof
Post by invisible-man on Jun 25^th, 2017, 1:14pm

woodsmith wrote on Jun 25^th, 2017, 7:41am:

Thanks so much for the info, also was going to mention that the rafters are approx. 450mm centres and ceiling joists about 600mm or so, therefore not side by side on wall plates, hence not directly connected to each other....is this also concerning even replacing roof like for like?

Thanks

Title: Re: Adding Support to Purlin for New Roof
Post by woodsmith on Jun 26^th, 2017, 9:17am

It's normal practice to align the ceiling joist with the rafters so that they act as a tie otherwise the rafters rely solely on the fixing into the wall plate which is far from ideal. Also with your roof the two braces under the purlins look like an afterthought.

You really need to get someone to look at this and it's just possible that your local councils building control officer would come out and have a look without charging you. They certainly used to but Councils are so strapped for cash these days there may be a charge, but even if they do charge I think it would still be the cheapest way to get some proper advice.

Title: Re: Adding Support to Purlin for New Roof
Post by thescruff on Jun 26^th, 2017, 4:23pm

Looks to me as though several braces/props have been removed. Did it used to be 2 houses at one time

Title: Re: Adding Support to Purlin for New Roof
Post by invisible-man on Jun 27^th, 2017, 9:10pm

thescruff wrote on Jun 26^th, 2017, 4:23pm:

Looks to me as though several braces/props have been removed. Did it used to be 2 houses at one time

No, never used to be 2 houses, no purlins have been removed.....

The strange thing is that both purlins have joins, like half join, one half holding the other half up........problem with one purlin is that the join is the wrong way round for the support, in other words the prop is holding up the half that is lapped over the other half.....

Have decided to take the easier route now, replace with imitation slate (weight for weight) of original covering.......

Thanks for all input, this has been invaluable in helping to reach a sensible decision !!!