Building inspection - roof structure not as per plan

[Harbourwoodwork]

Yes I agree RR, on some old agricultural or rustic buildings it's not uncommon to see a floor board used. It's not brilliant, but they have been up for 4-500 hundred years!

I have seen more and more of this doubling up of hips and valleys. A timber framed house going up near me has steels under the ridge of the main roof and the dormers; doubled valleys with steel plates sandwiched in between.

I thought a traditional roof was tried and tested technology, so how far do we go down this road? Stronger walls to hold up all the steel?

How about you architects or structural engineers to throw in your 10 pennorth?

 

[RonnyRaygun]

How about you architects or structural engineers to throw in your 10 pennorth?

I've worked as an SE for the last five years. No-one has yet given me a satisfactory explanation of hip member design. Nothing in any of the books I've got either.

As Tony says, they don't push out at the corners. I guess this is because they are restrained by the battens. The whole roof acts like a "shell" type structure once it's all nailed together.

 

[Harbourwoodwork]

I agree that the jack rafters hold up the hip to a great extent and as far as I know its OK to scarf a hip because of this, but if hips don't push out why traditionally is the hip tied into the tie brace with the dragon ?

 

[RonnyRaygun]

I agree that the jack rafters hold up the hip to a great extent and as far as I know its OK to scarf a hip because of this...

I'm not convinced that the jack rafters do provide much support to the hips.
Clearly common rafters will support a ridge board...take away the ridge board and the rafters remain in position. As long as they are adequately tied they will not spread.
But what happens to jack rafters if you take away the hip? Answer: They fall over! So what is doing the supporting? It seems to me it must be the hip member??

but if hips don't push out why traditionally is the hip tied into the tie brace with the dragon ?

I agree, there has to be some outward thrust. As I said before, I think a lot of restraint will be provided by the battens.

Maybe hip members have always been undersized in the past...

Maybe Tony will come along with some education for us!

 

[Harbourwoodwork]

yes RR were only left with speculation now ,more input needed ,who is Tony?

 

[catlad]

I have seen more than a few hips spread around Lancashire, generally end terrace. The dragon beam detail for the hip I have only seen in manuals, never in the flesh so to speak.

 

[Harbourwoodwork]

To be honest I haven't either, has anyone? But I have taken a dragon beam out of a loft running diagonally corner to corner in a loft.

 

[RonnyRaygun]

yes RR were only left with speculation now ,more input needed ,who is Tony?

See post 12 on page 1 of this thread, although not sure Tony knows the answer either...

 

[GCarnegi]

Okay, I've just had a proper look in the loft, and it seems the rafters we have actually in place are measuring 45x160@600ctrs, where the drawings says 50x200@400ctrs.

 

[RonnyRaygun]

Okay, I've just had a proper look in the loft, and it seems the rafters we have actually in place are measuring 45x160@600ctrs, where the drawings says 50x200@400ctrs.

If you let us know the length of the rafter between supports, and the length on plan (or the roof pitch) we can give you an idea as to whether they will be acceptible...although it doesn't sound good...

 

[GCarnegi]

The ridge is slightly nearest the back of the house than the front so the longest span is at to the front.
45 degree pitch with what looks like about 13-14ft horizontally from ridge to front wall. Can't actually see where rafters bear on wallhead as there is an inlay on the front. (ie building is 1&3/4 storey at the front.)
though it looks like it would be well strong enough to take the loads on it, whether it meets regulation loads I'm really not sure.
If it doesn't meet regs then it doesn't meet regs and if it needs fixed then it needs fixed. I'm just struggling to work out how to bring in additional rafters at this stage without major disruption. (we have just finished decorating the room directly below.) our budget for this extension is pretty much wiped out now, though we might be able to scrape together another 1-2k if need be, but that would have to get us back to the fully decorated state we are in Just now!

If it makes any difference, there are 50x100 vertical posts rising from floor joists meeting Rafter at around 8ft horizontally from ridge. So the 14ft Rafter span is not totally void of support.

 

[RonnyRaygun]

If it makes any difference, there are 50x100 vertical posts rising from floor joists meeting Rafter at around 8ft horizontally from ridge. So the 14ft Rafter span is not totally void of support.

14ft on horizontal is definitely too long for an unsupported length. However, if the joists are able to take the additional load from the rafters then you might get away with it. Depends on the size of the joists. Are they at 600mm centres as well, or are the posts supported on a sole plate of some description?
Give me the joist sizes, spans, and centres and I'll give you an idea.

If not, rather than put in more rafters, would it be possible to install a purlin?

 

[GCarnegi]

Thanks Ronnie.
Looks like the vertical ties bear down onto a double 50x200 joist. The span of this is approx 5.5m (excuse me mixing my metric and imperial!). They nail onto the SIDE of the rafter though, but I guess theres nothing stopping me putting in other vertical ties which brace directly underneath the rafter.
The joists pass their load onto the old granite wall of the house, and the timber structure supporting the new extension at the other end.

Just had a thought though. Most of these rafters are jack-rafters, so their span is going to be a fair bit less than 14ft. As you can see on the photo's, the roof is an extended hip, laid on top of an old hip, so there are jack rafters going from new hip to wall, a couple of full rafters, and then there are other rafters going from the lay rafter (on the old hip) UPWARDS to the new ridge. I guess bringing in additional of these upwards rafters should be easy enough as access should be no problem.

Hopefully the photo's will explain a little more.

Untitled

• GCarnegi
• 31 May 2010
• 3

Untitled

• GCarnegi
• 30 Sep 2011
• 2

Number of rafters above not accurate - but you get the idea.

Untitled

• GCarnegi
• 21 Jul 2012
• 1

Untitled

• GCarnegi
• 21 Jul 2012
• 1

Untitled

• GCarnegi
• 21 Jul 2012
• 1

also looks like one of those jack rafters (above) is a double. ie 2@45x160mm for some reason.

Untitled

• GCarnegi
• 21 Jul 2012
• 1

Untitled

• GCarnegi
• 21 Jul 2012
• 1

Untitled

• GCarnegi
• 21 Jul 2012
• 1

I think maybe I should measure each rafter individually and consider it's span and bearing points individually, rather than treating the whole roof the same. Guess it's out with the tape again in the morning then!

 

[noseall]

Okay, I've just had a proper look in the loft, and it seems the rafters we have actually in place are measuring 45x160@600ctrs, where the drawings says 50x200@400ctrs.

Who made the decision for such a drastic downsize and why?

600mm c/c is a dreadful span and i refuse to set ANY timbers to this. I have to say, the roof does appear to look 'spindly'.

 

[RonnyRaygun]

Have you got collar ties on every rafter? They reduce the span of the rafters significantly. As long as the rafters are also tied at the base you have effectively got trusses. Although that probably doesn't reduce the span enough...

A double 50x200 at 5.5m long is a total waste of time when it is carrying half your roof. Maybe the answer is to replace the 50x200 with a steel beam...as long as you could get it in.

I don't think there's a simple answer. You need to get an SE to do some sums for you