Pitched roof bracing

[anyone8542]

Hi,

I'm in the process of renovating my house.
Purchased in November, surveyor's report stated that the purlin was undersized, would need uprating and additional bracing installing.

It is a 1920's build, 3 bed semi.
I don't know how to technically describe the roof design so here's a picture.

http://dl.dropbox.com/u/11764772/001.jpg

My question's are
1. What size timber's should be used
2. What is the best way to join the lengths of purlin as i will have to put it in in 2 pieces.
3. Where should the braces go as i only have stud wall's upstairs and ceiling joists are only 3" x 2" and flex quite a bit.

I will get some picture's of the inside of the roof.
What do people need to see in the pictures??

Thanks in advance for your replies.

 

[noseall]

Measurements are what is important i.e. clear span of timber elements relative to the section size of said timber.

A photograph of the inside of the roof will reveal more.

The outside looks healthy no obvious tiredness.

 

[^woody^]

The roof looks OK for 90 years old, so why do the purlins need uprating?

If the purlins were not up to the job, the roof would have been sagging by 1950

 

[anyone8542]

Thanks for the replies.
Structural engineer guestimated that it had been re-roofed aprox 30 years ago.
There are no signs of sagging at the moment but as the purlin/lack thereof was brought up on our survey we don't want to be in the situation when we come to sell in 2 to 3 years of any buyer trying to knock us down on price.


My thought was that while i have the ceilings down and have space to upgrade the purlin it would be better done now than some one claiming that they would need scaffold and to remove part of the roof etc at the cost of 2k plus

Here's the photos and measurements.

Puriln 4" x 2"
Rafters 4" x 2"
Ceiling joists 3" x 2"
Wall plate to ridge board 3.4meters
Gable to party wall 6.6meters
Stud wall 4" x 2"

Pictures
Gable to party wall
http://dl.dropbox.com/u/11764772/Gable to party.jpg

Party wall to gable
http://dl.dropbox.com/u/11764772/party to gable.jpg

The small battens above and below the purlin were holding some rockwool insulation in place but this was removed as there was a condensation problem.

Joints in current purlin
http://dl.dropbox.com/u/11764772/purlin 1.jpg
http://dl.dropbox.com/u/11764772/purlin 2.jpg

I have no brick walls upstairs so can only brace off these studs
http://dl.dropbox.com/u/11764772/stud 1.jpg
http://dl.dropbox.com/u/11764772/stud 2.jpg

View's into the gabled section at the front of the house
http://dl.dropbox.com/u/11764772/front left.jpg
http://dl.dropbox.com/u/11764772/front right.jpg

 

[anyone8542]

Is anyone able to imparts some pearls of wisdom??

Could really do with some help on this one.

Thanks

 

[freddiemercurystwin]

Unfortunately you've gone from one extreme to the other, we've now got information overload! At the very least load your pics into your DIYnot album and make them accessible from within the thread so that we don't have to open 8 other tabs to seem them, that's just a pain in the butt.

 

[anyone8542]

Here are the pictures all in one place.

Gable to party wall

Untitled

• anyone8542
• 13 Mar 2011
• 1

Party to gable

Untitled

• anyone8542
• 13 Mar 2011
• 1

Purlins

Untitled

• anyone8542
• 13 Mar 2011
• 1

Untitled

• anyone8542
• 13 Mar 2011
• 1

Front section of roof

Untitled

• anyone8542
• 13 Mar 2011
• 1

Untitled

• anyone8542
• 13 Mar 2011
• 1

Stud walls

Untitled

• anyone8542
• 13 Mar 2011
• 1

Untitled

• anyone8542
• 13 Mar 2011
• 1

As i said before i've no solid walls upstairs.
I assume that i will have to brace off the studs but not sure they would take the weight.
Can i make the studs stronger??

 

[freddiemercurystwin]

They're not purlins! A purlin is a continuous structural support, you seem to have 2 or 3 individual members poorly joined together to form a tie that's doing no more than holding the rafters a set distance apart. As you say there doesn't look to be any evidence of any sagging, I think I'd be inclined to leave it alone as its stood the test of tie up to now. There are millions of roofs across the UK that have undersized this or that yet they sill function as well as when they were first built. You could get an SE's assessment as to what you could do although don't go asking for a structural report as that'll be big bucks. Frankly it smells a bit like your surveyor just covering his back.
Maybe worth asking mods to move your thread to the Building Section as a few SE's hang out there. OR PM Static and ask him to take a look.

 

[Static]

The so called purlins are definitely too small and would be suprised the roof hasn't spread.. there is some signs that the purlins are overstressed due to them bowing.. 30yrs may be an over estimation.. may have been the hurricane of '87 ment the roof needed replacement.. so could only be 20yrs..

Anyways.. you have a few options..
Can replace the purlins with something more suitable with bracing to load bearing partitions (so yes the partitions may need some additional studs if they even line up with walls below)..
You can turn the roof into trusses.. by adding diagonal members.. to each one rafter (may not be possible)..
You can replace the purlins with some steels which span gable-gable.. this would give you more options for future conversion of the loft space to habitable..
Or remove the roof complete and replace with trusses (may sound extreme but might be simpler)..

On another note where the roof comes out to the front elevation it looks like the ceiling rafters are 90deg to the roof rafters.. and the purlins have been cut.. this would need sorting at the same time..

Hopefully you took the surveyors comments and got a good price for the house..

 

[freddiemercurystwin]

Well there ya go!

 

[Peet]

It's great to see a decent set of pictures for once.

As a non-roofer may I stick my two penneth into this one. The 'purlins' to my mind serve little purpose. Probably their main purpose, as freddymercurystwin says, is to hold the rafters a fixed distance apart. Similar to the noggins between floor joists that stop them twisting under load and loosing their vertical alignment for load bearing. The slate batons you can't see above the roof felt tie the roof rafters one side, and these 'purlins' tie them the other.

Some comments on what I'd be looking at if it were my roof, and if anybody can see any reason why these aren't decent suggestions then do comment, in the name of my continued learning.


Check if the roof rafters, the 4x2s, are upto the job they're doing, ignoring the 'purlins' effect. If they're not, either sort the roof rafters or install something that would do a purlins job to assist them

If the roof rafters are strong enough for their current configuration, then check that the bottom end of them are are properly tied into the ceiling joists and that the ceiling joists are sound and continuous.

If the A frame shape of the roof is stable then all the walls see are vertical forces (ie the roof's weight) and things are happy. If the 'A' shape is allowed to spread due to the ceiling joists parting from the roof rafters, then unwanted horizontal forces/components come into play. Its critical that the 'A' shape is not allowed to spread.

Again to stop potential spreading, and Static mentions it; In the pictures http://dl.dropbox.com/u/11764772/front left.jpg and http://dl.dropbox.com/u/11764772/front right.jpg (or labelled 'Front section of roof' in your album) the ceiling rafters run at 90degrees to what I would expect to see. If the ceiling rafters are in the right place for your ceiling then leave them be, and install another set of rafters above them, at 90 degrees to them, to connect the bottom of each roof rafter to its corresponding partner on the other side of the roof. The existing ceiling rafters don't need to support their weight and its probably a good idea if they don't, so pack them off by say 1/2 inch during installation and then remove the packers when its all bolted up.

If the roof hasn't moved/bowed in all this time it looks fairly good, just make sure its intended design is stable.

Whilst you're there and doing all that work make sure the roof space ventilation needs are addressed to prevent condensation build up. Often when roofs were felted, the felt lapping right down to the eaves/gutters kills the air flow off.

Hope that helps