Acrow, push-pull or bottlejack for supporting purlin for repair

[pabrinex0]

Hi all, grateful for some advice.

Just moved into a c.1900 Victorian terrace. Quickly spotted, unnoticed by the surveyor, a crack in one of the purlins. You can see failure is a combination of the purlin strut being removed for hatch installation, knot in the purlin, and heavier roof tiles than originally built. You can see there is some deflection in the purlin, though nothing visible from the outside.

My repair plan is to jack the purlin from the central supporting wall - essentially fashioning a footplate where the existing purlin strut for a prop, and cutting a similar mouthed piece of wood for the purlin end. I think it probably only needs to move 5-10mm. Then glue and bolt at 300mm centres a 2.4m length of 2x4 CLS along the back of the purlin, before then removing the prop.

Originally was planning to use an acrow between two flat, parallel surfaces as above. Though realise I could use a push-pull prop or indeed a bottlejack. Hope everything is clear from the photos - the existing purlin struts are birdmouthed onto some timber sitting on the central loadbearing brick wall. The existing boarding in the loft from the previous owner has been notched around it.

Any advice welcome!

 

[blup]

Any of them provided the base is stable and load bearing. That said I temporarily supported a purlin by hammering a temporary strut under it (at an angle) the base being a length of 2x6 nailed across three or four rafters.

 

[^woody^]

You only need to stop the purlin from deflecting more, not try and jack it up - which you are unlikely to be able to do and would potentially cause more problems than it solves.

2x 600mm 12mm Ply strips over the split area, glued and screwed will do.

 

[tony1851]

Woody is correct. Timber under long-term load is subject to 'creep', whereby the timber keeps its deflected shape.
If you try to jack it straight, it might lift the rafters at the end.
As it's not showing externally. just reinforce it as it is by any convenient method, adding extra timber or plywood with plenty of screws and PVA.

 

[catlad]

8 ft length of 3 x2 or similar will catch half a dozen of those rafters if you sister it to the lower side of that puny purlin.
If you are screwing through that old purlin into any new timber then I recommend piloting the old timber first.

 

[pabrinex0]

Thanks all for the advice, much appreciated.

Won't try to move it, just glue and fix in place with long piece 2x4 behind and some ply over the front of the crack. Hopefully will get it done this weekend so will stick a completed photo up.

Cheers

 

[aveatry]

You only need to stop the purlin from deflecting more, not try and jack it up - which you are unlikely to be able to do and would potentially cause more problems than it solves.

2x 600mm 12mm Ply strips over the split area, glued and screwed will do.

Would a few bolts all the way through be a better way rather than screws. Would you recommend normal D4 wood glue or the PU wood glue. And maybe 20mm ply ?

 

[stevie888]

I would slightly differ with Woody's view inso far as I would cautiously jack it a bit to take out the elastic deflection (ie the spring).

We bought a flat as a rental and on survey found there had beena cannabis farm in the roof. Worked out a cost of £1200 to sort and offered to split with the owner who was happy to agree. Stripped out the plasterboard and moved insulation to ceiling level. Found the pair of purlin props that had been removed (1 cut in half) and the wee packing piece that sits between them. I cut a replacement prop and on offering it all up the bottom gap had closed about 40mm.

I borrowed a couple of Acrows (placed beside and parallel to the props and wound them up gently until it started to get hard to turn the handle any further without leverage. My bottom gap had opened up 20mm so I'd got half the deflection out and that felt like a good compromise. Shortened the packing piece enough to get a pair of folding wedges in to snug everything up and job's a good 'un.

Only cost a bit of time and a trip to the tip

 

[noseall]

Thanks all for the advice, much appreciated.

Won't try to move it, just glue and fix in place with long piece 2x4 behind and some ply over the front of the crack. Hopefully will get it done this weekend so will stick a completed photo up.

Cheers

As per above post. There's nothing wrong with taking a wee bit of slack up, regards propping. I'd want the repair to feel the tension in the roof rather than just fighting the existing sag.

 

[^woody^]

Would a few bolts all the way through be a better way rather than screws. Would you recommend normal D4 wood glue or the PU wood glue. And maybe 20mm ply ?

It's either/ivor for multiple screws or a few bolts. The benefit of screws is that the fixing is spread over a wider area of timber and not limited to just a few locations and axis in the timber. The screws also pull all the ply to the purlin, not just at the locations of the bolts.

PU tends to expand and so can push materials apart slightly forming a weaker joint, and I've always been under the impression that PVA was stronger and more flexible than PU, so had better chance of dealing with some movement without coming apart. But that's not based on science, just my view.

I don't think there is any need for the water-resistant qualities of a D4 glue in that location.

Ply is incredibly strong when used for vertical loads and thick sections are not needed. 9-12mm are adequate for this situation in house roofs.

 

[Notch7]

Thanks all for the advice, much appreciated.

Won't try to move it, just glue and fix in place with long piece 2x4 behind and some ply over the front of the crack. Hopefully will get it done this weekend so will stick a completed photo up.

Cheers

You can easily take up the slack by cutting a bit of say 4 x 2 a tiny bit longer than distance between underside of purlin and an offcut of say 4 x 2 on the loft floor -if it moves around screw it temporarily in place

you just put the length of timber in place on the angle and then use a hammer to knock it straight -or until you are happy its where you want it

 

[pabrinex0]

Thanks to all who provided some input! Some updates as promised

Finished the repair this morning. Used an acrow to gently jack the purlin - we think it probably moved about 20mm total, was relatively easy to wind the prop but perhaps got a bit harder towards when we arbitrarily decided to stop.

The 2x4 was then bolted with M10 carriage bolts with PVA along its surface. Tried to bolt it keeping 1/3 of purlin material on the outside.

Interestingly the wedge piece where the purlin sat on the party wall, which was previously loose because of the end deflecting upwards, is now tight again.

Now to try and keep the loft >10 degs C for enough time for the glue to go off.