Purlin struts

[chippy1970]

Originally sent as a pm but I couldnt send for some reason, maybe I haven't posted enough since the forum started up again. Anyway directed at Mike but feel free to chime in.


MikeG,

Just looking to get your view on this as an architect. I'm a carpenter/joiner have been for years but you can't know everything lol . So looking at your posts you seem more anal than me about doing things right , so I thought youd be the man to ask.

Over a few years I've been turning my loft (when materials have been free) into a storage room. I've got a plasterer coming tomorrow so the whole thing is nearly finished. Anyway I spotted the old existing struts supporting the purlin in the centre had splits coming away from the crook of the birds mouth. I decided to cut two new c16 4x2 in next to them then removed the old ones. I also tried to lift the purlin slightly with a plasterboard prop (i doubt this did much)

Straight away the new ones have split which I'm guessing is just where I made them a touch to tight and was caused when knocking them in. There's photos below excuse the left hand lower b/mouth I cut it upside down originally whoops measure twice and all that.

My thoughts are get another two 4 x 2 s and cut slightly smaller and take care this time getting them in then when the old ones are cut out the riff will settle down by a mm or 2 onto the new.

Another idea was to sandwich these ones with 18mm plywood on each side with polyurethane glue to stop and further splitting.

Or 3 rd thoughts were a bolt down through the 4x2 top to bottom with large washer and nut top and bottom to clamp it together to stop any splitting.

What's your thoughts , I think first option if it's not going to split again. That way it looks normal if a surveyor looks at it on future . It won't look cobbled together as such.

Cheers
Chris

 

[Mike G]

Hmmm.............thinking.........

Those 4x2s seem a bit skinny. Are they in the right place? In other words, that central beam on the floor is bearing on something? Did those timbers replace something other than a like-for-like purlin prop? Odd to be splitting along the centre-line. I'm just thinking out loud here........

 

[Mike G]

I hope you understand that this is no more than good layman's advice, without having seen the problem for myself. I would suggest you do the following:

-Revert to 6x2's, or to 4x4 oak, or a pair of 4x2's.

-Screw a 6x1 SW plate, or similar in ply, to either side of the struts, immediately above where they meet the beam on the floor. Make sure you screw fairly close to the outer (lower) edge of the struts, and not on the centreline.

-Ensure they are the right length, and not over-long in an attempt to straighten out the purlins.

All of the above assume that the central beam is actually load-bearing. If it is just sitting on the joists then we'll need to look at this all again a bit more fully.

You may well have something else going on here: broken rafters, an over-large dormer, a rotting wall plate...who knows. Without visiting the site I couldn't start to guess. If my suggested fixes show a similar problem, please ensure a structural engineer has a look.

I hope this helps a little.

 

[chippy1970]

Cheers Mike , no it is all straight forward nothing else is damaged or sagged. When I say lift the purlin it probably moved a mm if that lol. I didn't need to straighten the purlin it's ok but wanted to take a touch of weight off to help knock the new ones in.

My main thing was I didn't want anyone in the future to worry when they saw the split in the original struts. Then I go and split the new ones too, but I'm pretty sure that's down to me not being careful.

The double sounds an idea I had thought along them lines. Obviously a 4x4 would not go in at the angle but two 4x2s would work.

One thing that surprised me was the old struts were not fixed in any way . Very odd I was expecting a large rusty nail top and bottom but no nothing.

I know some people just take the struts out thinking they're not doing anything lol. Then wonder why their roof has collapsed whoops

I will post back when sorted

 

[Mike G]

The little plates at the bottom will help prevent that lower end splitting, and will hide any splits if they happen. There isn't such a fix for the top, though.

Some suppliers of those semi-finished studs, with the rounded-over edges, use wood which is over dry, or dried too quickly. This is particularly prone to splitting. Maybe, if you are buying new wood for the task, just let it stand outside for a week......not in the rain, but sheltered, yet exposed to the winter air.

 

[Mike G]

The other thing that crossed my mind is to make sure that your birdsmouths are cut ever-so-slightly open. If they are even a little bit closed, then the purlins are trying to prise the timber apart.

 

[chippy1970]

The other thing that crossed my mind is to make sure that your birdsmouths are cut ever-so-slightly open. If they are even a little bit closed, then the purlins are trying to prise the timber apart.

Yeah I thought that too, I reckon that's what happened as I knocked them in it put pressure on the birds mouth causing the split. Which wouldn't happen if it's slightly open as you say.