I have a tricky rafter problem

**hobey** · 22nd September 2012, 12:32 AM

Hi all

This is an interesting one and any help appreciated.

Have an old Edwardian. Discovered a split rafter, which I can sister from ridge beam to wall so that ok. But checking the span tables for the rafter length I have (3.6m under tiles 450mm spacing, no purlins) I should have 120x45 or even 150x45 in F17 at N1, N2. The existing rafters, including the split one, go back to 1920s are old hardwood 2x4s. Theres a few of them. So a little under sized but the roof is still on!

So what do I do? There really isnt scope to put in a 150x45 or even a 120x45 without raising the entire roof, replacing all the rafters etc. No thanks. If I birdmouth a 120x45 deep enough at the wall I can get it in under the battens but Id need to notch close to 40% of rafter height at the wall which is not allowed. I understand its 25% max on birdmouthing. Installing purlins is difficult as there are no walls to support the struts all the way along at 90 degrees to the rafter. If I were to put in a strutting beam it would have to span all the way from the party wall to the outer wall and again the roof is too low to attach the strutting beam at that outer wall.

At the wall the rafters sit on a wall plate which in turn sits on double brick. There isnt any scope to remove brick work here to fit in the larger rafter.

So thats my problem. All help or ideas appreciated.

**Acco** · 22nd September 2012, 12:51 AM

Suggest that you copy and paste your thread over to our sister site

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**rwbuild** · 22nd September 2012, 01:40 AM

Replace rafter with KD F17 same size as existing then fit either a cable truss under the rafter or a barrup rod but I would suggest that since it is an existing roof you need only match the existing because it is only one rafter and not a complete or partial roof rebuild

**rustynail** · 22nd September 2012, 04:28 PM

Fishplate the split rafter with 15 mm structural ply. Bonded and nailed or dry screwed at 300mm centres in two rows for full length of rafter. Stager end butt joints of ply on each side of rafter.

**rwbuild** · 22nd September 2012, 05:22 PM

Originally Posted by rustynail

Fishplate the split rafter with 15 mm structural ply. Bonded and nailed or dry screwed at 300mm centres in two rows for full length of rafter. Stager end butt joints of ply on each side of rafter.

rustynail is spot on. I had forgotten about that one, a common fix method for pre fab trusses and certified by design engineer, much easier than my suggestion.

**John Samuel** · 23rd September 2012, 08:50 AM

Rustynail's advice is good. In a previous life did a repair the same way ... glued and screwed 3/4 ply to the rafter (Selley's 308 used for strength and for gap filling).

Another fix I've used when the rafter was rotten was to replace it with a laminated beam made with kiln dried hardwood and Selley's 308 glue. The beam was made oversize with a slight upwards curve in it. You can then cut this curve out (to the desired size) once the glue had dried. I chose to leave the curve in and chock under the battens. It was far stronger than the original.

If you are really concerned about strength, yet another approach you could use is the put two of the same size timbers in, either side of the crook rafter.

**soundman** · 23rd September 2012, 11:29 AM

You can forget all ya modern tables and expectations, you can almost guarantee that none of the existing structure even remotely resembles the modern framing rules.....BUT these buildings still stay up.

One question is ...How long has the rafter been split? HMMM

One method that has not been commonly used for a very long time is the metal tape or wire wrap...depending on how it is split, this is one thing that would have been done in the 50's or earlier.

The split or a made scarf joint is wrapped tightly in metal tape or wire ( tape or strap is better) over the full length of the split or scarf and then some, being nailed off at regular intervals.

The tape is nailed off at the end, and is pulled tight at every oportunity with a crowbar with a spike thru the holes in the tape...pulled tight and nailed off.

The every roof trusse in my parents house had a taped scarf join.

I don't know how modern enginering would view this......But I can show streets of houses done like this.

cheers

**rustynail** · 23rd September 2012, 07:52 PM

bit hard wrapping tape round a rafter when the roof is already on.

**soundman** · 23rd September 2012, 11:29 PM

That depends on where the split is and how much room there is in the roof space.

cheers

**rustynail** · 24th September 2012, 03:02 PM

Once a rafter has developed a visible split, it is fair to assume that even though the damage may not be visible along the full length of the timber, there is every possibility the fibres may well have been compromised. Therefore, in the interest of carrying out a satisfactory repair, it would be advisable to reinforce the full length of the rafter. As the rafter is purlin free, it should be easy enough to get the necessary ply wood up to the rafter. If internal access is limited, the ply can be slipped in by removing a bottom tile.
A strapped or bandaged rafter doesnt meet the building code and could present problems with an insurer

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