Rules for sprocketing roofs?

[Penners]

I am in the middle of a planning application to build a garage. The design has been broadly approved by the planners - it's a timberframe structure on a brick plinth, with black weatherboarding. The roof is 40 deg pitch red clay pantiles.

However, I have been told that the bargeboards must be sprocketed and capped. I therefore have to revise my drawing.

I'm not sure how to draw the extent or angle of the sprocketing. Is there any hard-and-fast rule as to how far up the roof the sprocketing should extend? As this is a small roof, and pantiles are quite big, I was considering having the sprockets extending just one course of pantiles up the roof. Is this acceptable? Or should I go for two or more courses?

And what about the reduction in pitch of the sprocketing? Is there a formula that governs that, or is just a question of "that looks OK"?

Many thanks for any thoughts on this.

 

[Moo]

Judging by the number of viewings and the paucity of replies, I reckon you've got a lot of us stumped.

Mind you, it doesn't take much to stump me - I don't even know what sprockets are (some sort of widget?), or what capping is. Way above my head.

 

[Mr.C]

You normally sprocket eaves, not bargeboards.

I will attempt a sketch and post asap

 

[Mr.C]

Ok .. got the right diagram ... now how do I send it to this site !!!!!!!!!!!

 

[Mr.C]

On the basis that this might take hours, get hold of the following from a library or 'building' friend;


BUILDING CONSTRUCTION
W.B.McKAY
Volume One
Longmans

Reprint July 1944

Page 75 shows a double roof with sprocketted eaves, and gives the required angles for either slates or tiles.

Yes, an old construction book, but I still use it today for detailing!!

 

[GK]

If you only put a short sprocket on, then only the bottom course of pantiles gets kicked up which looks *rap. Make the sprockets long enough to tilt up 3 pantiles. About 30 inches. Or maybe if it's a really small roof two might look OK.

The barge board would continue in a straight line on the bottom edge and you plant a tapered fillet on the top edge so that it follows the line of the tiling. Effectively, it widens out towards the bottom.

Capping - not sure what this is - but it could be where you put a 3" wide or so flat board on top of the barge board which sails back over the pantiles a bit. I did this once with 2" or so back over the pantiles, about 1.5" forward of the bargeboard and stuck an ovolo beading under it, which visually reduces the depth of the bargeboard and adds some nice shadowing. Obviously the bargeboard has to be high enough to clear the peak of the pantiles.

Getting details like this right can be tricky - yes, it's all about aesthetics but get it right and you end up with a really nice looking roof.

 

[Penners]

Thanks for those thoughts, GK.

I take on board what you say about the length of the sprocketing, but I think 3 tiles would be too much on this roof - it would be something like 25% of the roof. However, I'll definitely follow your compromise suggestion of 2 tiles.

Thanks also for the detailing of how to sprocket the bargeboard.

Your guess about capping is dead right. I like the idea of sailing it over the bargeboard and popping a moulding underneath. You've even got me wondering whether to run a little moulding along the arris of the capping... but maybe that would be over-egging the pudding.

One further question - when it comes to carrying the capping over the ridge tile, do you just increase its pitch a bit before the ridge, to carry it high enough? My only concern about doing this is that - at the apex where the two cappings meet - there's going to be quite a gap underneath the cappings and above the ridge tile. Is this normal, and do you just fill it up with mortar?

Many thanks again for the tutorial.

 

[GK]

Hmmm. As I recall in my case the capping was simply mitred at the top and in fact I dressed a bit of lead over it as it is obviously a susceptible point. I think the bargeboard was high enough to clear the ridge as well but this was a biggish roof.

I did the whole thing dry, without mortar, though I always regarded it as a bit of a risky situation. Before the tiles and capping went on I took a widish piece of dpc and tacked it over the laths up the back edge of the bargeboard, so that when the capping went on it was held in a kind of tension and pressed itself against the back edge all the way up. Any water driven into the gap under the capping would safely run away but just at the point where the capping meets the top of the bargeboard it might have been possible for water to ingress a bit. Possibly if the gap isn't too big on your roof you might fillet it with mortar.

Thinking about how it looks I think the best appearance would be to hide the end of the ridges. Maybe you could fit a smaller fillet towards the apex to clear the ridges whilst keeping the general run tight to the top of the pantiles. Ah, that's kind of what you're suggesting, I think.

It's an interesting detail, around here it's rarely seen but where it is it seems to be on former estate properties, slightly romanticised cottages.

MrC's reference actually has a formula for the angle of the sprocket. It says "they are inclined at an angle which equals the difference between a right angle and the pitch of the roof". The example given is for a 55 degree pitch the sprockets would be 90 - 55 = 35 degrees. This doesn't seem to work for a 40 degree pitch roof though.

Nemesis will have to go and lie down now.

 

[Penners]

Nemesis will have to go and lie down now.

She's not the only one!

As you said, the sprocket angle formula wouldn't work for a 40 degree roof, because they would end up kicking down, rather than up. So I think I'll just go by eye on that one. Hope fully the planning officer will be too busy with important stuff to worry about my little garage!

Incidentally, I've redrawn the sprockets at two tiles length, and you're absolutely right. They look much better.

Many thanks again for all your help.

 

[biffvernon]

Blimey. Round here you nail a few sheets of corrugated iron up for a garage roof.

 

[NT]

so, what is a sprocket? Dictionaries say a toothed metal wheel, typicaly used with chain drives. And I doubt youre sticking one of those on the roof.

NT

 

[NT]

Blimey. Round here you nail a few sheets of corrugated iron up for a garage roof.

dam, luxury! I remember when I had that for a house roof. Could hear the rain before I could see it.

Next you'll be telling me you have windows too...

NT

 

[Penners]

so, what is a sprocket?

It's a short additional piece of rafter, added to the bottom of the main rafters. It descends at a shallower angle than the main rafters, so that the bottom few courses of tiles/slates, etc. are at a shallower pitch. You see that detail quite often in old East Anglian buildings.

Sprockets are used where the main rafters stand on top of the wall plate (rather than oversailing it), in order to carry the tiles over the plate.

I hope that's understandable - if we were having a pint at the pub I could sketch it on a fag packet... if I smoked... and if smoking were allowed in the pub... but I'm too bone idle to upload a drawing and then link it to this post.

The use of the same word as a gear tooth isn't as silly as it seems. The original gear sprockets were short sticks embedded in the rim of a wheel, so the link is the short bits of timber.

 

[Mr.C]

Penners,
Did you get my e-mail?

 

[Penners]

Penners,
Did you get my e-mail?

I sent you a reply about 20 mins ago, Mr C. Many thanks for an extremely useful drawing. Most kind of you.

 

[Mr.C]

You are welcome.

My e-mails have been going astray, hence the ask, and my incoming are soooooooooooooo slow!

 

[NT]

Thanks for the explanatin Penners. It now makes sense in my mind! Which does occasionally happen.


NT