Two pitched roofs converging - drainage?

[loply]

Hi folks,

I'm planning a DIY garage build which is (at long last) about to get underway.

Ideally I want to put a pitched roof on the garage which is attached to the side of the house. An existing extension has a small lean-to pitched roof, and the garage roof would meet this.

I guess I need something like a valley gutter but I'm struggling to figure out the detail of it, ie how I ensure both the garage and the adjoining room are kept watertight. Can I simply put the garage roof trusses on one leaf of the cavity wall, and the extension trusses on the other leaf, and a box gutter in the middle?

The garage roof trusses are quite big (7m span).

Would appreciate any advice.

Cheers,
Rich[/img]

 

[Blagard]

It is a valley and a traditional way to deal with it is a lead gutter to falls that is formed with steps in it to suit the normal maximum length of lead in that situation. around 1.5m to each step . It is a horrible thing to deliberately design because it requires regular inspections to ensure leaves, twigs moss etc don't block it.

Example
Another Example


I think I would prefer a narrow flat roof between the two pitched sides falling away from the middle to front and rear. The same principle but avoiding the rather narrow lead gutter and as DIY, done in something other than lead although I do believe it is the best material for the job it is highly skilled work. The second example demonstrate the need for lead rolls once the width gets bigger

 

[loply]

Thanks Blagard.

If I put a narrow flat roof what would it be supported on? I've worked out the detail below:-

I guess I can hang the scissor trusses for the garage roof on something resembling a joist hanger, and I can probably create a few inches of 'drop' in the gutter too by angling right?

Cheers,
Rich

 

[Blagard]

If I put a narrow flat roof what would it be supported on?

You would use bearers across and between the rafters of each roof slope with some vertical propping if necessary. The bearers would be at different heights to create the fall. You can see four such bearers in the first photo of "another example"but there would be one across every pair of rafters before the deck goes down to support the flat roof material. A timber like the one running front to back just under the bearers could be used if the rafters don't match up conveniently

Your drawing is not what I would expect to see, so I think you should get it drawn up by someone with better knowledge of construction. While your main house rafters could be built as drawn, it is usual to find the rafter bearing on the inner leaf and on a wall plate. Are you sure you have drawn what you actually have?

 

[noseall]

The project needs designing and detailing with particular attention paid to the valley.

As said, the valley will need to spill at one end or t'uther or both ends and this does require thought. Guessing on here won't help.

Your own drawing detail does not grasp basic building.

 

[indus]

It is a valley and a traditional way to deal with it is a lead gutter to falls that is formed with steps in it to suit the normal maximum length of lead in that situation. around 1.5m to each step . It is a horrible thing to deliberately design because it requires regular inspections to ensure leaves, twigs moss etc don't block it.

Example
Another Example


I think I would prefer a narrow flat roof between the two pitched sides falling away from the middle to front and rear. The same principle but avoiding the rather narrow lead gutter and as DIY, done in something other than lead although I do believe it is the best material for the job it is highly skilled work. The second example demonstrate the need for lead rolls once the width gets bigger

May I ask what the steps in the lead are for?

Many thanks

 

[noseall]

May I ask what the steps in the lead are for?

Large or long sections of lead should be laid in pre set pieces and never in one big piece, to allow for ex-con depending upon the lead thickness.

Where one piece of lead laps or joins another, there needs to be a weathering system.

On a flat roof this can be achieved with mop-stick and lead rolls. On a valley it is done by using steps so that each piece of lead upstands under the downturn piece from above.

 

[jeds]

Steps are very awkward to detail and get right. I'd go for a flat lead valley to a suitable fall with expansion joints instead of steps.

 

[loply]

Thanks for all your help folks.

I suppose the other option is to convert the extension to a flat roof to simplify everything, but I do think it looks better with a pitch. Mind, if it was flat I could store my spare tires/wheels/etc up there...

The pitched (lean-to) roof on the extension needs removed and rebuilt anyway so I'll build it again at the same time as doing the garage roof.

Would it be acceptable to have the garage trusses bear on the outer leaf, the lean-to trusses on the inner leaf, and the gutter running somewhere in between them (ie the gutter more or less in the cavity)?

Cheers,
Rich

 

[Blagard]

May I ask what the steps in the lead are for?

I appreciate noseall has given a reply, but I will expand on that.
Lead while being a first class material for roofing has one major issue. It does expand and contract a lot through temperature change. If you lay the lead in too big a piece what happens is it can't move enough and so finds a weak spot that gets flexed and stretched excessively and finally it will split.

The way to deal with it is to lay the lead in panels with joints that allow movement. Typical joints are created by steps across the running length or rolls to the sides. The step allows the top piece to slide over the lower bit with just a simple lap joint. If you lap it flat, water will leak back, so laps without steps only get used where there is a decent fall like pitched valleys. Even a simple lead flashing (code 4)should only be fitted in short lengths (no more than 1.8m) with lapped joints. If you ever see a long unjointed flashing, expect to find it split in a few years time!

Standard and proven details for leadwork can be found on the lead association web site http://leadsheet.co.uk/ although only samples are free now.

Of interest to me was the neoprene expansion joint when I checked - It seems a waste of lead to incorporate a material like neoprene that I would expect to fail decades before the lead!

http://leadsheet.co.uk/assets/files/LSA_Bulletin_Gutter.pdf

FYI the lead codes code 3, 4 etc are a hangover from imperial days as you will find that is the approximate weight of the lead in lbs per square foot

 

[munchingB]

i have a valley to repair and if i were building a new roof i would avoid having a valley at all costs.

just seems that the convergence of water from 2 roofs is asking for trouble and when they do fail the damage is also doubled as are the repairs. r
when youre doing your detailing try imagine where it will fail and where water will run off and how long it will be until youre aware of it..

 

[indus]

Many thanks

 

[loply]

Hi folks,

Thanks for all the advice. Been thinking about this for a while and I'm wondering if it might be advantageous to knock up a 'section' of flat roof as per the sketch below. I would make this 'box gutter' type thing from ply then fibreglass it and wrap the fibreglass under the tiles, and have it high at one end?

Really appreciate the help/advice.

Cheers,
Rich

 

[jeds]

I think you may be over thinking it a bit. The normal route would be to get the structure right and then just construct a valley gutter at the point where the roofs converge. This is easy if you don't have steps - it becomes a bit trickier if you do. Personally I would make a flat sloping gutter with expansion joints. Dead easy to do and makes finishing the bottom course of roof tiles a lot neater and tidier. And will service for many years without issue.

PS. The deep box section gutter you show will not be that easy to get right. Try drawing it in section and you'll see what I mean. Also, either way you need to be careful if the roof pitches are shallow - yours do look a bit shallow but could just be the sketch?