Advice re fitting inset valley boards versus ply over rafters

[RomanAbramovich]

On my bungalow extension I parted existing roof tiles and fixed diminishing trusses up the rafters (what a nightmare that was).

The trusses are at 600 centres, existing rafters are at 400 centres.

The roof intersection new to old forms two valleys, each 7 metres long.

Each roof structure has a different pitch, difference 10 degrees, therefore local roofing merchants sold me grp dry valley unit, to handle the pitch issue.

Installation instructions mention support for the grp units can be via inset valley boards or alternatively on 6mm ply laid over the rafters. Obviously with these valley lengths the ply overlaid would be the preferred option - otherwise I need to cut 28 valley boards for the existing rafters and 12 valley boards for between the diminishing trusses.

Is 6mm ply substantial enough to fix the ends of tile battens to? Assume the 6mm increase in tile batten height where it meets the ply board won't be enough to make the tiles kick up?

If anyone has tackled a similar 'challenge', your advice would be much appreciated.

 

[Bcg74]

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

 

[RomanAbramovich]

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

 

[Bcg74]

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

6mm would hardly be noitcable at all tbh.especially with the big upstand in the middle,which pertrudes above the tiles.if your using noggings you should be fine.or if you feel more comfortable, you could extend the nogging past the plywood ? So half of the nogging has the ply sat on it then the other half ,has your battens on it ?

 

[RomanAbramovich]

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

Valley boards inset between the rafters , would be my preference , make them wider than your GRP valley , then the extra will carry your battens. 6mm ply won't be sufficient enough to carry your battens, you would still need noggings !

Sorry I neglected to mention the ply would be supported on noggings - noggings are specified for support under the ply in the grp dry valley installation instructions.

I'm tempted with the ply over rafters option because the noggings will be much easier to cut and fix, when compared to inset valley boards. Just concerned the 6mm step in height of the ply board will cause the tiles to kick up, where they hit the valley - anyone of the opinion this tile kick up will be really noticeable?

6mm would hardly be noitcable at all tbh.especially with the big upstand in the middle,which pertrudes above the tiles.if your using noggings you should be fine.or if you feel more comfortable, you could extend the nogging past the plywood ? So half of the nogging has the ply sat on it then the other half ,has your battens on it ?

Great tip - cheers. Sketched out some designs and using your extended nogging idea think it's possible to work it so there isn't any 'step' due to the ply...

Tile battens on site are 50x25. If 6mm ply used, cut 200mm wide strips (grp valley unit is 400mm wide). Support with 4b2 nogging on edge, leaving 35mm/40mm extended past the plyboard. Buy and attach 19x38 batten to the top of the outside edge of the plyboard - this forms the counter batten (which fits under the grp unit profile). When the tile battens are butted up against the counter batten, and fixed down onto the nogging, both tile and counter batten SHOULD be the same height. Sound feasible?

 

[RomanAbramovich]

 

[noseall]

6mm ply is very flimsy especially in the spaces where the nogg's are missing. 18mm between supported on battens is much more robust and will not bounce when being fixed to.

Untitled

• noseall
• 6 Jun 2015
• 1

 

[Bcg74]

View attachment 81864

Yeah , Looks good to me , if your adamant , you want to use 6mm ply , I'm sure that will work

 

[RomanAbramovich]

Cheers - Had half a sheet of 18mm ply in the garage, so attempted the inlayed boards, as per noseall's picture. Been subjected to several heavy rain showers today, meaning most of the time been spent uncovering / covering the exposed roof void, just to get / weather proof at the valleys

Inbetween the uncovering / covering / rain, cut 7 'common' valley boards and nailed them to 50x25 noggings between the existing rafters...rock solid.

Only 33 more to cut and fix.