Architect or tile manufacturer? Who knows best?

[Jubilish]

We have planning permission to build a low pitched (15°) tiled roof. The tile manufacturer (Weinerberger, 'Sandtoft 2020') states the need for 'secret gutters' for all sloped abutments (and provides a sectional drawing). Our architect (experienced but close to retirement) states secret gutters are not necessary and to just use soakers with cover flashing. It feels like we might be being gaslighted, but then again manufacturer might be over specifying anyway. To complicate matters somewhat, one abutment is not at 90° (more like 115° shown in red) which means, discounting wind, most of the runoff will be directed away from this junction. The other party wall abutment (solid brick) is a standard 90° (shown in blue) but a boiler flue will puncture the roof near the party wall. Who would you trust the most, the architect or the manufacturer's resident expert?

 

 

The manufacturers secret gutter section is online here (left hand depiction which also omits the counter battens detail for slopes less than 22°). The manufacturers installation advice on side abutments is:

Side Abutments Generally: Weather side abutments using a secret gutter. In addition, to prevent the possibility of the secret gutter becoming blocked by debris over time, a cover flashing is also recommended.

The manufacturers response to my specific issue:

We would advise that the side abutment (secret gutter), top edge abutment and valley are all formed in lead – the detail required between side abutment, top of valley and top edge abutment is bespoke so we do not have anything drawn up for this but would suggest a lead welded saddle is used to weatherproof the junction.

My Architect has not drawn a section, but his wording for the pertinent roof covering / parapet wall sections are as follows: (all figures: metric mm)

9.11 Fix 350w x 50h stone copings to 225 wall laid in 1:1:6 cement:lime:sand mortar on 250w polyethylene damp proof course.

13.1 Install 125 thick insulation between rafters.

13.2 Lay breathable sarking membrane taught over whole roof laid parallel to the eaves with 100 horizontal laps, sealed with 75 double sided acrylic adhesive tape, and fixed with non-ferrous felt nails or staples to each rafter at 300 max centres. Fix underlay support tray at eaves and dress membrane over the underlay support tray and into gutter. Form valley gutter in 19 wpb (weather and boil-proof) plywood extending 225 each side of centre line of gutter with tilting fillets fixed 150 each side of centre line. Line valley gutter in code 4 lead (i.e. 1.8mm) laid directly onto ply base and dress membrane into gutter over tilting fillets. Lead lining to extend beyond fascia to discharge into gutter. Fix 12.5 wpb ply to existing rear wall and party wall to neighbours and to raking wall to side lane to form base for fixing membrane and flashings. Dress membrane 100 up abutments and seal to ply with 1.5 butyl rubber tape.

13.3 Fix sawn 50 x 25 softwood battens and 50 x 19 softwood counter battens, CCA or OS preservative treated, with 65 long aluminium or stainless-steel nails through 'Nilvent' membrane into rafters, battens fixed at 230 maximum gauge. Fix 25 deep over-fascia ventilator to top of fascia board.

13.4 Cover roof with 'Sandtoft 2020' tiles laid using tile-and-a-half tiles every alternate course with left hand verge tiles so each course is half bonded. Use tile-and-a-half tiles on every course each side of the valley gutter, neatly cut to form 150 wide gutter. Fix every tile with 45 x 3.35 aluminium ring shank clout nails and clips. Fix 1no. 'Sandtoft concealed tile ventilator', colour Flanders, with pipe adaptor, connected to duct from extract fan in kitchen. Form opening for gas boiler flue and seal with lead collar around flue pipe and lead flashing set under and over tiles.

13.5 Interlace tiles with code 4 lead soakers at side abutments dressed 75 up wall and fixed to ply base. Soakers to extend 100 over tiles

13.6 Cover soakers abutting side walls with code 4 lead flashings wedged and screwed into chases under copings on wall to lane and into raked out horizontal joints in brickwork on exposed party wall to neighbours, seal joints with mastic.

13.8 Cover top abutment to rear wall of house with code 4 lead abutment flashing fixed to ply upstand and dressed 100 over roof tiles; cover with code 4 lead cover flashing set into horizontal chase cut into existing wall, seal joint with mastic.

When I queried his omission of a secret gutter the architect responded:

Raked secret gutter is not necessary as the water sheds away from the raking parapet wall; normal soakers and cover flashing will suffice. The secret gutter would be applicable where water from a roof sheds towards a raking parapet wall.

I am really in a quandary about all this. I would prefer to have stepped flashing on both abutments too, which would imply building the top 1/5th of the inner cavity wall near the parapet in normal brick instead of blockwork.

Any advice or comments would be greatly appreciated, should I side with the tile manufacturers on this occasion? Thanks in advance.

[nod]

They are probably both right 

Soakers will probably be fine 

But a manufacturer will always go belt and braces 

 

We have had to drop the pitch on two low level 30 degree roofs Down to 27

The slate suppliers have recommended dropping the size of the slate to compensate 

 

[Temp]

I've got a feeling not all interlocking tiles can be done with soakers. The soaker messes with the way they interlock. Perhaps get a few samples and do a mockup?

 

Generally flat tiles must have lead soakers not just a flashing strip over the top. Pantiles can get away with just a strip on top if it's wide enough to cover one hump in the tile.

 

Their guide on low pitch roof also says..

 

Quote

There will be conditions, though, when snow or rain finds its way through the tile joints; particularly where the tiles may not be seated as closely as normal as they cross over, for example, flashings.

 

Edited January 4, 2023 by Temp

[PeterW]

I would go secret gutter and cover over - with that low pitch I’d want all the joints as tight as possible. 

[Jubilish]

 

22 hours ago, Temp said:

I've got a feeling not all interlocking tiles can be done with soakers. The soaker messes with the way they interlock. Perhaps get a few samples and do a mockup?

 

Generally flat tiles must have lead soakers not just a flashing strip over the top. Pantiles can get away with just a strip on top if it's wide enough to cover one hump in the tile.

 

Their guide on low pitch roof also says..

 

 

 

Thanks for your thoughts (and others' comments), very useful; the manufacturers supply LH and RH end tiles (plus 'tile and a half' versions), so interlocking tiles might be less of a problem, but the profile has lugs that (perhaps) can't just be lopped off as the lugs have corresponding indents on their upper surface.... 1.8mm lead soakers are reasonably thick, I suppose. I think I need to order a sample to see for myself.

I spoke to a cavity tray expert who just recommended dressing up the membrane up and under the coping to seal it up, but as I understand it, breathable membranes are not intended to be watertight as such.

 

[Temp]

I believe this is the tile. The lead soaker would cover the indents at the top but not much more or it would be visible. Is that enough to ensure its waterproof on a low pitch?