rafter height advice

[tom patton]

Hi all. I am setting out my 4 main Kerto beams for building my pyramid roof on-roof is zinc sheeted. The architect/structural engineers idea is that you cut two slots in the existing flat roof set up 2 beams 7 metres long on inner brick load bearing walls then you build the roof on top based around the other two which rest on the first two at right angles forming a 6.1 metre square. Problem is they did not check the slope of the existing flat roof the slotted in beams are below roof level the brickie-who I agree with says just put pads under the beams to raise them-they are 150x200 in cross section surely there is no problem in raising them by 40mm on structural C24 timber pads.

[BigWallop]

I'm glad you say you have engineers involved in this build, because a sloping roof on top of walls, even supporting ones, generates far more outward pushing forces than the flat roof does.

What I think your engineers are telling you is, cut slots in to the edges of the existing roof until you hit brickwork. Once you can see the outside corner of the top row of bricks, then you place the bottom edge of your new main purloin beams against it until you get the angle you need to meet both diagonal corners of the new roof equal. The height of the brickwork shouldn't matter, because what you need to achieve is the same angle on both side of the roof. Once you get the first diagonal pair of beams in place, then to other two should fall against them with no problems.

Roofs don't sit directly on top of the walls. A roof over-hangs the edges of the building to keep the rain from running directly down the walls. The height of all the walls could all be different, but the roof angle will remain the same all round because it only sits on the top of the building. Bracing timbers or strings stop the angle from opening up, so this is what holds the roof on.