Ok ... first off, a confession ... in winter, I burn a small amount of wood in an open fire from time to time. I apologize! I know this forum can get quite antsy on this subject. But I have loads of the stuff lying about the place, and I love the atmosphere ... especially the particulates etc.!

So we have a pumice flue liner all in place. The old 30's art deco house had no cowl at all ... just the flue open to the elements, with a rendered (& weathered) concrete surround. Seemed to work ok, but I want a cowl (to protect from the rain now the flue is completely vertical & from birds etc.) and I want to maintain the boxy, rendered look of the original chimney. Eg. I don't want a terracotta pot and traditional cowl etc.

So can anyone pick holes in the attached design I've come up with? In the picture I've half hidden the concrete capstone on top so as to show some of the internal detail ...

MUCH APPRECIATE any words of wisdom ....

On my travels through europe going ski-ing (mostly mountains so lots of rural houses that burn wood) nearly all the chimneys I have seen are similar to the one above but just a pillar in each corner so nothing to clog but flue covered to stop rain/snow and lots of air to breath. Its also what I would like for my new build (planners allowing).

Don't bother trying to calculate how much wind will be needed to lift the stone - just provide some mechanical attachment e.g. stainless steel bolts epoxied into the chimney and through the cap stone which can then be held by nuts.
I agree that the SS mesh is likely to clog, but I would also make the internal surfaces steeply sloping to stop birds nesting in the nice flat protected surface. The slope, if steep enough, will also be a bit self cleaning - especially when agitated with a chimney brush from below

Good point about the slope from the flue top down to the apertures ... will make an adjustment for that. Might mean four weep holes or similar, just so I can keep thin apertures for the actual smoke vents.

But re: the SS mesh - was thinking 25mm x 25mm apertures minimum - can't see that this is going to clog massively quickly, but then I've no experience yet of this. It's a flat roof though, and we'll be up there lots, so absolutely no sweat to check it now and again.

The reason I think the mesh could cause a problem is just the fact it will be a cold surface in the path of the exiting gases. Condensing of the creosote is quite likely IMO but it sounds like access is easy so cleaning would be no big deal.

Posted By: scrimperGood point about soot laden rain spoiling the render ... shall work on that! Thanks guys ... am pushing the 'go' button!

Put the apertures at the top and the slopes facing inwards so that anything blowing under the overhanging cap stone goes down the chimney, which is where any precipitate from combustion within the chimney will be going any way.

Won't the weep holes eventually clog up? When you clean the flue the action of pushing the brush upward will take with it some debris (soot and muck) this will overspill into the chamber. What about soot build up on the underside of the capstone?

How about, as a capping inserting/resin anchoring 4 x stainless 12mm pieces of studding on the support pillars, with pieces of spacer tube over and then have fabricated a stainless cap with small downstand all round. Drilled secured from above with stainless dome nuts with spring washers and a spot of locktite.