Help! Pergola roof problem.

[Martin]

I have just built a pergola for a customer the first one I have ever done with a plastic corrugated roof and think I have made a boo boo. Don't get me wrong it looks great and the customer was really pleased with it, or she was until she left a garbled message on my answering machine the other day saying it was leaking! I have not been able to contact her about it as she has gone on holiday, so I'm left to stew about it.

It's built into the corner of a house and looks a bit like a carport. It's 3.5 by 4 meters square with 3 by 2 rafters resting on a 6 by 2 wall plate at the back at the front the rafters slope down to 6 by 2 beams supported by 4 by 4 posts. The purlins on top of run this (running diagonally across the rafters are notched 2 by 2's end on the other wall plate (2 by 3 with 2 by 2 ledge running at a 14 degree incline). All in all it's a solid structure and can take the weight of a man on the roof (bar the plastic.)

I attached the wall plates with screw in thunder bolts making sure I injected the holes first with quality sealant I then sealed the top edge of the plates then butted up the plastic sheets against the plate and stuck self adhesive flashing from the painted wall onto the plate and overlapping onto the sheets.

The problem I think is this, the wall plate has probably warped pulling away the adhesive flashing (only11/2" inches wide stuck on the wall) water must be running down the wall under the back of the wall plate. My worry is that in time water might start to enter one of the bolt holes.

The problems I can see are as follows:

1.How do I get on the roof to make repairs without damaging the corrugated plastic?

2.If I used sealant to fill the gaps and put on a wide layer of self adhesive flashing would this be adequate say for 20 years use?

3.If I decided to use lead flashing could I cut a groove with an angle grinder into just the render which is about 20mm thick and fit it in.

4.Am I being overtly concerned seeing how I did inject the bolt holes?

Any help from an experienced person will be gratefully received.

[Owain]

Well, a pergola is a garden feature not a habitable room - was it expressly agreed it would be completely waterproof?

You /might/ be able to get away with putting cushions above the rafters (to spread the load on the plastic) and resting ladders on them.

I don't trust s/a flashings and wouldn't use one.

A synthetic flashing is probably adequate for this application, but it does need to be cut into the wall, and I would go for deeper than 20mm.

The lady customer will probably complain about the wall becoming stained and the wood starting to decay first.

Owain

[riccip]

A sheet of ply or MDF cut to the max size possible should spread your weight evenly enough to prevent damage. Otherwise you could rent a cherry-picker on a trailer.

No, definitely not. Self-adhesive flashing is nothing short of crap. It's unlikely to last the first winter.

Yes, that's the way to do it. Use a thin stone-cutting disk, cut a deep groove and seal with a good mastic rather than mortar. As an alternative to lead have you considered uPVC? That would be perfectly adequate, cheaper and much easier to fit. You should be able to fit it over the top of the adhesive flashing too. Your local uPVC specialist will have an extrusion just right for the job. Remember to fix it to the brickwork.

Probably. If it's leaking where you suspect then it wants sorting but it won't have done any harm as yet.

riccip

[andrewpreece]

I have some self-adhesive flashing ( bitumen -backed anodised aluminium ) on a slate on my roof. To my knowledge it has been there at least 10 years, and is stuck firmly in place. I have also used it on slate on a dovecote, and it has not deteriorated in two years. I think the criticism of it is a little overdone, though obviously real lead flashing would last a lifetime. I think the main concern is getting it to attach to dusty or irregular surfaces. They sell a primer to go along with it for that, but I have no experience of it. If you use it, I would say use it on a warm day, apply plenty of pressure when applying it and make sure it is applied to a dry, non-dusty surface.

Andy.

[martin]

Many thanks for all your usefull suggestions folks. Martin