Cement in the rain

[Adam]

We had some roofing contractors replacing the flashing on our flat roof earlier this week. They left the job unfinished, as it was raining at the time, and said that they couldn't cement the flashing in place while it was raining. This sounded reasonable, and I believed them.

They came back today to finish the job. It was raining again this morning, but they went ahead and did it anyway.

So, were they just looking for an excuse to skive off early when they were with us earlier in the week, or is the cement they put in place today not going to set properly?

Many thanks Adam

[Phil L]

It depends which way the rain is driving, the other day it could have been blowing against this wall, today it may be blowing in the opposite direction leaving this wall relatively dry...cement still sets in the rain, but running water will cause it to wash out, if it does this, you will be left with a cementy mess on your roof!

[Adam]

Thanks for that. So does that mean that if it looks OK in a couple of days' time and isn't a cementy mess all over the roof then it probably is OK?

Adam

[Phil L]

Basiclly, yes, and it doesn't need a few days, if it's alright after 12 hours it's not going to wash out, no matter how much it gets wet.

[Julian]

Assuming the 'cementy mess' hasn't washed off the roof, into the gutter and down the drain!

Julian.

[nightjar]

Cement sets by chemical reaction, so it will go off under water. Some large yachts even carry a sand and cement mix for running repairs if they get holed. The only problem is if it washes out of position before it goes off.

Colin Bignell