Cavity Trays

[Craig15]

Hello, I'm struggling to find any approved guidance for installation of cavity trays above the roof of an extension. Forums mention remove three bricks at a time, fit tray, pack and point brickwork then on to the next three - but then the weight is not uniformly distributed over the area of the entire brick, just the packer. Plus I thought slate packing was a no-no with most BCOs nowadays?

The best way I can think of would be to do in two phases - remove every other three bricks over the span, fit the trays and ram in the mortar. Allow a few days to go off and then do the rest. But to be confident of a full bed to spread the load from above, how can you confidently pack the joint without any snots getting into the cavity? Also none of the literature I've seen from the companies who sell the preformed trays show insulated cavities, any issue here?

Cheers,
Craig

 

[^woody^]

how can you confidently pack the joint without any snots getting into the cavity?

Do a seven-year apprenticeship?

Slate packing is a means for retrofitting a beam or such like internally, not this. This needs to be pointed, and fully filled.

You can take out alternate 4 or 5 bricks worth of wall normally, fit the tray and then replace the bricks and point with mortar. Just allow for the overlap and don't replace all the bricks you take out before removing the next section. Insulation needs to go in if the wall is above the roof or insulated ceiling.

 

[Craig15]

Haha but on a serious note how many 'replaced' bricks do you see on full beds - they are little more than a re-pointing job so that nothing gets into the cavity. I would like to see how many retrofit cavity tray jobs are full and without any snots. So using your example you would remove 4 bricks, and replace 3 giving for the overlap? One of the manufacturers have since replied to me and advised slate packing and pointing - which reinforces my point, I think the trays are a necessity and an important subject but IMO there is conflicting and limited knowledge around the subject, and when you are removing near a full course of brick and re-fitting there is potential for major structural issues. I think this is supported further by the amount of extensions I see with damp problems, where the trays have been omitted.

 

[^woody^]

It depends on how good the person doing it is. If he knows his job, it ends up with a full joint. There are also repointing guns which can help a lot.

The thing is, it's not impossible or unachievable, or even hard to do.

 

[Notch7]

Haha but on a serious note how many 'replaced' bricks do you see on full beds - they are little more than a re-pointing job so that nothing gets into the cavity. I would like to see how many retrofit cavity tray jobs are full and without any snots. So using your example you would remove 4 bricks, and replace 3 giving for the overlap? One of the manufacturers have since replied to me and advised slate packing and pointing - which reinforces my point, I think the trays are a necessity and an important subject but IMO there is conflicting and limited knowledge around the subject, and when you are removing near a full course of brick and re-fitting there is potential for major structural issues. I think this is supported further by the amount of extensions I see with damp problems, where the trays have been omitted.

I cant say Ive heard of structural issues as a result of cavity trays, I wonder if you are overthinking it.

 

[Craig15]

I probably am mate - "but ask a question and look like a tit for ten minutes" - in the end if it helps me improve and gives the customer a better job I'm happy. Thanks for replies.

 

[23vc]

Do you definitely need one? I didn’t (thank god) as existing pointing was decent, not mega exposed area etc.
But yeah I guess things like this make the difference between someone who “can lay bricks” and someone who really has the time served skill. Hence I wasn’t fancying doing it on mine even though I built the whole extension(s).

 

[^woody^]

You fill the frogs and butter the top of the brick before you slot it in. Thicker at the back, and taper the mortar. This stops you pushing the rest of the mortar straight through. Then, you slide semi dry mortar in and tamp it. Rinse and repeat.

 

[23vc]

I like the tapered mortar thing. Wouldn’t have thought to do that. By that logic do you taper the top of the brick in your hand with the thicker edge at the front then? If that makes sense

And why semi dry, because it’s less likely to make a mess, or cos it won’t slump?

 

[stuart45]

Depends on how much time you have to spend on it. You can use a wetter mortar to just fill a bit at the back, and then fill with a dryer mix the next day as it's easy to push it in tighter then. Most people don't have the time for this, so it's done as woody said.

 

[stuart45]

And why semi dry, because it’s less likely to make a mess, or cos it won’t slump?

Less shrinkage.

 

[^woody^]

I like the tapered mortar thing. Wouldn’t have thought to do that. By that logic do you taper the top of the brick in your hand with the thicker edge at the front then? If that makes sense

And why semi dry, because it’s less likely to make a mess, or cos it won’t slump?

Thicker at the back so that you can poke additional mortar in afterwards. And the mortar doesn't get pushed out as you slide the brick in, but it mostly fills the rear half of the joint at the back.

Semi dry is easier to use, packs in without it slumping, less shrinkage when drying, and yes less chance of staining.

 

[noseall]

The best results are more difficult to achieve but involves buttering the brick AND the recipient bricks. By buttering both, you are priming both surfaces and the results are so much better. Very tricky trying to get mortar to stay on the underside of the recipient brick though. We use water to dampen the surfaces.

Whenever I slot in the last brick on a run, I use this method, i.e. butter (perp's) both on the brick and his mates already laid with both sets of perps shaped so that they slot in together.

TIP: Lay some mortar on a brick to let it dry out for retro-pointing.

 

[Notch7]

Do you definitely need one? I didn’t (thank god) as existing pointing was decent, not mega exposed area etc

Cavity trays are sometimes a judgement call. Its impossible to make the retro fit invisible and some are as you say low risk where below big overhangs or sheltered aspects.

One thing I would say is any areas at risk of damp, avoid using bonding plaster -its like a sponge. Either use waterproof render as scratch coat or batten and use foil backed plasterboard. I always do that on old solid walls that could get damp.