Hipped roof to Gable conversion

IWWNCU

I am planning to build a Gable wall in place of the existing ,hipped roof section of my dormer bungalow in order to maximise the floor area of the bedroom underneath. My question is how do I build the new Gable without having to remove the existing cavity closer (block on flat). Is there an easier method ?.

RKQ

IMO it would be best to remove the cavity closer block.
This will allow better construction of plumb blockwork. It will also allow the new cavity insulation to be joined up with the existing insulation. Giving better overall insulation.

Blocks are cheap at circa 50c each and any other alternative, if one exists, will take longer to do and cost you more in the long run.

IWWNCU

I understand what you are saying. However the wall plate and joists are sitting on the cavity closer, so, in order to remove the block on flat I will have to support the joists from underneath which i think will complicate the construction and add to the cost. Is a stepped DPC at the cavity closer a viable option ?

RKQ

Ah! now I see where you are coming from! I still think a few Acro props circa €3 each per week, is a small price to pay.

Prop the floor, remove the wallplate and cavity closer and build the joists into the new blockwork wall.

What size are your existing ceiling joist / floor joists?

I don't think a stepped dpc will be 100% effective. What do you plan to do with the existing wallplate?

I'd try and "grab the bull by the horns" and save money and time in the long run. It will also result in a better building - structure wise, insulation wise and dpc wise.

IWWNCU

My concern is that if I do as you have recommended, I will invariably end up damaging the ceilings downstairs and adding cost and disruption . Is it physically difficult to remove the block on flat ?. My joists are standard 9x2.

RKQ

Its not physically difficult to remove the cavity closer. In your case it will be messy as you'll be removing part of the wall of the existing room below and the acro's will be a potential risk to the ceiling finish.

Of course removing the roof will open the ceilings to the elements for a period of time. So slight damage at the very least must be expected to the existing ceiling slabs.

On the positive side you will need to plaster upstairs in the new conversion, so any damage to plaster work can be repaired at a reasonable price!

It might be structurally possible to have a Timber frame company design new loadbearing gable walls and new roof etc that can be strapped to the existing structure / wallplate and joists. This could address the wall insulation. Its worth investigating.

What does your Architect / Engineer recommend?

Its difficult for me to help further as I'm not fully familiar with your existing structure. It worth getting professional advice.

Quazzie

How would the inside leaf carry on up to the new gable if you are leaving the cavity closer and wall plate there? The outer leaf might be doable (albeit structurually inept imo) but surely the inner leaf would be impossible. There would also be massive thermal brodging where the current cavity closer is and that is probably the last place you want it.

I'd recommend some seriosu structural advice that can only be given by a professional after inspection.

Poor Uncle Tom

When was the original dormer built?

IWWNCU

The house was built in 2001. The floor area involved is approx 32' x 15'. I actually had a reputable builder look at the job, he wasn't overly concerned with leaving the cavity closed although he agreed ideally it should be removed but that it could add significant cost to the renovation. He advised putting a stepped DPC in and to fit insulation at the cavity closer to prevent Thermal bridging. I'm interested in the timber frame option for the inner leaf, would this eliminate the need to open the cavity ?. I know this is a difficult question to answer but any ideas how much this job would cost, to complete to 'first-fix' finish - all the services are easily accessible for the job.

Stuxnet

the cheapest way to do it would be cut the rafters at the birds mouths at the wall plate, and build a timber frame gable, finished with wbp plywood, breathable membrane, concrete board, and plaster, if you could use a 6 inch stud aswell, you'd get a nice bit of kingspan or something similar to insulate the inside, its not a big job its light and perfectly good as opposed to pulling out joists and cavity closers which is alot of work very messy and expensive, same results imo
carpenter

Poor Uncle Tom

Because the house was built 8 years ago, and most of the settlement has occurred and all the mortar joints are fully cured, I would take a different approach.

I would leave the cavity closer block in place and pour a reinforced beam over, incorporating a stepped dpc, to spread the load evenly. After 8 years you will not get a good blend of mortar joints between old and new and with new point loadings you will get cracking in the existing walling.

The wall plate should be removed and the joists cut back, hangers should be incorporated in the beam.

I wouldn't even hazard a guess at pricing the works without seeing the job first hand.

IWWNCU

Bobby, is the 'concrete board' different to a conventional concrete block external leaf ?.

Will I be using the existing wall plate as the sole plate for the timberframe ?.

Is this work something a roofer would be able to undertake without much fuss ?

RKQ

The best thing to do is retain a professional to answer these questions, inspect construction and ultimately Certify the construction. It is extremely difficult to answer your query without inspecting the existing dwelling.

I really think you need to get professional advice.:)

IWWNCU

Had an engineer look at this saturday evening. He was happy to go with stepped dpc option. I get the feeling that ultimately I will have to make the call on this myself. I was hoping that somebody out there has done something very similiar and would share their knowledge.

muffler

Listen folks, I dont want to put a dampener on things but please be aware of one point that is contained in the charter concerning giving advice on structural elements - Its not allowed.